Online English Summarizer tool, free and accurate!
Distributed Systems
Introduction
Venus Samawi
Isra University 1
Content
• Distributed Computing & Distributed System
• Advantages and Disadvantages of Distributed Systems
• Characteristics of Distributed Systems
• Examples of distributed systems
• Resource Sharing & The Web
• WWW
2
Distributed Computing & Distributed System
• Early computing was performed on a single processor. Uni
processor computing can be called centralized computing.
• Twoadvances in technology began to change the situation
• Thedevelopment of powerful microprocessors
• Theinvention of high-speed computer networks
• Distributed computing is computing performed in a distributed
system.
• ADistributed system is:
• Acollection of independent computers, interconnected via a network, capable
of collaborating on a task,
• One in which components located at networked computers communicate and
coordinate their actions only by message passing,
• Acollection of independent computers, that appears to the users of the system
as a single computer.
3
Advantages of Distributed Systems
• The motivation for constructing and using distributed systems
stems from a desire to share resources.
• Economics: distributed systems allow the pooling of resources,
including
• CPUcycles, data storage, input/output devices, and services.
• Distributed systems have a better price/performance ratio
• Concurrency: we can solve the problem more quickly using
several processors concurrently.
• Some applications are inherently distributed : In some problems
the most natural solution is to use separate parallel processes to
perform the subtasks of the given problem.
• Computer Supported Cooperative Work
4
Advantages of Distributed Systems
(Cont.)
• Communication: make human-to-human communication
easier, for example by electronic mail.
• Reliability: a distributed system allow replication of resources
and/or services, thus reducing service outage due to failures.
• Incremental growth: computing power can be added in small
• increments.
5
Disadvantages of Distributed Computing
• Multiple Points of Failures: the failure of one or more
participating computers, or one or more network links, can
spell trouble.
• Security Concerns: In a distributed system, there are more
opportunities for unauthorized attack.
6
Characteristics of Distributed Systems
• Concurrency of components (or Parallel activities)
• Autonomous components executing concurrent tasks
• Lack of a global clock (only limited precision for processes to
synchronize)
• No global state: No single process can have knowledge of the
Current global state of the system
• Independent failures of components
7
Examples of distributed systems
• The following examples are based on familiar and widely used
computer networks: the Internet, intranets and the emerging
technology of networks based on mobile devices.
• Internet: A huge interconnected collection of computer
networks of many different types.
• The Internet is a vast interconnected collection of computer
networks.
• Programs running on computers connected to it interact by passing
messages.
• It is a "network of networks" that consists of millions of private
and public, academic, business, and government networks of local
to global scope that are linked by copper wires, fiber-optic cables,
wireless connections, and other technologies.
• It enables users to make use of services such as the World
Wide Web, e-mail and file transfer.
8
Examples of Distributed Systems—Cont.
• Intranet: A portion of the Internet (a network of computers and
workstations within an organization) managed by an organization
that can be configured to enforce local security policies.
• Composed of several LANs linked by backbone connections.
• Isolated from the Internet via a protective device (a firewall).
• Mobile and ubiquitous computing: the integration of small and
portable computing devices into distributed systems. These
devices include: Laptop computers, Handheld devices (PDAs,
mobile phones, pagers, video cameras and digital cameras), smart
watches, devices embedded in appliances such as cars, washing
machines.
• Ubiquitous is intended to mean that small computing devices will
eventually become so pervasive in everyday objects that they are
scarcely noticed. 9
Internet
• It enables users to use services such as WWW, email, file transfer,
Multimedia services, etc.
• Programs running on the computers connected to the Internet
interact by passing messages.
• The design and construction of the Internet protocols enables a
program running anywhere to address messages to programs
anywhere else.
Note: Some times Web is incorrectly used to mean the Internet.
• Internet Service Providers (ISPs) are companies that provide
modem links and other types of connection to users and small
organizations, enabling them to
• Access services in the internet as well as
• Providing local services such as email and web hosting
10
11
Intranets
12
13
Mobile and Ubiquitous Computing
14
• Gateways adjust traffic between dissimilar networks, while routers adjust traffic
between similar networks
• Since TCP/IP is the main Internet protocol, a router could be used to connect the
network to the Internet.
15
Resource Sharing & The Web
16
Resource Sharing & The Web (cont.)
• AService is a distinct part of a computer system that manages
a collection of related resources and presents their functionality
to users and applications.
• For example, shared files are accessed through a file service.
• The service access is via the set of operations that it exports.
• e.g. A file service provides read, write and delete operations on
files.
• Server refers to a process (running program) on a network
computer that accepts requests from programs running on other
computers to perform a service and responds appropriately.
• The requesting processes are referred to as a clients.
17
Resource Sharing & The Web (cont.)
• Requests are sent in massages from clients to a server
• Replies are sent in massages from the server to the clients.
The client invokes an operation upon the server means, the client
sends a request for an operation to be carried out by a server.
• Remote invocation: A complete interaction between a client and
server, from the point when the client sends the request until
receiving the server’s response,
• Although in everyday parlance the terms ‘client’ and ‘server’ refer to
the computers themselves.
• In this topic, the terms ‘client’ and ‘server’ refer to processes rather
18
than the computers that they execute upon.
Resource Sharing & The Web (cont.)
• In a distributed system written in an object-oriented language,
• Resources may be encapsulated as objects and accessed by client
objects, in which case we speak of a client object invoking a method
upon a server object.
• An executing web browser is an example of a client.
• The web browser communicates with a web server, to request web
pages from it.
19
The World Wide Web (WWW)
20
Web servers and web browsers
21
Distributed Systems
Architecture & Challenges
Venus Samawi
Isra University 1
Content
• Networks vs. Distributed Systems
• Architectures of Distributed Systems
• How to characterize a distributed system?
• Distributed Systems: Challenges
2
Networks vs. Distributed Systems
• Networks: A media for interconnecting local and wide area
computers and exchange messages based on protocols.
• Network entities are visible and they are explicitly addressed (IP
address).
• Distributed System: existence of multiple autonomous computers is
transparent
• However,
• many problems (e.g., openness, reliability) in common, but at
different levels.
• Networks focuses on packets, routing, etc., whereas distributed systems
focus on applications.
• Every distributed system relies on services provided by a computer
network.
Distributed Systems
Computer Networks
Architectures of Distributed Systems
• Multiprocessor systems
• Shared memory
• Tightly coupled system
• Easier to program
• Bus-based interconnection network
• E.g. SMPs (symmetric multiprocessors) with two or more CPUs
• Multicomputer systems / Clusters
• No shared memory
• Homogeneous in hard- and software
• Massively Parallel Processors (MPP)
• Loosely coupled system
• PC/Workstation clusters(each has its own memory
And CPU.
• High-speed networks/switches-based connection.
How to characterize a distributed system?
• Computers in distributed systems may be on separate
continents, in the same building, or the same room. DSs
have the following consequences:
• Concurrency – each system is autonomous.
• Carry out tasks independently
• Tasks coordinate their actions by exchanging messages.
• Heterogeneity
• No global clock
• Independent Failures
• Prime motivation: to share resources
6
Selected application domains &
Associated networked applications
Finance and commerce eCommercee.g. Amazon and eBay, PayPal, online
banking and trading
The information society Web information and search engines, ebooks, Wikipedia; social
networking: Facebookand Twitter.
Creative industries and
entertainment
Online gaming, music and film in the home, user
generated content, e.g. YouTube, Flickr
Healthcare Health informatics, on online patient records, monitoring
patients.
Education e-learning, virtual learning environments; distance
learning. e.g., Coursera
Transport and logistics GPS in route finding systems, map services: Google
Maps, Google Earth
Science and Engineering Cloud computing as an enabling technology for
collaboration between scientists (LHC, LIGO)
Environmental management Sensor networks to monitor earthquakes, floods or
tsunamis (Bureau of Meteorology flood warning system)
Business Example –Challenges
• What if
• Your customer uses a completely different hardware? (PC, MAC, iPad,
Mobile…)
• … a different operating system? (Windows, Unix,…)
• … a different way of representing data? (ASCII,…)
• Heterogeneity
• Or
• You want to move your business and computers to the Caribbean (because
of the weather or low tax)?
• Your client moves to the Caribbean (more likely)?
• Distribution transparency
Middleware – software layer providing:
• masking heterogeneity of:
Characterization: Challenges
(Difficulties and Threats)
• Widely varying models of use
• High variation of workload, partial disconnection of components,
or poor connection.
• Wide range of system environments
• Heterogeneous hardware, operating systems, network, and
performance.
• Internal problems
• Non synchronized clocks, conflicting updates, various hardware
and software failures.
• External threats
• Attacks on data integrity, secrecy, and denial of service.
Characterization: Dealing with
Challenges
• Observations
• Widely varying models of use
• The structure and the organization of systems allow for distribution
of workloads, redundant services, and high availability.
• Wide range of system environments
• A flexible and modular structure allows for implementing different
solutions for different hardware, OS, and networks.
• Internal problems
• The relationship between components and the patterns of interaction
can resolve concurrency issues,
• Structure and organization of component can support failover
mechanisms.
• External threats
• Security has to be built into the infrastructure and it is fundamental
for shaping the relationship between components.
Physical Models
• A representation of the underlying H/W elements of a
DS that abstracts away specific details of the
computer/networking technologies.
• Baseline physical model – minimal physical model of
a distributed system as an extensible set of computer
nodes interconnected by a computer network for the
required passing of messages
Three Generations of DSs
(Distributed Systems)
• Three generations of distributed systems
1- Early distributed systems
• 10 and 100 nodes interconnected by a local area network
• limited Internet connectivity
• supported a small range of services e.g.
