1. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 RPC Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms 2/e

2. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Communication in distributed systems Always based on low-level message passing as offered by the Under lying network widely-used models for communication – Remote procedure call(RPC) –Aims at hiding most of the intricacies of message passing –Low level communication facilities of computer networks are not suitable Lack of distribution transparency

3. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Layered protocols Lower-level protocols – physical layer is transmitting the 0s and 1s – data link is putting bit pattern and computing a checksum by adding up all the bytes in the frame in a certain way –Network layer is a part of the internet protocol suite. IP packet exchanging Transport protocols – forms the last part of what could be called a basic network protocol stack, some packets can be lost on the way from the send to receiver. Some application can handle their own error recovery –Is to provide this service. Additional transport protocols are regularly proposed; Real-time Transport protocol(RTP)

4. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Layered protocols Higher-level protocols –Session layer is essentially and enhanced version of the transport layer. It provides dialog control, to keep track of which party is currently talking and synchronization –Presentation layer is concerned with the meaning of the bit; structured information –Application layer become the container for all applications –Form the perspective virtually all distributed systems are just applications. Application-specific protocols ; http, ftp… Middleware protocols – lives in application layer but more specific applications –A distinction can marked between high-level communication protocols and protocols for establishing's various middleware service.

5. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Layered Protocols (1) Layers, interfaces, and protocols in the OSI model.

6. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Layered Protocols (2) A typical message as it appears on the network.

7. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Middleware Protocols An adapted reference model for networked communication.

8. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Types of Communication Viewing middleware as an intermediate (distributed) service in application-level communication. Email service

9. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Client and Server Stubs Principle of RPC between a client and server program.

10. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Client and Server Stubs Client Server stub The idea behind RPC is to make procedure call look as much as possible like a local one Call to a READ in the code to get the data, the read routine is extracted form the library and insert into the object program. The READ procedure(Stub) is a kind of interface between the user code and local OS. The call to send, the client stub calls receive, blocking itself until the reply comes back When the message arrives at the server, the server’s OS passes it up to a server stub. A server stub is the server-side equivalent of a client stub

11. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Remote Procedure Calls (1) A remote procedure call occurs in the following steps: 1.The client procedure calls the client stub in the normal way. 2.The client stub builds a message and calls the local operating system. 3.The client’s OS sends the message to the remote OS. 4.The remote OS gives the message to the server stub. 5.The server stub unpacks the parameters and calls the server. Continued …

12. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Remote Procedure Calls (2) A remote procedure call occurs in the following steps (continued): 6.The server does the work and returns the result to the stub. 7.The server stub packs it in a message and calls its local OS. 8.The server’s OS sends the message to the client’s OS. 9.The client’s OS gives the message to the client stub. 10.The stub unpacks the result and returns to the client.

13. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Passing Value Parameters (1) Figure 4-7. The steps involved in a doing a remote computation through RPC.

14. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Parameter Specification and Stub Generation. (a) A procedure. (b) The corresponding message. Caller and callee agree on the format of the messages they exchange and they follow the same steps when it comes to passing complex data structures Both sides in an RPC should follow the same protocol otherwise the RPC will not work correctly

15. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 RPC (1) The interaction between client and server in a traditional RPC. As interface is usually available in the same programming language as the one in which the client or sever is written By means of an Interface Definition Language(IDL)

16. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Asynchronous RPC (2) –When a client calls a remote procedure the client will block until are replies returned –Unnecessary when there is no result return; adding entries into a database, transferring money from one account to another, batch processing… –To support this situation, RPC systems may provide facilities for what are called asynchronous RPCs. (Server’s acknowledgement for client requirement)

17. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Asynchronous RPC (2) The interaction using asynchronous RPC. It also called as deferred synchronous RPC

18. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Asynchronous RPC (3) A client and server interacting through two asynchronous RPCs.

19. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Ex. DCE RPC RPCs have been widely adopted as the basis of middleware and distributed systems Good example Distributed computing enviroment(DEC) –-Developed by the Open Software Foundations(OSF) –-Ref Sun RPC vs MS’ Decom It consists of many components; language, libraries, daemons and utility programs. Together these make it possible to write clients and servers. In CS system, everything tied together with Interface Definition Language

20. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Writing a Client and a Server (1). The steps in writing a client and a server in DCE RPC.

21. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Writing a Client and a Server (2) When the IDL file is complete, the IDL compiler is called to process it. Three files output by the IDL compiler: A header file (e.g., interface.h, in C terms).- contains the unique identifier, type definition constant definition and function prototypes The client stub. – contains the actual procedures that the client program will call The server stub. – contains the procedures called by the runtime system on the server

22. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Binding a Client to a Server (1) To allow a client to call a server, necessary that the server be registered and prepared to accept incoming calls. Registration of a server makes it possible for a client to locate the server and bind to it. Server location is done in two steps: 1.Locate the server’s machine. 2.Locate the server on that machine. -second step is subtle and client needs to know an end point -In DCE a tale of pairs is maintained by DEC Daemon

23. Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007 Prentice-Hall, Inc. All rights reserved. 0-13-239227-5 Binding a Client to a Server (2) Client-to-server binding in DCE.