RPC Model, Stubs and Skeletons Divya Nampalli

l Assume that you are developing a client-server application: n How to let the two processes (client and server) located on two machines communicate with each other? u Socket programming: using functions like connect(sd, (struct sockaddr *)&sin, sizeof(sin)), write(sd, buf, strlen(buf)) etc. Client-Server Communication

Remote Procedure Calls (RPC) Avoid explicit message exchange between processes Basic idea is to allow a process on a machine to call procedures on a remote machine –Make a remote procedure possibly look like a local one

How are parameters passed in a local procedure call –E.g., #include sys/types.h #include unistd.h... char buf[20]; size_t nbytes; ssize_t bytes_read; int fd;... nbytes = sizeof(buf); bytes_read = read(fd, buf, nbytes);... Conventional Procedure Call

Figure 4-5. (a) Parameter passing in a local procedure call: the stack before the call to read. (b) The stack while the called procedure is active.

Remote Procedure Calls (RPC) How are parameter passed in a remote procedure call, while making it look like a local procedure call?

Remote Procedure Calls (RPC) Suited for Client-Server structure. Combines aspects of monitors and synchronous message passing: ∗ Module (remote object) exports operations, invoked with call. ∗ call blocks (delays caller) until serviced. call causes a new thread to be created on remote (server). Client-server synchronization and communication is implicit.

Terminology / Notation Server module: operations, (shared) variables, local procedures and threads for servicing remote procedure calls. Interface (specification): describes the operations, parameter types and return types. Server process: thread created by call to service an operation. Background process: threads running in a module that aren’t created in response to call.

Lookup and registration How does the client find the server? Often server registers (binds) with a naming service (registry). Client obtains information (lookup) about server from this server. This changes the question to: How does the client find the registry?

Synchronization Synchronization within a module (server). Two approaches: 1.Assume mutual exclusion in server (only one server process/background process executing at a time). ∗ Similar to monitors. ∗ Still need conditional synchronization. 2. Program it explicitly (i.e., using semaphores, monitors etc.)

Client and Server Stubs Principle of RPC between a client and server program.

Steps of a Remote Procedure Call 1.Client procedure calls client stub in normal way 2.Client stub builds message, calls local OS 3.Client's OS sends message to remote OS 4.Remote OS gives message to server stub 5.Server stub unpacks parameters, calls server 6.Server does work, returns result to the stub 7.Server stub packs it in message, calls local OS 8.Server's OS sends message to client's OS 9.Client's OS gives message to client stub 10.Stub unpacks result, returns to client

Skeletons and Stubs

1.‘Stub’ objects implement the same interface as the server objects. (Proxy pattern) 2. (0), (5) Client threads call a stub local to their JVM instance. 3. (1), (4) Stub messages (TCP/IP) to Skeleton object in remote JVM & waits for reply. 4. (2), (3) Skeleton creates a new server thread which calls the server. 5. Stub and skeleton classes are synthesized by a RMI Compiler (rmic).

Passing Value Parameters (1) Steps involved in doing remote computation through RPC 2-8

Passing reference parameters –What is Call By Value and Call By Refernce? –Example: call foo(int, int * ) or read(fd, buf, nbytes) –Call by copy/restore –The dreaded “ pointer problem ” Linked list Complex graph a b a’ b’ foo(a, &b ) Call foo(a, &b’ ) Copy value a and contents of loc b into a’ and loc b’ Return Copy contents of loc b’ into b Machine A Machine B

Marshalling Values must cross the network Machine formats differ –Integer byte order Little-endian or big-endian –Floating point format IEEE 754 or not Marshalling  transferring data structure used in remote procedure call from one address space to another.

RPC: The basic mechanism Client routines Client stub RPC runtime Network routines Server routines Server stub RPC runtime Network routines Process kernel Process kernel Client process Server process 1.Client calls a local procedure on the client stub 2.The client stub acts as a proxy and marshalls the call and the args. 3.The client stub send this to the remote system (via TCP/UDP) 4.The server stub unmarshalls the call and args from the client 5.The server stub calls the actual procedure on the server 6.The server stub marshalls the reply and sends it back to the client

Example1: A Time Server Interface struct time { int seconds; int minutes; int hours; int day; int month; int year; char timezone[4]; } int gettime(t); struct time *t; int settime(t); struct time *t;

Example1: Client Stub for Settime int settime(t); struct time *t; { char *p, message[32]; int stat; p = message; p = put_int(p, SETTIME); p = put_int(p, t->seconds); p = put_int(p, t->minutes); p = put_int(p, t->hours); p = put_int(p, t->day); p = put_int(p, t->month); p = put_int(p, t->year); p = put_string(p, t->timezone, 4); stat = do_operation(“time_server”, message, 32); if(stat == SUCCESS) get_int(message, &stat); return(stat); }

Example1: Server Stub (1) void main_loop() { char *p, message[32]; int len, op_code; struct time t; for(;;) { len = receive_request(message, 32); if(len < 4) { /* error handling code */ } p = message; p = get_int(p, op_code); switch(op_code) { case SETTIME: if (len < 32) { /* error handling code */ } p = get_int(p, &t.seconds); p = get_int(p, &t.minutes); p = get_int(p, &t.hours); p = get_int(p, &t.day); p = get_int(p, &t.month); p = get_int(p, &t.year); p = get_string(p, &t.timezone, 4); len = settime(&t); put_int(message, len); len = 4; break; case GETTIME: /* code for unmarshalling and calling gettime */ } send_reply(message, len); }

LPC v.s. RPC Global variables Client and server fail independently –RPC: requires code to deal with server crashes

When Things Go Wrong Semantics of remote procedure calls –Local procedure call: exactly once How many times a remote procedure call may be called? A remote procedure call may be called: –0 time: server crashed or server process died before executing server code –1 time: everything worked well –1 or more: due to excess latency or lost reply from server, client retransmitted Exactly once may be difficult to achieve with RPC

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