• shared local printers
• File servers
• email
• file transfer across the Internet
Three Generations of DSs(Distributed Systems)
2- Internet-scale distributed systems
• Extensible set of nodes interconnected by a network of networks (the
Internet)
3- Contemporary DS with hundreds of thousands nodes + emergence of:
• Mobile computing
• laptops or smart phones may move from location to location – need
for added capabilities (service discovery; support for spontaneous
interoperation)
• Ubiquitous computing
• Computers are embedded everywhere
• Cloud computing
• pools of nodes that together provide a given service
Architectural model
• An Architectural model of a distributed system is concerned with the
placement of its parts and relationship between them. Examples:
• Client-Server (CS) and Peer Process models.
• CS can be modified by:
• The partitioning of data/replication at cooperative
servers(performance and reliability reasons)
• The caching of data by proxy servers or clients
• The use of mobile code and mobile agents
• The requirements to add or remove mobile devices.
Fundamental Models
• Fundamental Models are concerned with a formal
description of the properties that are common in all of the
architectural models
• Models addressing time synchronization, message delays,
failures, security issues are addressed as:
• Interaction Model– deals with performance and the difficulty of
setting of time limits in a distributed system.
• Failure Model– specification of the faults that can be exhibited
by processes
• Security Model– discusses possible threats to processes and
communication channels.
Architectural
Models
13
Architectural Models
Architectural Elements
• What are the entities that are communicating in the distributed system?
• How do they communicate, or, more specifically, what communication
paradigm (model) is used?
•What (potentially changing) roles and responsibilities do they have in the
overall architecture?
• How are they mapped on to the physical distributed infrastructure (what is
their placement)?
Architectural Models –Cont.
• The architecture of a system is its structure in terms of separately specified
components.
• Its goal is to meet present and likely future demands(What might suit 10 users is
terrible for 10,000 users).
• Major concerns are making the system reliable, manageable, adaptable, and cost
effective.
• Architectural Model:
• Simplifies and abstracts the functions of individual components
• The placement of the components across a network of computers – define patterns
for the distribution of data and workloads.
• The interrelationship between the components – i.e., functional roles and the
patterns of communication between them.
• A
communication pattern is a pattern on messages exchanged in a distributed
computation.
• An
input pattern is a vector made up of the input parameters of the processes
involved in a distributed computation.
Architectural Models –Cont.
• Architectural Model - simplifies
and abstracts the functions of
individual components:
• An initial simplification is
achieved by classifying
processes as:
• Server processes
• Client processes
client
server
• Peer processes
peer
• Cooperate and communicate in a
symmetric manner to perform a task.
peer
Software Architecture &Layers
• The term software architecture referred:
• Originally to the structure of software as layers or modules in a single
computer.
• More recently in terms of services offered and requested between processes
in the same or different computers.
• Breaking up the complexity of systems by designing them through
layers and services
• Layer: a group of related functional components
• Service: functionality provided to the next layer.
Layer N
…
Layer 2
(services offered to above layer)
Layer 1
Software and hardware service
layers in distributed systems
Applications, services
Computer and network hardware
Platform
Operating system
Middleware
Platform
• The lowest hardware and software layers are often
referred to as a platform for distributed systems and
applications.
• These low-level layers provide services to the layers
above them, which are implemented independently in
each computer.
• Major Examples
• Intel x86/Windows
• Intel x86/Linux
• Intel x86/Solaris
• PowerPC/MacOS
• iPhone/iOS
• Samsung Galaxy/Android
Middleware
• A layer of software whose purpose is to mask heterogeneity
present in distributed systems and to provide a
convenient programming model to application
developers.
• Major Examples:
• Sun RPC (Remote Procedure Call)
• OMGCORBA (Common Object Request Broker
Architecture)
• Microsoft D-COM (Distributed Components Object
Model)
• Sun Java RMI (Remote Method Invocation)
• Modern Middleware Examples:
• ManjrasoftAneka– for Cloud computing
• IBM WebSphere
• Microsoft .NET
• Sun J2EE
• Google AppEngine
• Microsoft Azure
System Architecture
• The most evident aspect of DS design is
the division of responsibilities between
• System components (applications, servers,
and other processes) and
• The placement of the components on
computers in the network.
• It has major implication for:
• Performance, reliability, and security of the
resulting system.
Client-Server Basic Model:
Clients invoke individual servers
Client
invocation
result
Client
invocation
Server
Key:
result
Process:
Server
Computer:
• Client processes interact with individual server processes in a separate
computer in order to access data or resource. The server in turn may use
services of other servers.
• Example:
• A Web Server is often a client of file server.
• Browser search engine -> crawlers other web servers.
Client-Server Architecture Types
(Tier arch compliments layer architecture)
• Two-tier model (classic)
client
server
• Three-tier (when the server, becomes a client)
client
Server/client
server
• Multi-tier (cascade model)
client
Server/client
server
Server/client
server
Clients and Servers
• General interaction between a client and a server.
A service provided by multiple
servers
Service
Client
Server
Client
Server
Server
• Services may be implemented as several server processes in separate host computers.
• Example: Cluster based Web servers and apps such as Google, parallel databases Oracle
Proxy servers (replication transparency)
and caches: Web proxy server
Client
Client
• A cache is a store of recently used data.
Web
Proxy
server
server
Web
server
Peer Processes: A distributed
application based on peer processes
Peer 2
Peer 1
Application
Sharable
objects
Application
Peer 3
Application
Peer 4
Application
Peers 5 .... N
• All of the processes play similar roles, interacting cooperatively as peers to perform
distributed activities or computations without distinction between clients and servers.
E.g., music sharing systems Napster, Gnutella, Kaza, BitTorrent.
P2P with a Centralized Index Server
(e.g. Napster Architecture)
peer
peer
peer
peer
peer
peer
peer
Variants of Client Sever Model: Mobile
code and Web applets
a) client request results in the downloading of applet code
Client
Applet code
b) client interacts with the applet
Client
Applet
Web
server
Web
server
An applet is any small application which,
• performs one specific task that
• runs within the scope of a dedicated widget engine (software platform on which
desktop or web widgets run
) or a larger program, often as a plug-in
• Applets downloaded to clients give good interactive response
• Mobile codes such as Applets are potential security threat, therefore the
browser gives applets limited access to local resources (e.g. NO access
to local/user file system).
Variants of Client Sever Model:
Mobile Agents
• A running program (code and data) that travels from one
computer to another in a network carrying out an autonomous
task, usually on behalf of some other process
• advantages: flexibility, savings in communications cost
• virtual markets, software maintain on the computers within an organisation.
• Potential security threat to the resources in computers they visit.
The environment receiving agent should decide which of the local
resource to allow. (e.g., crawlers and web servers).
• Agents themselves can be vulnerable – they may not be able to
complete task if they are refused access.
• Example technology:
• Java Agent Development Framework (JADE)
Thin clients and compute
servers
Network computer or PC
Thin
Client
Compute server
network
Application
Process
• Network computer: download OS and applications from the network
and run on a desktop (solve up-gradation problem) at runtime.
• Thin clients: work by connecting remotely to a server-based computing
environment where most applications, sensitive data, and memory, are
stored.
• The server does most of the work, which can include
launching software programs, performing calculations, and storing data.
• Windows-based UI on the user machine and application execution on a
remote computer. E.g, X-11 system.
Mobile devices and spontaneous networking
[3rd Generation Distributed System]
• The world is increasingly populated by small and portable computing
devices.
• W-LAN needs to handle constantly changing heterogeneous, roaming
devices
• Need to provide discovery services: (1) registration service to enable
servers to publish their services and (2) lookup service to allow clients
to discover services that meet their requirements.
Summary -Models and Implications
• The use of CS (Client-Server) has impact on the
software architecture followed:
• Distribution of responsibilities
• Synchronization mechanisms between client and server
• Admissible types of requests/responses
• Basic CS model, responsibility is statically allocated.
• File server is responsible for file, not for web pages.
• Peer process model, responsibility is dynamically
allocated:
• In fully decentralized music file sharing system, search
process may be delegated to different peers at runtime.
Design Requirements/Challenges of Distributed
Systems
• Performance Issues
• Responsiveness
• Support interactive clients
• Use caching and replication
• Throughput
• Load balancing and timeliness
• Quality of Service:
• Reliability
• Security
• Adaptive performance.
• Dependability issues:
• Correctness, security, and fault tolerance
• Dependable applications continue to work in the presence of
faults in hardware, software, and networks.
Presentation Outline
• Introduction
• Architectural Models
• Software Layers
• System Architectures
• Client-Server
• Clients and a Single Sever, Multiple Servers, Proxy Servers with Caches,
Peer Model
• Alternative Client-Sever models driven by:
• Mobile code, mobile agents, network computers, thin clients, mobile
devices and spontaneous networking
• Design Challenges/Requirements
• Fundamental Models –formal description
• Interaction, Failure, and Security models.
• Summary
Fundamental Models at Glance
• Fundamental Models are concerned with a formal description of the
properties that are common in all of the architectural models
• All architectural models are composed of processes that
communicate with each other by sending messages over a computer
networks.
• Models addressing time synchronization, message delays, failures,
security issues are addressed as:
• Interaction Model – deals with performance and the difficulty of setting of
time limits in a distributed system.
• Failure Model – specification of the faults that can be exhibited by processes
• Security Model – discusses possible threats to processes and
communication channels.
