Understanding Remote Procedure Calls: Fundamentals and Challenges in Network Abstractions

1. CPS110: More Networks Landon Cox March 27, 2008

2. Network abstractions • We’ve been using send/receive • Client sends a request to the server • Server receives request • Server sends response to client • What else in CS is this interaction like? • Calling a function

3. Remote procedure call (RPC) • RPC makes request/response look local • Provide a function call abstraction • RPC isn’t a really a function call • In a normal call, the PC jumps to the function • Function then jumps back to caller • This is similar to request/response though • Stream of control goes from client to server • And then returns back to the client

4. RPC stub functions • Key to making RPC work • Stub functions

5. RPC stub functions call send Client stub recv return send return Server stub call recv

6. RPC stub functions • Client stub 1) Builds request message with server function name and parameters 2) Sends request message to server stub • (transfer control to server stub) 8) Receives response message from server stub 9) Returns response value to client • Server stub 3) Receives request message 4) Calls the right server function with the specified parameters 5) Waits for the server function to return 6) Builds a response message with the return value 7) Sends response message to client stub

7. RPC example • Client calls produce(5) Client stub: Server stub: // must be named produce! int produce (int n) { int status; send (sock, &n, sizeof(n)); recv (sock, &status, sizeof(status)); return status; } // could be named anything void produce_stub () { int n; int status; recv (sock, &n, sizeof(n)); // produce func on server! status = produce (n); send (sock, &status, sizeof(status)); } Server stub code can be generated automatically (C/C++: rpcgen, Java: rmic) What info do you need to generate the stubs? Input parameter types and the return value type.

8. Java RMI example • Remote calculator program

9. Problems with RPC • How is RPC different from local function calls? • Hard to pass pointers (and global variables) • What happens if server dereferences a passed-in pointer? • Pointer will access server’s memory (not client’s) • How do we solve this? • Send all data reachable from pointer to the server • Change the pointers on the server to point to the copy • Copy data back when server function returns

10. Example RPC with pointers • On client: int a[100]; • Want to send “a” (a pointer to an array) • Copy entire array to server • Have server’s pointer point to copy of a • Copy array back to client on return • What if a is more complicated? A linked list? • Have to marshal the transitive closure of pointer

11. Problems with RPC • Data may be represented differently • Machines have different “endianness” • Byte 0 may be least or most significant • Must agree on standard network representation • RPC has different failure modes • Server or client can fail during a call • In local case, client and server fail simultaneously

12. Finishing RPC • Where have you used RPC in CPS 110? • Project 2 infrastructure • C library interface to system calls is similar • Makes something look like a function call

13. Structuring a concurrent system • Talked about two ways to build a system

14. Alternative structure • Can also give cooperating threads an address space • Each thread is basically a separate process • Use messages instead of shared data to communicate • Why would you want to do this? • Protection • Each module runs in its own address space • Reasoning behind micro-kernels • Each service runs as a separate process • Mach from CMU (influenced parts Mac OS X) • Vista’s handling of device drivers

15. Course administration • Project 2 scores (out of 89) • 88, 86, 86, 86, 86, 84, 84, 84, 84, 71, 66, 50 • Great! • Project 3 out next week (I hope) • Questions?