CPS110:
More Networks
Landon Cox
March 27, 2008
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
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
RPC stub functions
 Key to making RPC work
 Stub functions
RPC stub functions
call
return
return
call
Client
stub
Server
stub
send
recv
send
recv
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
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.
Java RMI example
 Remote calculator program
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
Example RPC with pointers
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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
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
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
Structuring a concurrent system
 Talked about two ways to build a system
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
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?