Interaction Model
• Computation occurs within processes;
• The processes interact by passing messages,
resulting in:
• Communication (information flow)
• Coordination (synchronization and ordering of activities)
between processes.
• Two significant factors affecting interacting
processes in a distributed system are:
• Communication performance is often a limiting
characteristic.
• It is impossible to maintain a single global notion of time.
Interaction Model:
Performance of Communication Channel
• Communication over a computer network has
performance characteristics:
• Latency:
• A delay between the start of a message’s transmission from one
process to the beginning of reception by another.
• Bandwidth:
• the total amount of information that can be transmitted over in
a given time.
• Communication channels using the same network, have to
share the available bandwidth.
• Jitter
• The variation in the time taken to deliver a series of messages.
It is very relevant to multimedia data.
• Is the variation in the time between data packets arriving,
caused by
• network congestion, or
• route changes.
• The longer data packets take to transmit, the more jitter affects
audio quality. The standard jitter measurement is in
milliseconds (ms)
Interaction Model:
Computer clocks and timing events
• Each computer in a DS has its own internal clock, which
can be used by local processes to obtain the value of the
current time.
• Therefore, two processes running on different computers
can associate timestamp with their events.
• However, even if two processes read their clocks at the
same time, their local clocks may supply different time.
• This is because computer clock drifts from perfect time
and their drift rates differ from one another.
• Even if the clocks on all the computers in a DS are set to
the same time initially, their clocks would eventually vary
quite significantly unless corrections are applied.
Interaction Model:
Two variants of the interaction model
• In a DS it is hard to set time limits on the time taken for process
execution, message delivery or clock drift.
• Synchronous DS – hard to achieve:
• The time taken to execute a step of a process has known lower
and upper bounds.
• Each message transmitted over a channel is received within a
known bounded time.
• Each process has a local clock whose drift rate from real time has
known bound.
• Asynchronous DS: There is NO bounds on:
• Process execution speeds
• Message transmission delays
• Clock drift rates.
Interaction Model:
Event Ordering
• In many DS applications we are interested in knowing
whether an event occurred
• before,
• after, or
• concurrently with another event at other processes.
• The execution of a system can be described in terms of
events and their ordering despite the lack of accurate
clocks.
Failure Model
• In a DS, both processes and communication channels
may fail – i.e., they may depart from what is considered
to be correct or desirable behavior (define and classify the
faults).
• Types of failures:
• Omission Failure(refer to cases where the process or
communication channel fails to perform a requested
action)
• Arbitrary Failure(where any type of error can occur.
Corrupt data, unexpected responses)
• Timing Failure(related to synchronous messages,
where a set bound [clock drift, message ack, process
execution time] exceeds defined bounds)
Processes and channels
process p
send
process q
m
receive
Communication channel
Outgoing message buffer
Incoming message buffer
• Communication channel produces an omission failure if it does
not transport a message from “p”s outgoing message buffer to
“q”’s incoming message buffer. This is known as “dropping
messages” and is generally caused by
• a lack of buffer space at the receiver or at gateway or by a network
transmission error.
Omission and arbitrary failures
Class of failure Affects Description
Fail-stop Process Process halts and remains halted. Other processes may
detectthis state.
Crash Process Process halts and remains halted. Other processes may
not be able to detect this state.
Omission Channel A message inserted in an outgoing message buffer never
arrivesat the other end’s incoming message buffer.
Send-omission Process A process completes a send, but the message is not
put in its outgoing message buffer.
Receive-omission Process A message is put in a process’s incoming message
buffer, but that process does not receive it.
Arbitrary
(Byzantine)
Process or
channel
Process/channel exhibits arbitrary behaviour: it may
send/transmit arbitrary messages at arbitrary times,
commit omissions; a process may stop or take an
incorrect step.
Timing failures
Class of Failure Affects Description
Clock Process Process’s local clock exceeds the bounds on its
rate of drift from real time.
Performance Process Process exceeds the bounds on the interval
between two steps.
Performance Channel A message’s transmission takes longer than the
stated bound.
Masking Failures
• It is possible to construct reliable services from
components that exhibit failures.
• For example, multiple servers that hold replicas of data can
continue to provide a service when one of them crashes.
• A knowledge of failure characteristics of a component
can enable a new service to be designed to mask the
failure of the components on which it depends:
• Checksums are used to mask corrupted messages.
Security Model
• The security of a DS can be achieved by securing the processes
and the channels used in their interactions and by protecting
the objects that they encapsulate against unauthorized access.
Protecting Objects: Objects and
principals
invocation
Client
Access rights
result
Principal (user)
Netw ork
Server
Principal (server)
Object
• Use “access rights” that define who is allowed to perform operation on a
object.
• The server should verify the identity of the principal (user) behind each
operation and checking that they have sufficient access rights to perform the
requested operation on the particular object, rejecting those who do not.
The enemy
Copy of m
Process p
The enemy
m
m’
Communication channel
Process q
• To model security threats, we postulate an enemy that is
capable of sending any process or reading/copying
message between a pair of processes
• Threats form a potential enemy: threats to processes,
threats to communication channels, and denial of service.
Defeating security threats:
Secure channels
Principal A
Process p
Secure channel
Principal B
Process q
• Encryption and authentication are use to build secure channels.
• Each of the processes knows the identity of the principal on whose
behalf the other process is executing and can check their access
rights before performing an operation.
Summary
• Most DSs are arranged accordingly to one of a
variety of architectural models:
• Client-Server
• Clients and a Single Sever, Multiple Servers, Proxy Servers
with Cache, Peer Model
• Alternative Client-Sever models driven by:
• Mobile code, mobile agents, network computers, thin clients,
mobile devices and spontaneous networking
• Fundamental Models – formal description
• Interaction, failure, and security models.
• The concepts discussed in the module play an
important role while architecting DS and apps.
Distributed Systems
Communication in Distributed
Systems
Venus Samawi
Isra University 1
Content
• Network layers of the model
• Summary of OSI layers
• TCP/IP and OSI model
• The interaction between layers in the OSI model
• An exchange using the OSI model
• Port Addresses
• Computer Networks
• TCP VsUDP
• Software and hardware service layers in distributed systems
• Operating systems (OS)
• Network OS
• Distributed OS
• Middleware
2
In previous lectures
• So far, We have discussed the communication in distributed
system form internet protocol’s point of view by studying the
layered implementation.
• We are going to study the communication in distributed system
at higher level (processes and application)
Figure 2.2 Seven layers of the OSI model
2.4
Figure 2.15 Summary of layers
2.5
TCP/IP and OSI model
2.6
The interaction between layers in the OSI model
2.7
An exchange using the OSI model
2.8
Port addresses
2.9
Port Address Translation (PAT)
• Port Address Translation (PAT) is an extension of Network Address Translation
(NAT)
• permits multiple devices on a LAN to be mapped to a single public IP address
to conserve IP addresses.
• PAT is similar to port forwarding except that an incoming packet with
destination port (external port) is translated to a packet different destination port
(an internal port).
• The Internet Service Provider (ISP) assigns a single IP address to the edge
device.
• When a computer logs on to the Internet, this device assigns the client a
port number that is appended to the internal IP address, giving the
computer a unique IP address.
• If another computer logs on the Internet, this device assigns it the same
public IP address, but a different port number.
• Although both computers are sharing the same public IP address, this
device knows which computer to send its packets, because the device uses
the port numbers to assign the packets the unique internal IP address of the
computers.
Computer Networks
Difference: Connection oriented and Connectionless service
What is an Operating System? • A program that acts as an intermediary between a user of a computer and the computer hardware • Operating system goals: • Execute user programs and make solving user problems easier • Make the computer system convenient to use • Use the computer hardware in an efficient manner Computer System Structure Computer system can be divided into four components: • Hardware –provides basic computing resources • CPU, memory, I/O devices • Operating system • Controls and coordinates use of hardware among various applications and users • Application programs –define the ways in which the system resources are used to solve the computing problems of the users • Word processors, compilers, web browsers, database systems, video games • Users • People, machines, other computers Operating System Supporting In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the kernel of the operating system on which it is executed. Distributed OS Distributed OS Cont Network OS Vs Distributed System OS The main difference between these two operating systems (Network Operating System and Distributed Operating System) is that • In network operating system each node or system can have its own operating system • Indistribute operating system each node or system have same operating system which is opposite to the network operating system. Network OS Vs Distributed System OS Cont. Network OS • Main objective is to provide the local services to remote client. • Communication takes place on the basis of files. • more scalable than Distributed Operating System. • Limited fault tolerance • High Rate of autonomy • Ease of implementation • All nodes can have different operating system. Distributed System OS • Main objective is to manage the hardware resources. • Communication takes place on the basis of messages and shared memory. • Less scalable than network Operating System. • High fault tolerance • Limited Rate of autonomy • More complicated implementation • All nodes have same operating system.
Distributed Systems
Inter-process Communication
(IPC)
Venus Samawi
Isra University 1
Content
• Interprocess Communication (IPC)
• Message passing
• Buffer
2
InterprocessCommunication
Process Cooperation
• There are several reasons to provide process cooperation environment
• Information sharing
• Computation speed up: If we want a particular task to run faster, we must
break it into subtasks, each of which will be executing in parallel with the
others.
• Modularity: dividing the system functions into separate processes or threads
• Convenience: Even an individual user may work on many tasks at the same
time. For instance, a user may be editing, printing, and compiling in parallel.
