CS6223: Distributed Systems Remote Procedure Call (RPC)

2 An Example of Local Procedure Call /* save a string to file msg.dat */ savemsg(char *msg) { FILE *fp; fp = fopen("msg.dat", "w+"); fprintf(fp, "%s\n", msg); fclose (fp); } main() { char *str = "This is an example of LPC."; savemsg(str); }

3 Moving the local procedure to a remote machine main() { char *str = "This is an example of RPC."; char *remote_host = "sus1.cs.cityu.edu.hk"; CLIENT *clnt; clnt = clnt_create(remote_host, MSGPROG, MSGVER, "netpath"); if (clnt == (CLIENT *) NULL) { clnt_pcreateerror(host); exit(1); } savemsg_1(&str, clnt); } ………… savemsg_1(char **argp; struct svc_req *rqstp) { FILE *fp; fp = fopen("msg.dat", "w+"); fprintf(fp, "%s\n", *argp); fclose (fp); } server: client:

4 Remote Procedure Call 1984: Birrell & Nelson –Mechanism to call procedures on other machines –Process on machine A can call a procedure on machine B A is suspended Execution continues on B When B returns, control passed back to A Goal: Make a remote procedure call looking the same as a local call to programmers.

5 RPC implementation The RPC compiler auto-generates stub/skeleton routines to make an RPC to the user as if it is a local call. What stub routines do: marshalling / unmarshalling parameters and return values handling different data formats between different machines detecting and handling failures of client/server processes and the networks service routines server skeleton client stub client program message passing client server

6 Compilation in SUN RPC server skeleton (main) service routines data conversion client program (main) client stub RPC compiler interface definition C compiler server executable C compiler client executable

7 Demo of RPC Interface file msg.x: program MSGPROG { version MSGVER{ int SAVEMSG(string)= 1; string READMSG(int)= 2; } = 2; } = ; Generate stub routines by: rpcgen –a msg.x Compile program by: cc –o object xxx.c

8 Steps in a RPC client program (msg_client.c) service routines client stub (msg_clnt.c) server skeleton network routines 1. Client calls stub (push parameters onto stack)

9 Steps in a RPC client programservice routines client stub (msg_clnt.c) network routines 2. Clnt_stub marshals parameters to message & makes an OS call (client blocked) server skeleton

10 Steps in a RPC client program client stub network routines 3. Network message sent to server server skeleton service routines

11 Steps in a RPC client program client stub network routines 4. Deliver message to server stub & unblock server server skeleton (msg_svr.c) service routines

12 Steps in a RPC client program client stub network routines 5. Svr-stub unmarshals parameters & calls service routine (local call) server skeleton (msg_svr.c) service routines (msg_server.c)

13 Steps in a RPC client program client stub network routines 6. Return to the stub from service routine server skeleton service routines

14 Steps in a RPC client program client stub network routines 7. Svr_stub marshals return-value & requests OS to send out message server skeleton service routines

15 Steps in a RPC client program client stub network routines 8. Transfer message over network server skeleton service routines

16 Steps in a RPC client program client stub network routines 9. Deliver message to client (unblock client) server skeleton service routines

17 Steps in a RPC client program client stub network routines 10. Clnt_stub unmarshals return-value & returns to client program… server skeleton service routines

18 Compilation in SUN RPC server skeleton (main) service routines data conversion client program (main) client stub RPC compiler interface definition C compiler server executable C compiler client executable

19 Writing the programs Programmers need to write two pieces of programs: Client program –Specify server’s location –Parameters and return value of RPC are pointers Service routines –Generally the same as local procedures, except the parameters and parameter types

20 Interface definition (1) Define machine/OS/language-independent data types and data structures Specify interfaces: program # and version # Define remote procedures: procedure #, parameters, return type constant and type definitions /*optional */ program IDENTIFIER { version VERSION_ID { procedure list } = value; … } = value; program MSGPROG { version MSGVER{ int SAVEMSG(string)= 1; string READMSG(int)= 2; } = 3; } = ;

21 Interface definition (2) one program block in an interface definition one or more sets of versions in the program block one or more sets of procedures in each version block one argument in each procedure by default –If more than one parameter is required, use a struct to group them –“–N” option in “rpcgen” allows multiple arguments

22 Incompatible data representation (1) Different byte ordering –Big endian: Most significant byte in low memory (e.g. Sun Sparcs) –Little endian: Most significant byte in high memory (e.g. Pentiums) The problem was solved in the IP protocol suite by forcing everyone to use big endian byte ordering for all 16 and 32 bit fields by htons (host-to-network-short) and htonl functions. main() { unsigned int n; char *a = (char *) &n; n = 0x ; printf("%02x, %02x, %02x, %02x\n", a[0], a[1], a[2], a[3]); } Output on a Sparc: 11, 22, 33, 44 Output on a Pentium: 44, 33, 22, 11

23 Incompatible data representation (2) Different sizes of integers and other types Different floating point representations Different character sets Alignment requirements Need standard data representation and associated encoding /decoding functions to enable communication between heterogeneous systems –e.g. Sun’s RPC uses XDR (eXternal Data Representation)

24 Interface Definition Language (IDL) SUN XDR (eXternal Data Representation) A subset of ASN.1 (ITU), a standard data representation for message exchanges between clients and servers. Data types and structs are translated into C language by rpcgen (see XXX.h, generated by rpcgen). A set of encode/decode routines (see XXX_xdr.c) to convert between XDR and the local representations.

25 Student registration system: regist.x const MAXSUBJ = 64; typedef char id[8]; typedef char code[8]; struct registargs { code subj_code; id stud_id; }; struct status { code subj_code; int regist_stat; }; struct regist_status { int total_regist; struct status subjs_status ; }; program REGISTPROG { version REGISTVER { int REGIST(registargs) = 1; int DEREGIST(registargs) = 2; regist_status VIEW_STATUS(id) = 3; } = 1; } = ;

26 Important data types simple data types: similar to C array fixed-length: type-name identifier[n]; variable-length: type-name identifier ; type-name identifier<>; array of strings string identifier ; string identifier<>; different from array of char in representation (explained later)

27 Important data types constant/enumeration/type defintion/struct: similar to C const MAXSIZE = 512; enum state { BUSY=1, IDLE=2, TRANSIT=3 }; typedef long counter; typedef char code[8]; struct status { code subj_code; int regist_stat; };

28 Data representation/alignment (1) The basic data item size is 4 bytes. All variables or data structures should be aligned in 4-bytes. Variable-length objects, e.g., variable-length arrays, structures, and strings, have a length in front of them.

29 Data representation/alignment (2) simple data types –int, unsigned int, char, bool, float, etc.: 4 bytes –double, etc.: 8 bytes array –sequence of representations of individual elements –variable-length array has a 4-byte length in the front string –an integer of string length, followed by a sequence of chars, one for a byte –a residue of (n mod 4) bytes are stuffed to make the total byte count a multiple of 4, e.g. string “exam-paper_01” is represented as: