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Communication

Communication. Chapter 2. IPC. Inter-Process Communication is the heart of all DSs. Processes on different machines. Always based on low-level message passing. In this chapter: RPC RMI MOM (Message Oriented MiddleWare) Streams (due to the advent of Multimedia DSs).

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Communication

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  1. Communication Chapter 2

  2. IPC • Inter-Process Communication is the heart of all DSs. • Processes on different machines. • Always based on low-level message passing. • In this chapter: • RPC • RMI • MOM (Message Oriented MiddleWare) • Streams (due to the advent of Multimedia DSs)

  3. Layered Protocols (1) • Layers, interfaces, and protocols in the OSI model. 2-1

  4. Layered Protocols • Protocol • Connection Oriented • Connectionless • Protocol Stack • Description of the layers, Unit of exchange.

  5. Layered Protocols (2) • A typical message as it appears on the network. 2-2

  6. Data Link Layer • Discussion between a receiver and a sender in the data link layer. 2-3

  7. Transport Protocols • Makes the underlying layers usable by the application layer. • Provide a reliable or unreliable connection for the upper layer. • UDP :: TCP • RTP for real-time systems.

  8. Client-Server TCP • Normal operation of TCP. • Transactional TCP. 2-4

  9. Middleware Protocols • An adapted reference model for networked communication. 2-5

  10. RPC • PC? • R…………….PC? • Simple idea • Complexity in provision

  11. Conventional Procedure Call • Parameter passing in a local procedure call: the stack before the call to read Count = read (fd, buf, nbytes); • The stack while the called procedure is active

  12. Issues • Calling Method? • Call by value • Call by reference • Call by Copy/Restore • Call by name

  13. Client and Server Stubs • Principle of RPC between a client and server program. • The read stub is called on behalf of the real read procedure!

  14. Steps of a Remote Procedure Call • Client procedure calls client stub in normal way • Client stub builds message, calls local OS • Client's OS sends message to remote OS • Remote OS gives message to server stub • Server stub unpacks parameters, calls server • Server does work, returns result to the stub • Server stub packs it in message, calls local OS • Server's OS sends message to client's OS • Client's OS gives message to client stub • Stub unpacks result, returns to client

  15. Passing Value Parameters (1) • Steps involved in doing remote computation through RPC • It works fine, while the scenario is simple and straightforward; but …. 2-8

  16. Passing Value Parameters (2) • Different character set standards (ASCII vs EBCDIC) • Little-Endian vs Big-Endian Architecture. • Original message on the Pentium (L. E.) • The message after receipt on the SPARC (B. E.) • The message after being inverted. The little numbers in boxes indicate the address of each byte

  17. Call by Reference Parameter Passing ???

  18. Parameter Specification and Stub Generation • Both sides should agree on the content of passing data structures. • Example in the next slide. • The way a message including the parameters is interpreted is the main issue!! • Client and server should agree on the representation of simple data structures. • Agreement on the actual exchange of the messages (connection-oriented or connection-less)

  19. Parameter Specification and Stub Generation • A procedure • The corresponding message. • Interface Definition Language  compiling into client stub and server stub

  20. Extended RPC Models • RPC becoming as de facto standard for comm. in DSs. • Popularity due to simplicity. • Two extensions • Doors • Async RPC.

  21. Doors • Equivalent to RPC for processes located on the same machine. • A door is a name for a procedure in the address space of a server process, called by colocated processes within the server. • Idea was originally from the Spirit OS (1994) • Same as LightWeight RPC. • The server process must register a door before use (calling door-create)

  22. Doors • The principle of using doors as IPC mechanism.

  23. Asynchronous RPC (1) • The interconnection between client and server in a traditional RPC • The interaction using asynchronous RPC 2-12

  24. Asynchronous RPC (2) • A client and server interacting through two asynchronous RPCs 2-13

  25. Writing a Client and a Server • The steps in writing a client and a server in DCE RPC. 2-14

  26. Binding a Client to a Server • Client-to-server binding in DCE. 2-15

  27. Performing an RPC • The whole scenario! • Semantics • At-most-once operation • Idempotency

  28. Remote Object Invocation • OO technology in centralized systems. • Promoting the idea of RPC to the OO technology. • Proxy as the client delegate == Client stub. • Skeleton == server stub • The object state is normally not distributed  remote object instead of distributed object

  29. Distributed Objects • Common organization of a remote object with client-side proxy. 2-16

  30. Message-Oriented Communication • Sometimes both RPC and RMI is not appropriate • Synchronous nature of RPC and RMI! •  Messaging.

  31. Berkeley Sockets (2) • Connection-oriented communication pattern using sockets.

  32. The Message-Passing Interface (MPI) • Some of the most intuitive message-passing primitives of MPI.

  33. Stream-Oriented Communication • Till now, focus was on exchanging one or more independent and complete units of info. • However, consider an audio stream, CD quality is also required  the original sound has been sampled at 44100 Hz a sample in each 1/44100 Sec is required to re-produce the original sound. • Time-dependent and continuous media is required :: Temporal relationship between data items are crucial.

  34. Data Stream (1) • Setting up a stream between two processes across a network. • Data stream is a sequence of data units.

  35. Transmission Modes • Async Trans Mode: Sending regardless of time • Synch Trans Mode: There is a max end-to-end delay for each unit: Sensor info! • Isochronous Trans Mode: Data units should be transferred on time:: A max and min end-to-end delay (bounded jitter).

  36. Data Stream (2) • Setting up a stream directly between two devices. 2-35.2

  37. Data Stream (3) • An example of multicasting a stream to several receivers.

  38. QoS • Time-Dependent requirement:: QoS • Next slide as a sample QoS specification • Formulation based on the token bucket algorithm • Basic idea is that tokens are generated at a constant rate. • Token is a fixed # of bytes, an application is allowed to pass to the network.

  39. Specifying QoS (2) • The principle of a token bucket algorithm.

  40. Setting Up a Stream • The basic organization of RSVP (Resource reSerVation Protocol) for resource reservation in a distributed system.

  41. Synchronization Mechanisms (1) • The principle of explicit synchronization on the level data units.

  42. Synchronization Mechanisms (2) • The principle of synchronization as supported by high-level interfaces. 2-41

  43. End of Chapter 2

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