4
Models for InterprocessCommunication
Models for interprocess communication
could be through:
• Shared memory
• Message passing
Message -passing Systems
5
Message Size
6
7
Naming
Communication between processes either Direct or Indirect
8
9
Example
10
Message passing (Part2)
Communication link must exists between processes
11
12
Buffer Size Vs Blocking and non-Boloking
13
Distributed Systems
Inter-process Communication
(IPC)
Venus Samawi
Isra University 1
Content
• Interprocess Communication (IPC)
• Sockets
• RPC
• RMI
2
Middleware layers
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5
Pearson Education 2012
Sockets (used for Communication in Client -Server systems)
4
Sockets
5
Sockets (Cont.)
6
Sockets and ports
socket
client
Internet address = 138.37.94.248
any port
agreed port
message
other ports
socket
server
Internet address = 138.37.88.249
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5
Pearson Education 2012
• Adaemonis a long-running background process that answers requests for services.
• In multitasking computer operating systems, a daemon is a computer program that runs as a background
process, rather than being under the direct control of an interactive user.
• Astub in distributed computing is a piece of code that converts parameters passed between client and server during a remote procedure call (RPC). • The main idea of an RPC is to allow a local computer (client) to remotely call procedures on a different computer (server). • "marshalling" refers to the process of converting the data or the objects into a byte stream, • "unmarshalling" is the reverse process of converting the byte-stream back to their original data or object.
• Adaemonis a long-running background process that answers requests for services.
• In multitasking computer operating systems, a daemon is a computer program that runs as a background
process, rather than being under the direct control of an interactive user.
• Binding information includes the protocol that clients use to communicate with the
site, the site's IP address, the port number, and a host header. The
RPC
RMIVS
RPC
RPC does not provide
any security.
Although it provides
client-level security.
18
Key Differences Between RPC and RMI
• RPC supports procedural programming paradigms thus is C based, while RMI supports
object-oriented programming paradigms and is java based.
• The parameters passed to remote procedures in RPC are the ordinary data structures. On
the contrary, RMI transits objects as a parameter to the remote method.
• RPCcanbeconsidered as:
• Theolder version of RMI,
• it is used in the programming languages that support procedural programming,
• it can only use pass by value method.
As against, RMI facility is devised based on modern programming approach, which could
use
• passbyvalue or reference.
• Anotheradvantage of RMIis that the parameters passed by reference can be changed.
• RPCprotocol generates more overheads than RMI.
• The parameters passed in RPC must be “in-out” which means that the value passed to the
procedure and the output value must have the same datatypes. In contrast, there is no
compulsion of passing “in-out” parameters in RMI.
• In RPC, references could not be probable because the two
processes have the distinct
address
space, but it is possible in case of RMI.
19
Parameter Passing in RPC
• Functions in an application that runs in a single process may collaborate via
parameters and/or global variables.
• Functions in an application that runs in multiple processes on the same host may
collaborate via message passing and/or non-distributed shared memory
• In RPC, passing parameters is the only way that , clients and servers share
information
• Parameters that are passed by value are fairly simple to handle
• The client stub copies the value from the client and packages into a network
message
Ex:
• Consider a remote procedure, sum(i, j), which takes two integer parameters and
returns their arithmetic sum.
• The client stub takes its two parameters and puts them in a message, and Puts the name
or number of the procedure to be called in the message.
• When the message arrives at the server, the stub examines the message to see
which procedure is needed, and then makes the appropriate call.
•When the server has finished execution, it takes the result provided by the server
and packs it into a message.
This message is sent back to the client stub, which unpacks it and returns the
value to the client procedure
20
RPC: Parameters Passed by Reference
• Parameters passed by reference are much harder:
• For example distributed systems with distributed shared-memory
mechanisms can allow passing of parameters by reference.
• A pointer is meaningful only within the address space of the
process in which it is being used.
Suppose there are two parameters to be passed, if the second parameter
is the address of the buffer which is 1000 on the client, one cannot just
pass the number 1000 to the server and expect it to work. Address 1000
on the server might be in the middle of the program text.
For that, call by reference is not practical in RPC and massage passing
is used.
21
Remote Method Invocation (RMI)
• Remote Method Invocation (developed in 1980’s) allows remote
method calls, at which objects in different programs can
communicate
• RMI is based on RPC (RMI is Java’s implementation of RPC).
• It is object-oriented version of RPC
• Methods calls appear same as those in same program.
• RMI performs networking and marshaling of data (converting
the data or the objects into a byte-stream)
• Interface definition language is required to describe functions.
• RMI is an approach that provides remote communication between
the application using two objects:
• stub
22
• skeleton
Stub
Whenastub'smethodisinvoked, itdoes
thefollowing:
• initiates aconnectionwith the remote
JVMcontainingtheremoteobject,
•marshals (writes and transmits) the
parameterstotheremoteJVM,
•waits for the result of the method
invocation,
•Unmarshals(reads) thereturnvalueor
exceptionreturned,and
• returnsthevaluetothecaller.
Skeleton
Whenaskeletonreceivesanincomingmethod
invocationitdoesthefollowing:
• unmarshals (reads) the parameters for the
remotemethod,
• invokes themethod on the actual remote
objectimplementation,and
•marshals (writes and transmits) the result
(returnvalueorexception)tothecaller.
RMICont.
ThecommondifferencebetweenRPCandRMIisthat
• RPConlysupportsproceduralprogrammingwhereasRMIsupportsobject
orientedprogramming.
• Client calls the local method (stub) to perform this operation
• The stub on computer client calls RMI Registry to know
whether thatmethod exists and the correct way to call it
• Client stub calls this method on server
• Server skeleton receives this request and communicate with the
software on server, when it receives the response, sends this
response to the client stub.
Client
RMISystem Layers
The RMI system consists of 3 layers:
• The stub/Skeleton layer
• Client side stubs (proxies)
• Server side skeletons
• The Remote Reference Layer
• Transport Layer
RMI Architecture
Remote Reference layer
• The remote reference layer deals with the lower-level transport interface.
• It is also responsible for carrying out a specific remote reference
protocol which is independent of the client stubs and server skeletons.
• Each remote object implementation chooses its own remote reference
subclass that operates on its behalf.
• Various invocation protocols can be carried out at this layer.
Examples are:
• Unicast point-to-point invocation.
• Invocation to replicated object groups.
• Support for a specific replication strategy.
• Support for a persistent reference to the remote object (enabling
activation of the remote object).
• Reconnection strategies (if remote object becomes inaccessible).
27
Remote Reference layer (Cont.)
The remote reference layer has two cooperating components:
• the client-side
• server-side components.
The client-side component
• contains information specific to the remote server (or servers, if the remote
reference is to a replicated object)
• communicates via the transport to the server-side component.
During each method invocation, the client and server-side components
perform the specific remote reference semantics.
• For example,
If a remote object is part of a replicated object, the client-side component
can forward the invocation to each replica rather than just a single
remote object.
28
Transport Layer
The transport layer of the RMI system is responsible for:
• Setting up connections to remote address spaces.
• Managing connections.
• Monitoring connection "liveness."
• Listening for incoming calls.
• Maintaining a table of remote objects that reside in the
address space.
• Setting up a connection for an incoming call.
• Locating the dispatcher (لسرملا) for the target of the remote
call and passing the connection to this dispatcher.
Transport Layer (Cont.)
• The concrete representation of a remote object reference consists of
• Anendpoint
• Anobject identifier.
This representation is called a live reference.
• Given a live reference for a remote object,
• a transport can use the endpoint to set up a connection to the address space in
which the remote object resides.
• Onthe server side, the transport uses the object identifier to look up the target
of the remote call.
The transport for the RMI system consists of four basic abstractions:
• Anendpoint: is the abstraction used to denote an address space or Java
virtual machine.
• In the implementation, an endpoint can be mapped to its transport. That is, given an
endpoint, a specific transport instance can be obtained.
• Achannel: is the abstraction for a conduit ( )
هانق between two address spaces.
• it is responsible for managing connections between the local address space and the
remote address space for which it is a channel.
• Aconnection: is the abstraction for transferring data (performing
input/output).
• The transport abstraction manages channels.
30
• Each channel is a virtual connection between two address spaces.
Distributed Systems
Introduction
Venus Samawi
Isra University 1
Content
• Distributed Computing & Distributed System
• Advantages and Disadvantages of Distributed Systems
• Characteristics of Distributed Systems
• Examples of distributed systems
• Resource Sharing & The Web
• WWW
2
Distributed Computing & Distributed System
• Early computing was performed on a single processor. Uni
processor computing can be called centralized computing.
• Twoadvances in technology began to change the situation
• Thedevelopment of powerful microprocessors
• Theinvention of high-speed computer networks
• Distributed computing is computing performed in a distributed
system.
• ADistributed system is:
• Acollection of independent computers, interconnected via a network, capable
of collaborating on a task,
• One in which components located at networked computers communicate and
coordinate their actions only by message passing,
• Acollection of independent computers, that appears to the users of the system
as a single computer.
3
Advantages of Distributed Systems
• The motivation for constructing and using distributed systems
stems from a desire to share resources.
• Economics: distributed systems allow the pooling of resources,
including
• CPUcycles, data storage, input/output devices, and services.
• Distributed systems have a better price/performance ratio
• Concurrency: we can solve the problem more quickly using
several processors concurrently.
• Some applications are inherently distributed : In some problems
the most natural solution is to use separate parallel processes to
perform the subtasks of the given problem.
• Computer Supported Cooperative Work
4
Advantages of Distributed Systems
(Cont.)
