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Princess Nora Bint Abdulrahman University College of computer and information sciences Networks department. Distributed systems (NET 422). Prepared by Dr. Samia Chelloug E-mail: samia_chelloug@yahoo.fr. Content. Characterization of distributed systems. System models.

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Princess Nora Bint Abdulrahman UniversityCollege of computer and information sciencesNetworks department

Distributed systems (NET 422)

Prepared by Dr. Samia Chelloug

E-mail: samia_chelloug@yahoo.fr

content
Content
  • Characterization of distributed systems.
  • System models.
  • Inter-process communication (Internet protocols, external data representation, client-servereur, group communication…).
  • Distributed objects and remote invocation (RPC, events and notifications, Java, RMI).
  • Distributed file systems (architecture, case study Sun Nfs).
  • Name services (concepts, DNS, case study X.5000).
  • P2P systems and middleware.
  • Time and global state ( synchronization of physical and logical clocks).
  • Coordination and agreement.
  • Replication (system model, fault-tolerant services).
  • Web services (service description and IDL for web services, case study: the grid…)
textbooks
Textbooks
  • George Coulouris, Jean Dollimore, Tim Kindberg, Gordon Blair, ‘Distributed Systems. Concepts and Design’, Fifth Edition, 2012.
  • Kenneth P.Birman, ‘Reliable Distributed systems: technologies, web services and applications’, Springer
  • Andrew S.Tanenbaum, Marteen Van Steen, ‘Distributed Systems: Principles and paradigms’, second edition, 2006.
  • Arno Puder, Kay Romer, Frank Pilttofer, ‘Distributed Systems Architecture: a middle approach’, Elsevier, 2006.
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Part 2: System models
  • Architectural model
  • Fundamental model
  • Failure model
system models
System models
  • How to build distributed systems?
  • We need a descriptive model that can capture and analyze design issues and properties.
  • The architecture of a system and its structure in terms of separately specified components and their interrelationships.
  • The overall goal is to ensure that the structure will meet present and future demands.
  • What are the entities that are communicating in the distributed system?
  • How do they communicate? What communication paradigm is used ?

Architectural model:

system models1
System models
  • What roles (potentially changing) and responsabilities do they have in the overall architecture?
  • How are they mapped on to the physical distributed infrastructure?

Specific issues for distributed systems:

  • Entities that communicate in a distributed system are processes but some systems do not support processes abstraction (sensor networks) and hence the entities are nodes.
  • Objects: have been introduced to enable and encourage the use of oriented –object programming approaches in distributed systems.
system models2
System models
  • Objects are accessed via interfaces.
system models3
System models
  • Web services : closely related to objects. They are intrinsically integrated into the www. A software application identified by a URI, whose interface and binding are capable of being defined, described and discovered as XML.
  • Communications:
  • Interprocess communication: refers to the relatively low-level support for communication between processes in distributed systems, including message passing primitives, and direct access to the API.
  • Remote invocation: results in the calling of a remote operation, procedure or method.
system models4
System models

-Remote procedure call makes a remote call looks like a local call.

-Supports many transparencies and heterogeneities.

-Remote method call extends RPC to object-oriented programming.

Request-reply: involves a pairwise exchange of messages from client to server and from server to client.

  • Indirect communications:
  • Group communication is concerned with the delivery of messages to a set of recipients (one to many). Relies on a group identifier.
system models5
System models
  • Publish-subscribe:

Large number of producers (publishers) distribute information : items of interest (events) .

  • Message queue:

Producer processes can send messages to a specified queue and consumer processes can receive messages from the queue.

  • Distributed shared memory:

Enables sharing data between processes that do not share physical memory. A copy is provided in a timely manner.

system models6
System models
    • Roles and responsabilities:(System architecture)

What role does a given entity take?

  • Client/server:

Client processes interact with individual servers processes. Servers may in turn be clients of other servers.

system models7
System models
  • Peer - to peer:

All processes involved in a task play similar roles, interacting cooperatively as peers without any distinction between client and server processes.

system models8
System models
  • Caching:

a store of recently used data objects that is closer to one client or a particular set of clients.

  • When an object is needed by a client process, the caching service first checks the cache and supplies the object from there if an up-to date copy is available. If not, an up-to-date copy is fetched.
  • Web browsers maintain a cache of recently visited web pages.
system models10
System models
  • Mobile code:

client requests a result in the downloading of applet code. Applets are a well-known and widely used example of mobile code.

  • The user running a browser, selects a link to an applet whose code is stored on a web browser.
  • The code is downloaded to the browser and runs there.
  • Good interactive response (does not suffer from the delays and variability of bandwidth ).
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System models
    • Placement:
  • The final issue to beconsideredis how entities (objects, services, …) mapinto the underlyingdistributed infrastructure whichwillconsist of a potentially large number of machines interconnected by a network.
  • No optimal solutions. Onlystrategies to help
  • Layeringis a vertical organization of services into service layers.
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System models

  • Tiered architecture is complementary to layering. It enables organizing functionnality of a given layer and place this functionnality into appropriate servers and, as a second consideration, on to physical nodes.
system models12
System models

Fundamental model:

Fundamental model allows us to be more specific about some characteristics of distribted systems and the failures and security risks they might exhibit.

  • Interaction model:
  • The rate at which each process proceeds and the timing of the transmission of messages between them cannot in general be predicted.
  • Latency is defined as the delay between the start of message ‘s transmission from one process and the begining of its receipt by another.
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System models
  • Computer clocks and timing events:
  • Each computer has its own internal clock, which can be used by local processes to obtain the value of the current time.
  • Two variants of interaction models: synchronous and asynchronous ones.
  • Synchronous: time to execute each step of a process has known lower and upper bounds. Each message is recived with a known bound time.
  • In some distributed systems, we are interested in knowing whether an event at one process occured before, after or concurrently with another event.
system models14
System models
  • If the clocks on X’s, Y’s and Z’s computers could be synchronized, each message could carry the time on local computer’s clock. t1 < t2 < t3
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System models
  • Failure model:
  • Both processes and communication channels may fail.
  • Failure model defines the way in which failure may occur.
  • Omission failures:
  • Process p performs a send by inserting the message m in its outgoing message queue.
  • The communication channel is responsible for transporting m to q’s incomming message queue.
  • Process q perfomrs a receive by taking m from its incoming message buffer.
system models16
System models
  • Omission failure is known as « dropping messages ».
  • Omission failure is caused by lack of buffer space at the receiver or at any intermediate gateway.