COMP 655: Distributed/Operating Systems

1. COMP 655:Distributed/Operating Systems Summer 2011 Dr. Chunbo Chu Week 2, 3: Communication Distributed Systems - COMP 655

2. Agenda • 6:00 - 6:30, system summary presentation • 6:30 - 8:00, Lab 1 • 8:00 - 8:15, Break • 8:15 – 9:30, Communication • Wrap up Distributed Systems - COMP 655

3. Communication essentials • Communication patterns • Communication structure: OSI reference model and TCP/IP coverage • Major middleware communication services • RPC • RMI • http • I hate to wait … • Message passing • (skip) Streams Distributed Systems - COMP 655

4. Communication patterns Producer Consumer query response Distributed Systems - COMP 655

5. Communication patterns with broker Consumer 1 Producer 1 Subscription service Consumer 2 Producer 2 Consumer 3 query Server 1 Request broker Client 1 query response Server 2 Client 2 response Server 3 Distributed Systems - COMP 655

6. Communication patterns -other variations • Blocking or non-blocking • Connection-based or connectionless • Transient or persistent Examples: • blocking transient query-response == RPC • non-blocking persistent producer/consumer == message queuing Distributed Systems - COMP 655

7. A note about blocking • Blocking communication blocks the thread that makes the call • Other work may continue in other threads in the same process • More about threads and processes next week Distributed Systems - COMP 655

8. Communication essentials • Communication patterns • Communication structure: OSI reference model and TCP/IP coverage • Major middleware communication services • RPC • RMI • http • I hate to wait … • Message passing Distributed Systems - COMP 655

9. OSI Reference Model TCP/IP HTTP, FTP, … 2-1 not used not used TCP, UDP connectionless IP e.g. Ethernet sending bits Distributed Systems - COMP 655

10. Layered Protocols • A typical message as it appears on the network.

11. Middleware Protocols 2-5 RPC RMI Distributed Systems - COMP 655

12. Where does http fit? • Originally, it looked like an application-level protocol, where the application was fetching and viewing HTML pages • As the Web has matured, it has been used increasingly as middleware. • It’s the foundation for • Web applications • Web services Distributed Systems - COMP 655

13. Communication essentials • Communication patterns • Communication structure: OSI reference model and TCP/IP coverage • Major middleware communication services • RPC • RMI • http • I hate to wait … • Message passing Distributed Systems - COMP 655

14. RPC is all about • Allowing a client to make a procedure call that is processed on a remote machine • Without the client’s having to care (much) Distributed Systems - COMP 655

15. 2.3 0.98 Ordinary procedure call process caller callee (callee is atanh) Distributed Systems - COMP 655

16. Conventional Procedure Call Parameter passing in a local procedure call: the stack before the call to read. (b) The stack while the called procedure is active.

17. 0.98 0.98 client stub server stub message Atanh, 0.98 RPC: call the procedure across a network client process server process caller callee network Distributed Systems - COMP 655

18. 2.3 2.3 client stub server stub message OK, 2.3 RPC: call the procedure across a network client process server process caller callee network Distributed Systems - COMP 655

19. RPC Summary • client stub pretends to be the callee client process server process • server stub pretends to be the caller • caller and callee are written as if they were on the same machine caller callee client stub server stub • location transparency!! (almost) network Distributed Systems - COMP 655

20. Warning: passing parameters and results can be tricky Distributed Systems - COMP 655

21. Question • Parameter passing mechanisms • By value • By reference • By value-results • … Distributed Systems - COMP 655

22. Passing Value Parameters (1) Distributed Systems - COMP 655

23. Passing Value Parameters (2) • Original message (JILL, 5) on the Pentium (little endian) • The message after receipt on the SPARC (big endian) • The message after being inverted. The little numbers in boxes indicate the address of each byte Number is backwards String is backwards Distributed Systems - COMP 655

24. Distributed Systems - COMP 655

25. Conclusion from the byte order problem • You have to explicitly define your interfaces. • This problem is one of the fundamental motivators for the use of Interface Definition Languages (IDL) • WSDL, OMG IDL, … • Passing value parameters is a key component of access transparency. Distributed Systems - COMP 655

26. Interface definition Interface definitions are critical factors in • Openness • Flexibility • Access transparency • Enabling proxy construction in middleware Distributed Systems - COMP 655

27. Activity – what can go wrong?(with an RPC) Brainstorm things that can go wrong when a program on machine A tries to make an RPC to a server on machine B Network A B Distributed Systems - COMP 655

28. Pick the top three • The instructor will pick some of the failures you brainstormed • Multi-vote on the ones in whose solutions you are most interested • You will use the top three vote-getters in the next activity Distributed Systems - COMP 655

29. Activity – what could you do about it? Brainstorm approaches to dealing with the top three failure modes Network A B Distributed Systems - COMP 655

30. Communication patterns -reminder Parameters: • Query-response or producer-consumer • Blocking or non-blocking • Connection-based or connection-less • Persistent or transient Examples: • blocking transient query-response == RPC • non-blocking persistent producer/consumer == message queuing Distributed Systems - COMP 655

31. Finding the server or peer • In all cases, the process that initiates a communication has to find the process it wants to communicate with • The address could be in • Source code (simplest, least flexible) • Configuration file • A network directory service (requires client to know a name for a server) (e.g. DNS) • A series of directories (e.g. LDAP, then RMI Registry) • Interaction with the directory is usually blocking request/response Distributed Systems - COMP 655

32. Communication essentials • Communication patterns • Communication structure: OSI reference model and TCP/IP coverage • Major middleware communication services • RPC • RMI • http • I hate to wait … • Message passing Distributed Systems - COMP 655

33. Focus on Remote Objects • Here, object == state + methods + interface • Usually, only the interface is distributed • “Remote objects” refers to approaches in which, for each object there is some server where all of it state resides Distributed Systems - COMP 655

34. Distributed Objects • Common organization of a remote object with client-side proxy. 2-16 Distributed Systems - COMP 655

35. Basic Java RMI Interface.class _Stub.class * • Server registers its name and remote object • Client gets reference (a stub)  to remote object from rmiregistry • Client calls server Client 2. rmiregistry** 3. * = not needed if Client and Server are both Java 1.5 or newer 1. Server ** = RMI Registry can be a separate process or an object inside the Server process. If separate, must be on same machine. Interface.class _Skel.class * Distributed Systems - COMP 655

36. Passing objects around … • In RMI, parameters and return values can be object references • If the parameter refers to a local object, do you copy the object or pass only a reference? • If the parameter refers to a remote object, what do you pass? Distributed Systems - COMP 655

37. Local, remote objects in Java RMI • Local objects are passed by value • Remote objects by reference • Passing a remote object by reference means passing a proxy by value Distributed Systems - COMP 655

38. Parameter Passing in RMI Distributed Systems - COMP 655