• Communication: make human-to-human communication
easier, for example by electronic mail.
• Reliability: a distributed system allow replication of resources
and/or services, thus reducing service outage due to failures.
• Incremental growth: computing power can be added in small
• increments.
5
Disadvantages of Distributed Computing
• Multiple Points of Failures: the failure of one or more
participating computers, or one or more network links, can
spell trouble.
• Security Concerns: In a distributed system, there are more
opportunities for unauthorized attack.
6
Characteristics of Distributed Systems
• Concurrency of components (or Parallel activities)
• Autonomous components executing concurrent tasks
• Lack of a global clock (only limited precision for processes to
synchronize)
• No global state: No single process can have knowledge of the
Current global state of the system
• Independent failures of components
7
Examples of distributed systems
• The following examples are based on familiar and widely used
computer networks: the Internet, intranets and the emerging
technology of networks based on mobile devices.
• Internet: A huge interconnected collection of computer
networks of many different types.
• The Internet is a vast interconnected collection of computer
networks.
• Programs running on computers connected to it interact by passing
messages.
• It is a "network of networks" that consists of millions of private
and public, academic, business, and government networks of local
to global scope that are linked by copper wires, fiber-optic cables,
wireless connections, and other technologies.
• It enables users to make use of services such as the World
Wide Web, e-mail and file transfer.
8
Examples of Distributed Systems—Cont.
• Intranet: A portion of the Internet (a network of computers and
workstations within an organization) managed by an organization
that can be configured to enforce local security policies.
• Composed of several LANs linked by backbone connections.
• Isolated from the Internet via a protective device (a firewall).
• Mobile and ubiquitous computing: the integration of small and
portable computing devices into distributed systems. These
devices include: Laptop computers, Handheld devices (PDAs,
mobile phones, pagers, video cameras and digital cameras), smart
watches, devices embedded in appliances such as cars, washing
machines.
• Ubiquitous is intended to mean that small computing devices will
eventually become so pervasive in everyday objects that they are
scarcely noticed. 9
Internet
• It enables users to use services such as WWW, email, file transfer,
Multimedia services, etc.
• Programs running on the computers connected to the Internet
interact by passing messages.
• The design and construction of the Internet protocols enables a
program running anywhere to address messages to programs
anywhere else.
Note: Some times Web is incorrectly used to mean the Internet.
• Internet Service Providers (ISPs) are companies that provide
modem links and other types of connection to users and small
organizations, enabling them to
• Access services in the internet as well as
• Providing local services such as email and web hosting
10
11
Intranets
12
13
Mobile and Ubiquitous Computing
14
• Gateways adjust traffic between dissimilar networks, while routers adjust traffic
between similar networks
• Since TCP/IP is the main Internet protocol, a router could be used to connect the
network to the Internet.
15
Resource Sharing & The Web
16
Resource Sharing & The Web (cont.)
• AService is a distinct part of a computer system that manages
a collection of related resources and presents their functionality
to users and applications.
• For example, shared files are accessed through a file service.
• The service access is via the set of operations that it exports.
• e.g. A file service provides read, write and delete operations on
files.
• Server refers to a process (running program) on a network
computer that accepts requests from programs running on other
computers to perform a service and responds appropriately.
• The requesting processes are referred to as a clients.
17
Resource Sharing & The Web (cont.)
• Requests are sent in massages from clients to a server
• Replies are sent in massages from the server to the clients.
The client invokes an operation upon the server means, the client
sends a request for an operation to be carried out by a server.
• Remote invocation: A complete interaction between a client and
server, from the point when the client sends the request until
receiving the server’s response,
• Although in everyday parlance the terms ‘client’ and ‘server’ refer to
the computers themselves.
• In this topic, the terms ‘client’ and ‘server’ refer to processes rather
18
than the computers that they execute upon.
Resource Sharing & The Web (cont.)
• In a distributed system written in an object-oriented language,
• Resources may be encapsulated as objects and accessed by client
objects, in which case we speak of a client object invoking a method
upon a server object.
• An executing web browser is an example of a client.
• The web browser communicates with a web server, to request web
pages from it.
19
The World Wide Web (WWW)
20
Web servers and web browsers
21
Distributed Systems
Architecture & Challenges
Venus Samawi
Isra University 1
Content
• Networks vs. Distributed Systems
• Architectures of Distributed Systems
• How to characterize a distributed system?
• Distributed Systems: Challenges
2
Networks vs. Distributed Systems
• Networks: A media for interconnecting local and wide area
computers and exchange messages based on protocols.
• Network entities are visible and they are explicitly addressed (IP
address).
• Distributed System: existence of multiple autonomous computers is
transparent
• However,
• many problems (e.g., openness, reliability) in common, but at
different levels.
• Networks focuses on packets, routing, etc., whereas distributed systems
focus on applications.
• Every distributed system relies on services provided by a computer
network.
Distributed Systems
Computer Networks
Architectures of Distributed Systems
• Multiprocessor systems
• Shared memory
• Tightly coupled system
• Easier to program
• Bus-based interconnection network
• E.g. SMPs (symmetric multiprocessors) with two or more CPUs
• Multicomputer systems / Clusters
• No shared memory
• Homogeneous in hard- and software
• Massively Parallel Processors (MPP)
• Loosely coupled system
• PC/Workstation clusters(each has its own memory
And CPU.
• High-speed networks/switches-based connection.
How to characterize a distributed system?
• Computers in distributed systems may be on separate
continents, in the same building, or the same room. DSs
have the following consequences:
• Concurrency – each system is autonomous.
• Carry out tasks independently
• Tasks coordinate their actions by exchanging messages.
• Heterogeneity
• No global clock
• Independent Failures
• Prime motivation: to share resources
6
Selected application domains &
Associated networked applications
Finance and commerce eCommercee.g. Amazon and eBay, PayPal, online
banking and trading
The information society Web information and search engines, ebooks, Wikipedia; social
networking: Facebookand Twitter.
Creative industries and
entertainment
Online gaming, music and film in the home, user
generated content, e.g. YouTube, Flickr
Healthcare Health informatics, on online patient records, monitoring
patients.
Education e-learning, virtual learning environments; distance
learning. e.g., Coursera
Transport and logistics GPS in route finding systems, map services: Google
Maps, Google Earth
Science and Engineering Cloud computing as an enabling technology for
collaboration between scientists (LHC, LIGO)
Environmental management Sensor networks to monitor earthquakes, floods or
tsunamis (Bureau of Meteorology flood warning system)
Business Example –Challenges
• What if
• Your customer uses a completely different hardware? (PC, MAC, iPad,
Mobile…)
• … a different operating system? (Windows, Unix,…)
• … a different way of representing data? (ASCII,…)
• Heterogeneity
• Or
• You want to move your business and computers to the Caribbean (because
of the weather or low tax)?
• Your client moves to the Caribbean (more likely)?
• Distribution transparency
Middleware – software layer providing:
• masking heterogeneity of:
Characterization: Challenges
(Difficulties and Threats)
• Widely varying models of use
• High variation of workload, partial disconnection of components,
or poor connection.
• Wide range of system environments
• Heterogeneous hardware, operating systems, network, and
performance.
• Internal problems
• Non synchronized clocks, conflicting updates, various hardware
and software failures.
• External threats
• Attacks on data integrity, secrecy, and denial of service.
Characterization: Dealing with
Challenges
• Observations
• Widely varying models of use
• The structure and the organization of systems allow for distribution
of workloads, redundant services, and high availability.
• Wide range of system environments
• A flexible and modular structure allows for implementing different
solutions for different hardware, OS, and networks.
• Internal problems
• The relationship between components and the patterns of interaction
can resolve concurrency issues,
• Structure and organization of component can support failover
mechanisms.
• External threats
• Security has to be built into the infrastructure and it is fundamental
for shaping the relationship between components.
Physical Models
• A representation of the underlying H/W elements of a
DS that abstracts away specific details of the
computer/networking technologies.
• Baseline physical model – minimal physical model of
a distributed system as an extensible set of computer
nodes interconnected by a computer network for the
required passing of messages
Three Generations of DSs
(Distributed Systems)
• Three generations of distributed systems
1- Early distributed systems
• 10 and 100 nodes interconnected by a local area network
• limited Internet connectivity
• supported a small range of services e.g.
• shared local printers
• File servers
• email
• file transfer across the Internet
Three Generations of DSs(Distributed Systems)
2- Internet-scale distributed systems
• Extensible set of nodes interconnected by a network of networks (the
Internet)
3- Contemporary DS with hundreds of thousands nodes + emergence of:
• Mobile computing
• laptops or smart phones may move from location to location – need
for added capabilities (service discovery; support for spontaneous
interoperation)
• Ubiquitous computing
• Computers are embedded everywhere
• Cloud computing
• pools of nodes that together provide a given service
Architectural model
• An Architectural model of a distributed system is concerned with the
placement of its parts and relationship between them. Examples:
• Client-Server (CS) and Peer Process models.
• CS can be modified by:
• The partitioning of data/replication at cooperative
servers(performance and reliability reasons)
• The caching of data by proxy servers or clients
• The use of mobile code and mobile agents
• The requirements to add or remove mobile devices.
Fundamental Models
• Fundamental Models are concerned with a formal
description of the properties that are common in all of the
architectural models
• Models addressing time synchronization, message delays,
failures, security issues are addressed as:
• Interaction Model– deals with performance and the difficulty of
setting of time limits in a distributed system.
• Failure Model– specification of the faults that can be exhibited
by processes
• Security Model– discusses possible threats to processes and
communication channels.
