CMSC621: Advanced Operating Systems Advanced Operating Systems Nilanjan Banerjee Associate Professor, University of Maryland Baltimore County nilanb@umbc.edu http://www.csee.umbc.edu/~nilanb/teaching/621/ Slide Credit: Dr. Ying Lu Advanced Operating Systems

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

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 Original paper on RPC: A. Birrell, B Nelson, “Implementing Remote Procedure Calls”, ACM Symposium on Operating System Principles, 1984

Conventional Procedure Call 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 http://linux.die.net/man/2/read 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?

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

Steps of a Remote Procedure Call Client procedure calls client stub in normal way Client stub builds message, calls local OS Client's OS sends message to remote OS Remote OS gives message to server stub Server stub unpacks parameters, calls server Server does work, returns result to the stub Server stub packs it in message, calls local OS Server's OS sends message to client's OS Client's OS gives message to client stub Stub unpacks result, returns to client

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

Passing Value Parameters (2) The usual contrast is between most versus least significant byte first, called big-endian and little-endian respectively.

Passing Value Parameters (3) Original message on the Pentium (little-endian) The message after receipt on the SPARC (big-endian) Note: the little numbers in boxes indicate the address of each byte

Passing Value Parameters (3) Original message on the Pentium (little-endian) The message after receipt on the SPARC (big-endian) The message after being inverted (integer 5, string: “LLIJ”) Note: the little numbers in boxes indicate the address of each byte

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 match 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. Define a “network format”, for example following XDR (eXternal Data Representation) standard http://www.ietf.org/rfc/rfc1832.txt

RPC: The basic mechanism Client calls a local procedure on the client stub The client stub acts as a proxy and marshalls the call and the args. The client stub send this to the remote system (via TCP/UDP) The server stub unmarshalls the call and args from the client The server stub calls the actual procedure on the server The server stub marshalls the reply and sends it back to the client Client process Server process Client routines Server routines 1 5 Client stub Server stub 2 4 RPC runtime RPC runtime Process Process kernel kernel 3 6 Network routines Network routines Source: R. Stevens, Unix Network Programming (IPC) Vol 2, 1998

Binding a Client to a Server (1) 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: Locate the server’s machine. Locate the server on that machine.

Binding a Client to a Server (2) Figure 4-13. Client-to-server binding in DCE.

Asynchronous RPC (1) 2-12 The interconnection between client and server in a traditional RPC The interaction using asynchronous RPC

A client and server interacting through two asynchronous RPCs 2-13 One-Way RPC: The client invokes a remote procedure but it does not block or wait until it receives a return.

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

Semantics of remote procedure calls 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

Most RPC systems will offer either: RPC Semantics Most RPC systems will offer either: at least once semantics or at most once semantics Understand application: Illustrate some applications that “at least once” is suitable? Idempotent functions: may be run any number of times without harm Illustrate some applications that “at most once” is suitable? At-most-once call semantics are for those RPC applications which require a guarantee that multiple invocations of the same RPC call by a client will not be processed on the server. An example of such an application is inventory control. Consider several point-of-sale (POS) workstations and a server which maintains inventory records. Each POS workstation makes an RPC call to the server when an item is sold. The call causes a count of the number of items left in the inventory to be decremented on the server. If a call indicating that five of the item X was sold is processed on the server more than once, then the inventory record for item X will be in error.