Architectural
Models
13
Architectural Models
Architectural Elements
• What are the entities that are communicating in the distributed system?
• How do they communicate, or, more specifically, what communication
paradigm (model) is used?
•What (potentially changing) roles and responsibilities do they have in the
overall architecture?
• How are they mapped on to the physical distributed infrastructure (what is
their placement)?
Architectural Models –Cont.
• The architecture of a system is its structure in terms of separately specified
components.
• Its goal is to meet present and likely future demands(What might suit 10 users is
terrible for 10,000 users).
• Major concerns are making the system reliable, manageable, adaptable, and cost
effective.
• Architectural Model:
• Simplifies and abstracts the functions of individual components
• The placement of the components across a network of computers – define patterns
for the distribution of data and workloads.
• The interrelationship between the components – i.e., functional roles and the
patterns of communication between them.
• A
communication pattern is a pattern on messages exchanged in a distributed
computation.
• An
input pattern is a vector made up of the input parameters of the processes
involved in a distributed computation.
Architectural Models –Cont.
• Architectural Model - simplifies
and abstracts the functions of
individual components:
• An initial simplification is
achieved by classifying
processes as:
• Server processes
• Client processes
client
server
• Peer processes
peer
• Cooperate and communicate in a
symmetric manner to perform a task.
peer
Software Architecture &Layers
• The term software architecture referred:
• Originally to the structure of software as layers or modules in a single
computer.
• More recently in terms of services offered and requested between processes
in the same or different computers.
• Breaking up the complexity of systems by designing them through
layers and services
• Layer: a group of related functional components
• Service: functionality provided to the next layer.
Layer N
…
Layer 2
(services offered to above layer)
Layer 1
Software and hardware service
layers in distributed systems
Applications, services
Computer and network hardware
Platform
Operating system
Middleware
Platform
• The lowest hardware and software layers are often
referred to as a platform for distributed systems and
applications.
• These low-level layers provide services to the layers
above them, which are implemented independently in
each computer.
• Major Examples
• Intel x86/Windows
• Intel x86/Linux
• Intel x86/Solaris
• PowerPC/MacOS
• iPhone/iOS
• Samsung Galaxy/Android
Middleware
• A layer of software whose purpose is to mask heterogeneity
present in distributed systems and to provide a
convenient programming model to application
developers.
• Major Examples:
• Sun RPC (Remote Procedure Call)
• OMGCORBA (Common Object Request Broker
Architecture)
• Microsoft D-COM (Distributed Components Object
Model)
• Sun Java RMI (Remote Method Invocation)
• Modern Middleware Examples:
• ManjrasoftAneka– for Cloud computing
• IBM WebSphere
• Microsoft .NET
• Sun J2EE
• Google AppEngine
• Microsoft Azure
System Architecture
• The most evident aspect of DS design is
the division of responsibilities between
• System components (applications, servers,
and other processes) and
• The placement of the components on
computers in the network.
• It has major implication for:
• Performance, reliability, and security of the
resulting system.
Client-Server Basic Model:
Clients invoke individual servers
Client
invocation
result
Client
invocation
Server
Key:
result
Process:
Server
Computer:
• Client processes interact with individual server processes in a separate
computer in order to access data or resource. The server in turn may use
services of other servers.
• Example:
• A Web Server is often a client of file server.
• Browser search engine -> crawlers other web servers.
Client-Server Architecture Types
(Tier arch compliments layer architecture)
• Two-tier model (classic)
client
server
• Three-tier (when the server, becomes a client)
client
Server/client
server
• Multi-tier (cascade model)
client
Server/client
server
Server/client
server
Clients and Servers
• General interaction between a client and a server.
A service provided by multiple
servers
Service
Client
Server
Client
Server
Server
• Services may be implemented as several server processes in separate host computers.
• Example: Cluster based Web servers and apps such as Google, parallel databases Oracle
Proxy servers (replication transparency)
and caches: Web proxy server
Client
Client
• A cache is a store of recently used data.
Web
Proxy
server
server
Web
server
Peer Processes: A distributed
application based on peer processes
Peer 2
Peer 1
Application
Sharable
objects
Application
Peer 3
Application
Peer 4
Application
Peers 5 .... N
• All of the processes play similar roles, interacting cooperatively as peers to perform
distributed activities or computations without distinction between clients and servers.
E.g., music sharing systems Napster, Gnutella, Kaza, BitTorrent.
P2P with a Centralized Index Server
(e.g. Napster Architecture)
peer
peer
peer
peer
peer
peer
peer
Variants of Client Sever Model: Mobile
code and Web applets
a) client request results in the downloading of applet code
Client
Applet code
b) client interacts with the applet
Client
Applet
Web
server
Web
server
An applet is any small application which,
• performs one specific task that
• runs within the scope of a dedicated widget engine (software platform on which
desktop or web widgets run
) or a larger program, often as a plug-in
• Applets downloaded to clients give good interactive response
• Mobile codes such as Applets are potential security threat, therefore the
browser gives applets limited access to local resources (e.g. NO access
to local/user file system).
Variants of Client Sever Model:
Mobile Agents
• A running program (code and data) that travels from one
computer to another in a network carrying out an autonomous
task, usually on behalf of some other process
• advantages: flexibility, savings in communications cost
• virtual markets, software maintain on the computers within an organisation.
• Potential security threat to the resources in computers they visit.
The environment receiving agent should decide which of the local
resource to allow. (e.g., crawlers and web servers).
• Agents themselves can be vulnerable – they may not be able to
complete task if they are refused access.
• Example technology:
• Java Agent Development Framework (JADE)
Thin clients and compute
servers
Network computer or PC
Thin
Client
Compute server
network
Application
Process
• Network computer: download OS and applications from the network
and run on a desktop (solve up-gradation problem) at runtime.
• Thin clients: work by connecting remotely to a server-based computing
environment where most applications, sensitive data, and memory, are
stored.
• The server does most of the work, which can include
launching software programs, performing calculations, and storing data.
• Windows-based UI on the user machine and application execution on a
remote computer. E.g, X-11 system.
Mobile devices and spontaneous networking
[3rd Generation Distributed System]
• The world is increasingly populated by small and portable computing
devices.
• W-LAN needs to handle constantly changing heterogeneous, roaming
devices
• Need to provide discovery services: (1) registration service to enable
servers to publish their services and (2) lookup service to allow clients
to discover services that meet their requirements.
Summary -Models and Implications
• The use of CS (Client-Server) has impact on the
software architecture followed:
• Distribution of responsibilities
• Synchronization mechanisms between client and server
• Admissible types of requests/responses
• Basic CS model, responsibility is statically allocated.
• File server is responsible for file, not for web pages.
• Peer process model, responsibility is dynamically
allocated:
• In fully decentralized music file sharing system, search
process may be delegated to different peers at runtime.
Design Requirements/Challenges of Distributed
Systems
• Performance Issues
• Responsiveness
• Support interactive clients
• Use caching and replication
• Throughput
• Load balancing and timeliness
• Quality of Service:
• Reliability
• Security
• Adaptive performance.
• Dependability issues:
• Correctness, security, and fault tolerance
• Dependable applications continue to work in the presence of
faults in hardware, software, and networks.
Presentation Outline
• Introduction
• Architectural Models
• Software Layers
• System Architectures
• Client-Server
• Clients and a Single Sever, Multiple Servers, Proxy Servers with Caches,
Peer Model
• Alternative Client-Sever models driven by:
• Mobile code, mobile agents, network computers, thin clients, mobile
devices and spontaneous networking
• Design Challenges/Requirements
• Fundamental Models –formal description
• Interaction, Failure, and Security models.
• Summary
Fundamental Models at Glance
• Fundamental Models are concerned with a formal description of the
properties that are common in all of the architectural models
• All architectural models are composed of processes that
communicate with each other by sending messages over a computer
networks.
• Models addressing time synchronization, message delays, failures,
security issues are addressed as:
• Interaction Model – deals with performance and the difficulty of setting of
time limits in a distributed system.
• Failure Model – specification of the faults that can be exhibited by processes
• Security Model – discusses possible threats to processes and
communication channels.
Interaction Model
• Computation occurs within processes;
• The processes interact by passing messages,
resulting in:
• Communication (information flow)
• Coordination (synchronization and ordering of activities)
between processes.
• Two significant factors affecting interacting
processes in a distributed system are:
• Communication performance is often a limiting
characteristic.
• It is impossible to maintain a single global notion of time.
Interaction Model:
Performance of Communication Channel
• Communication over a computer network has
performance characteristics:
• Latency:
• A delay between the start of a message’s transmission from one
process to the beginning of reception by another.
• Bandwidth:
• the total amount of information that can be transmitted over in
a given time.
• Communication channels using the same network, have to
share the available bandwidth.
• Jitter
• The variation in the time taken to deliver a series of messages.
It is very relevant to multimedia data.
• Is the variation in the time between data packets arriving,
caused by
• network congestion, or
• route changes.
• The longer data packets take to transmit, the more jitter affects
audio quality. The standard jitter measurement is in
milliseconds (ms)
Interaction Model:
Computer clocks and timing events
• Each computer in a DS has its own internal clock, which
can be used by local processes to obtain the value of the
current time.
• Therefore, two processes running on different computers
can associate timestamp with their events.
• However, even if two processes read their clocks at the
same time, their local clocks may supply different time.
• This is because computer clock drifts from perfect time
and their drift rates differ from one another.
• Even if the clocks on all the computers in a DS are set to
the same time initially, their clocks would eventually vary
quite significantly unless corrections are applied.
Interaction Model:
Two variants of the interaction model
• In a DS it is hard to set time limits on the time taken for process
execution, message delivery or clock drift.
• Synchronous DS – hard to achieve:
• The time taken to execute a step of a process has known lower
and upper bounds.
• Each message transmitted over a channel is received within a
known bounded time.
• Each process has a local clock whose drift rate from real time has
known bound.
• Asynchronous DS: There is NO bounds on:
• Process execution speeds
• Message transmission delays
• Clock drift rates.
Interaction Model:
Event Ordering
• In many DS applications we are interested in knowing
whether an event occurred
• before,
• after, or
• concurrently with another event at other processes.
• The execution of a system can be described in terms of
events and their ordering despite the lack of accurate
clocks.
Failure Model
• In a DS, both processes and communication channels
may fail – i.e., they may depart from what is considered
to be correct or desirable behavior (define and classify the
faults).
• Types of failures:
• Omission Failure(refer to cases where the process or
communication channel fails to perform a requested
action)
• Arbitrary Failure(where any type of error can occur.
Corrupt data, unexpected responses)
• Timing Failure(related to synchronous messages,
where a set bound [clock drift, message ack, process
execution time] exceeds defined bounds)
Processes and channels
process p
send
process q
m
receive
Communication channel
Outgoing message buffer
Incoming message buffer
• Communication channel produces an omission failure if it does
not transport a message from “p”s outgoing message buffer to
“q”’s incoming message buffer. This is known as “dropping
messages” and is generally caused by
• a lack of buffer space at the receiver or at gateway or by a network
transmission error.
Omission and arbitrary failures
Class of failure Affects Description
Fail-stop Process Process halts and remains halted. Other processes may
detectthis state.
Crash Process Process halts and remains halted. Other processes may
not be able to detect this state.
Omission Channel A message inserted in an outgoing message buffer never
arrivesat the other end’s incoming message buffer.
Send-omission Process A process completes a send, but the message is not
put in its outgoing message buffer.
Receive-omission Process A message is put in a process’s incoming message
buffer, but that process does not receive it.
Arbitrary
(Byzantine)
Process or
channel
Process/channel exhibits arbitrary behaviour: it may
send/transmit arbitrary messages at arbitrary times,
commit omissions; a process may stop or take an
incorrect step.
Timing failures
Class of Failure Affects Description
Clock Process Process’s local clock exceeds the bounds on its
rate of drift from real time.
Performance Process Process exceeds the bounds on the interval
between two steps.
Performance Channel A message’s transmission takes longer than the
stated bound.
Masking Failures
• It is possible to construct reliable services from
components that exhibit failures.
• For example, multiple servers that hold replicas of data can
continue to provide a service when one of them crashes.
• A knowledge of failure characteristics of a component
can enable a new service to be designed to mask the
failure of the components on which it depends:
• Checksums are used to mask corrupted messages.
Security Model
• The security of a DS can be achieved by securing the processes
and the channels used in their interactions and by protecting
the objects that they encapsulate against unauthorized access.
Protecting Objects: Objects and
principals
invocation
Client
Access rights
result
Principal (user)
Netw ork
Server
Principal (server)
Object
• Use “access rights” that define who is allowed to perform operation on a
object.
• The server should verify the identity of the principal (user) behind each
operation and checking that they have sufficient access rights to perform the
requested operation on the particular object, rejecting those who do not.
The enemy
Copy of m
Process p
The enemy
m
m’
Communication channel
Process q
• To model security threats, we postulate an enemy that is
capable of sending any process or reading/copying
message between a pair of processes
• Threats form a potential enemy: threats to processes,
threats to communication channels, and denial of service.
Defeating security threats:
Secure channels
Principal A
Process p
Secure channel
Principal B
Process q
• Encryption and authentication are use to build secure channels.
• Each of the processes knows the identity of the principal on whose
behalf the other process is executing and can check their access
rights before performing an operation.
Summary
• Most DSs are arranged accordingly to one of a
variety of architectural models:
• Client-Server
• Clients and a Single Sever, Multiple Servers, Proxy Servers
with Cache, Peer Model
• Alternative Client-Sever models driven by:
• Mobile code, mobile agents, network computers, thin clients,
mobile devices and spontaneous networking
• Fundamental Models – formal description
• Interaction, failure, and security models.
• The concepts discussed in the module play an
important role while architecting DS and apps.
Distributed Systems
Communication in Distributed
Systems
Venus Samawi
Isra University 1
Content
• Network layers of the model
• Summary of OSI layers
• TCP/IP and OSI model
• The interaction between layers in the OSI model
• An exchange using the OSI model
• Port Addresses
• Computer Networks
• TCP VsUDP
• Software and hardware service layers in distributed systems
• Operating systems (OS)
• Network OS
• Distributed OS
• Middleware
2
In previous lectures
• So far, We have discussed the communication in distributed
system form internet protocol’s point of view by studying the
layered implementation.
• We are going to study the communication in distributed system
at higher level (processes and application)
Figure 2.2 Seven layers of the OSI model
2.4
Figure 2.15 Summary of layers
2.5
TCP/IP and OSI model
2.6
The interaction between layers in the OSI model
2.7
An exchange using the OSI model
2.8
Port addresses
2.9
Port Address Translation (PAT)
• Port Address Translation (PAT) is an extension of Network Address Translation
(NAT)
• permits multiple devices on a LAN to be mapped to a single public IP address
to conserve IP addresses.
• PAT is similar to port forwarding except that an incoming packet with
destination port (external port) is translated to a packet different destination port
(an internal port).
• The Internet Service Provider (ISP) assigns a single IP address to the edge
device.
• When a computer logs on to the Internet, this device assigns the client a
port number that is appended to the internal IP address, giving the
computer a unique IP address.
• If another computer logs on the Internet, this device assigns it the same
public IP address, but a different port number.
• Although both computers are sharing the same public IP address, this
device knows which computer to send its packets, because the device uses
the port numbers to assign the packets the unique internal IP address of the
computers.
Computer Networks
Difference: Connection oriented and
Connectionless service
What is an Operating System?
• A program that acts as an intermediary between a user of a computer
and the computer hardware
• Operating system goals:
• Execute user programs and make solving user problems easier
• Make the computer system convenient to use
• Use the computer hardware in an efficient manner
Computer System Structure
Computer system can be divided into four components:
• Hardware –provides basic computing resources
• CPU, memory, I/O devices
• Operating system
• Controls and coordinates use of hardware among various applications and
users
• Application programs –define the ways in which the system resources are used to
solve the computing problems of the users
• Word processors, compilers, web browsers, database systems, video games
• Users
• People, machines, other computers
Operating System Supporting
In computing, a system call
(commonly abbreviated to
syscall) is the programmatic
way in which a computer
program requests a service
from the kernel of the
operating system on which
it is executed.
Distributed OS
Distributed OS Cont
Network OS Vs Distributed System OS
The main difference between these two operating systems (Network Operating
System and Distributed Operating System) is that
• In network operating system each node or system can have its own
operating system
• Indistribute operating system each node or system have same operating
system which is opposite to the network operating system.
Network OS Vs Distributed System OS Cont.
Network OS
• Main objective is to provide the
local services to remote client.
• Communication takes place on the
basis of files.
• more scalable than Distributed
Operating System.
• Limited fault tolerance
• High Rate of autonomy
• Ease of implementation
• All nodes can have different
operating system.
Distributed System OS
• Main objective is to manage the
hardware resources.
• Communication takes place on the
basis of messages and shared
memory.
• Less scalable than network
Operating System.
• High fault tolerance
• Limited Rate of autonomy
• More complicated implementation
• All nodes have same operating
system.
Distributed Systems
Inter-process Communication
(IPC)
Venus Samawi
Isra University 1
Content
• Interprocess Communication (IPC)
• Message passing
• Buffer
2
InterprocessCommunication
Process Cooperation
• There are several reasons to provide process cooperation environment
• Information sharing
• Computation speed up: If we want a particular task to run faster, we must
break it into subtasks, each of which will be executing in parallel with the
others.
• Modularity: dividing the system functions into separate processes or threads
• Convenience: Even an individual user may work on many tasks at the same
time. For instance, a user may be editing, printing, and compiling in parallel.
4
Models for InterprocessCommunication
Models for interprocess communication
could be through:
• Shared memory
• Message passing
Message -passing Systems
5
Message Size
6
7
Naming
Communication between processes either Direct or Indirect
8
9
Example
10
Message passing (Part2)
Communication link must exists between processes
11
12
Buffer Size Vs Blocking and non-Boloking
13
Distributed Systems
Inter-process Communication
(IPC)
Venus Samawi
Isra University 1
Content
• Interprocess Communication (IPC)
• Sockets
• RPC
• RMI
2
Middleware layers
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5
© Pearson Education 2012
Sockets (used for Communication in Client -Server systems)
4
Sockets
5
Sockets (Cont.)
6
Sockets and ports
socket
client
Internet address = 138.37.94.248
any port
agreed port
message
other ports
socket
server
Internet address = 138.37.88.249
Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5
© Pearson Education 2012
• Adaemonis a long-running background process that answers requests for services.
• In multitasking computer operating systems, a daemon is a computer program that runs as a background
process, rather than being under the direct control of an interactive user.
• Astub in distributed computing is a piece of code that converts parameters passed
between client and server during a remote procedure call (RPC).
• The main idea of an RPC is to allow a local computer (client) to remotely call
procedures on a different computer (server).
• "marshalling" refers to the process of converting the data or the objects into a byte
stream,
• "unmarshalling" is the reverse process of converting the byte-stream back to their
original data or object.
• Adaemonis a long-running background process that answers requests for services.
• In multitasking computer operating systems, a daemon is a computer program that runs as a background
process, rather than being under the direct control of an interactive user.
• Binding information includes the protocol that clients use to communicate with the
site, the site's IP address, the port number, and a host header. The
element contains two attributes to configure the binding
information: bindingInformation and protocol
RPC
RMIVS
RPC
RPC does not provide
any security.
Although it provides
client-level security.
18
Key Differences Between RPC and RMI
• RPC supports procedural programming paradigms thus is C based, while RMI supports
object-oriented programming paradigms and is java based.
• The parameters passed to remote procedures in RPC are the ordinary data structures. On
the contrary, RMI transits objects as a parameter to the remote method.
• RPCcanbeconsidered as:
• Theolder version of RMI,
• it is used in the programming languages that support procedural programming,
• it can only use pass by value method.
As against, RMI facility is devised based on modern programming approach, which could
use
• passbyvalue or reference.
• Anotheradvantage of RMIis that the parameters passed by reference can be changed.
• RPCprotocol generates more overheads than RMI.
• The parameters passed in RPC must be “in-out” which means that the value passed to the
procedure and the output value must have the same datatypes. In contrast, there is no
compulsion of passing “in-out” parameters in RMI.
• In RPC, references could not be probable because the two
processes have the distinct
address
space, but it is possible in case of RMI.
19
Parameter Passing in RPC
• Functions in an application that runs in a single process may collaborate via
parameters and/or global variables.
• Functions in an application that runs in multiple processes on the same host may
collaborate via message passing and/or non-distributed shared memory
• In RPC, passing parameters is the only way that , clients and servers share
information
• Parameters that are passed by value are fairly simple to handle
• The client stub copies the value from the client and packages into a network
message
Ex:
• Consider a remote procedure, sum(i, j), which takes two integer parameters and
returns their arithmetic sum.
• The client stub takes its two parameters and puts them in a message, and Puts the name
or number of the procedure to be called in the message.
• When the message arrives at the server, the stub examines the message to see
which procedure is needed, and then makes the appropriate call.
•When the server has finished execution, it takes the result provided by the server
and packs it into a message.
This message is sent back to the client stub, which unpacks it and returns the
value to the client procedure
20
RPC: Parameters Passed by Reference
• Parameters passed by reference are much harder:
• For example distributed systems with distributed shared-memory
mechanisms can allow passing of parameters by reference.
• A pointer is meaningful only within the address space of the
process in which it is being used.
Suppose there are two parameters to be passed, if the second parameter
is the address of the buffer which is 1000 on the client, one cannot just
pass the number 1000 to the server and expect it to work. Address 1000
on the server might be in the middle of the program text.
For that, call by reference is not practical in RPC and massage passing
is used.
21
Remote Method Invocation (RMI)
• Remote Method Invocation (developed in 1980’s) allows remote
method calls, at which objects in different programs can
communicate
• RMI is based on RPC (RMI is Java’s implementation of RPC).
• It is object-oriented version of RPC
• Methods calls appear same as those in same program.
• RMI performs networking and marshaling of data (converting
the data or the objects into a byte-stream)
• Interface definition language is required to describe functions.
• RMI is an approach that provides remote communication between
the application using two objects:
• stub
22
• skeleton
Stub
Whenastub'smethodisinvoked, itdoes
thefollowing:
• initiates aconnectionwith the remote
JVMcontainingtheremoteobject,
•marshals (writes and transmits) the
parameterstotheremoteJVM,
•waits for the result of the method
invocation,
•Unmarshals(reads) thereturnvalueor
exceptionreturned,and
• returnsthevaluetothecaller.
Skeleton
Whenaskeletonreceivesanincomingmethod
invocationitdoesthefollowing:
• unmarshals (reads) the parameters for the
remotemethod,
• invokes themethod on the actual remote
objectimplementation,and
•marshals (writes and transmits) the result
(returnvalueorexception)tothecaller.
RMICont.
ThecommondifferencebetweenRPCandRMIisthat
• RPConlysupportsproceduralprogrammingwhereasRMIsupportsobject
orientedprogramming.
• Client calls the local method (stub) to perform this operation
• The stub on computer client calls RMI Registry to know
whether thatmethod exists and the correct way to call it
• Client stub calls this method on server
• Server skeleton receives this request and communicate with the
software on server, when it receives the response, sends this
response to the client stub.
Client
RMISystem Layers
The RMI system consists of 3 layers:
• The stub/Skeleton layer
• Client side stubs (proxies)
• Server side skeletons
• The Remote Reference Layer
• Transport Layer
RMI Architecture
Remote Reference layer
• The remote reference layer deals with the lower-level transport interface.
• It is also responsible for carrying out a specific remote reference
protocol which is independent of the client stubs and server skeletons.
• Each remote object implementation chooses its own remote reference
subclass that operates on its behalf.
• Various invocation protocols can be carried out at this layer.
Examples are:
• Unicast point-to-point invocation.
• Invocation to replicated object groups.
• Support for a specific replication strategy.
• Support for a persistent reference to the remote object (enabling
activation of the remote object).
• Reconnection strategies (if remote object becomes inaccessible).
27
Remote Reference layer (Cont.)
The remote reference layer has two cooperating components:
• the client-side
• server-side components.
The client-side component
• contains information specific to the remote server (or servers, if the remote
reference is to a replicated object)
• communicates via the transport to the server-side component.
During each method invocation, the client and server-side components
perform the specific remote reference semantics.
• For example,
If a remote object is part of a replicated object, the client-side component
can forward the invocation to each replica rather than just a single
remote object.
28
Transport Layer
The transport layer of the RMI system is responsible for:
• Setting up connections to remote address spaces.
• Managing connections.
• Monitoring connection "liveness."
• Listening for incoming calls.
• Maintaining a table of remote objects that reside in the
address space.
• Setting up a connection for an incoming call.
• Locating the dispatcher (لسرملا) for the target of the remote
call and passing the connection to this dispatcher.
Transport Layer (Cont.)
• The concrete representation of a remote object reference consists of
• Anendpoint
• Anobject identifier.
This representation is called a live reference.
• Given a live reference for a remote object,
• a transport can use the endpoint to set up a connection to the address space in
which the remote object resides.
• Onthe server side, the transport uses the object identifier to look up the target
of the remote call.
The transport for the RMI system consists of four basic abstractions:
• Anendpoint: is the abstraction used to denote an address space or Java
virtual machine.
• In the implementation, an endpoint can be mapped to its transport. That is, given an
endpoint, a specific transport instance can be obtained.
• Achannel: is the abstraction for a conduit ( )
هانق between two address spaces.
• it is responsible for managing connections between the local address space and the
remote address space for which it is a channel.
• Aconnection: is the abstraction for transferring data (performing
input/output).
• The transport abstraction manages channels.
30
• Each channel is a virtual connection between two address spaces.
Summarize English and Arabic text using the statistical algorithm and sorting sentences based on its importance
You can download the summary result with one of any available formats such as PDF,DOCX and TXT
ٌYou can share the summary link easily, we keep the summary on the website for future reference,except for private summaries.
We are working on adding new features to make summarization more easy and accurate
تعتبـــر التغذية الصحية مهمة جدا خلال الســـنتين الاولى من عمر الطفل حيث يتطور النمو العقلي والجســـ...
ﻦ ﷲ، إﻻ إﻟﮫ ﻻ ﯾﺎﻣﻮﺳﻰ: ﻗُﻞ ْ ﻗﺎل: ﺑﮫ، وأدﻋُﻮك َ أذﻛﺮُك َ ﺷﯿﺌًﺎ ﻋَﻠﱠﻤﻨﻲ ؟ ھﺬا ﯾﻘﻮﻟﻮن ﻋ ِ ﺒﺎدِك َ ﻛﻞ ﱡ ...
معايير التقييم الأساسية المهارة النسبة الفهم السمعي 20% التعبير الشفهي 25% القراءة والفهم 20% الكت...
التحسّس المبكّر لأمراض الكلى ضروري لمنع أو تأخير تطور المرض إلى مراحله النهائية. يشتمل التشخيص المبك...
عـهـدنـا كـنـزنـا حلم سـيـنــمـو فـينـا درب طـويــل و عـزمـنـا جــبـال فــيـنــا اهـدؤوا و ابـدؤو...
تحسن معدلات النجاة عالميًا: بفضل برامج التطعيم، وتحسن الرعاية الصحية الأولية، وانخفاض معدل الفقر. ...
. أوبين فلم إطا الوية واماعلى الإساة غير عاوية زى بلغ الزاع ر الهدة والتظيم تجلد خاضأو لأحكام القانو...
I have a request: whenever we make an appointment and it's an automated call reminder about the appo...
• في الدعائم ذات البنية المغلقة أو الشكل المصمت، يقتصر التحلل غالباً على السطح الخارجي، ما يؤدي إلى ...
• في الدعائم ذات البنية المغلقة أو الشكل المصمت، يقتصر التحلل غالباً على السطح الخارجي، ما يؤدي إلى ...
بموجب هذا العقد، يتفق البائع والمشتري على أن يقوم المشتري بدفع إيجار دوري للمبنى أو العقار المتفق عل...
1) المرونة: يستطيع الأطفال في هذه المرحلة مواجهة المخاطر والتحديات، مثل المشاكل الأسرية، والمشاكل ال...