1. Remote Procedure Call and Remote Method Invocation
INF 123 – Software architecture

2. Outline
RPC theory
Sun RPC
XML RPC
RMI

3. RPC: theory

4. The Problem 1980s: Internet and web servers
How can a client make a server do something?
Sockets are used to transmit data, not functions
Sending function code as text is a bad idea
Client in Python, server in C#?
Server runs eval(evil_code)? Eval is evil …

5. Dr. Eval
eval(readline()), eval(message), …

6. Reminder: main and subroutines
Main calls subroutines
Subroutines return values
Subroutines have no side-effects
AKA “call and return”
“Not a real architecture” <- really? Why?

7. RPC
Transparently call a procedure located in another process or machine
“make the invocation of a procedure on a remote machine appear as though it were local”

8. Solution: RPC Local Procedure Calls Remote Procedure Calls host host 1
program
program
caller
caller
r = foo(a, b)
r = foo(a, b)
Caller stub
host 2
callee
program
define foo(a, b) …
end
Callee stub
define foo(a, b) …
end
callee

9. RPC with words The local procedure calls a caller stub
The caller stub serializes the call (function name + arguments) and sends it to the callee stub on a remote machine
The callee stub calls the remote procedure
The remote procedure does its job, and gives the return value to the callee stub
The callee stub sends the value back to the caller stub
The caller stub passes that value back to the local procedure

10. Stubs Caller = client (initiate, active end)
Callee = server (listen, passive end)
The stubs are coupled
Client stub must know server stub’s IP and port
Sometimes, a port is reserved for RPC
Stubs are usually generated by tools
Rpcgen for Sun RPC, wscompile for JAX-RPC, …
Other naming: server stub = skeleton

11. (in Interface Def Language)
RPC
Usual code
RPC code
RPC belongs to the application layer
Caller program
(in client language)
Procedure Interface Definition
(in Interface Def Language)
Callee Program
(in server language)
call
generate
call
Client Stub
(in client language)
Server Stub
(in server language)
Serialize and send
Deserialize and receive
Network OS
Network OS

12. RPC Request-reply protocol, and RPC serialization format
RPC as middleware
Middleware: software between OS and application
Application
RPC stubs
Middleware layers
RPC Request-reply protocol, and RPC serialization format OS

13. RPC is not P2P
In both cases, nodes are clients and servers to each other
But in RPC, the goal is to execute a function remotely
A client and a server could use RPC
Peers could use RPC with each other
Peers could simply exchange data, and not use RPC
RPC stubs use a client-server paradigm
With a request-response messaging pattern

14. Sun RPC: DATE/Time server example IN C
From Unix Network Programming, by Stevens et al.
Sun RPC: DATE/Time server example IN C

15. Sun RPC AKA Open Network Computing Created in the mid 80s by Sun
The developer writes:
interface.x, the interface definition, in Interface Definition Language
callee.c, the server with the remote procedure
caller.c, the client calling the remote procedure
The rpcgen compiler generates:
interface.h, to be imported by caller.c and callee.c
callee_stub.c
caller_stub.c

16. XDR eXternal Data Representation Created in the mid 80s by Sun
Binary encoding
The data format exchanged between stubs
Serialization format used by:
Network File System (created by Sun)
SpiderMonkey (Firefox JS engine, XDR used to exchange compiled JS)
R (stats tool)

17. Sun RPC generated Hand-written Import function signature
caller.c
interface.h
callee.c
Generate
Call
interface.x
Call
caller_stub.c
callee_stub.c
Generate
Send XDR
Recv XDR
network

18. caller.c (I adapted it)
server = ‘ ’; client = create_client(server, DATE_PROG, DATE_VERS, "udp"); bin_date = get_binary_date(NULL, client); str_date = bin2str_date(bin_date, client); printf("time on server %s: %s", server, *str_date);

19. caller_stub.c (generated and adapted)
#include "date.h" static struct timeval TIMEOUT = { 25, 0 }; long * get_binary_date(void *argp, CLIENT *clnt) { static long clnt_res; // return value is a long memset((char *)&clnt_res, 0, sizeof (clnt_res)); // allocate memory cli_call = client_call(clnt, GET_BIN_DATE, // RPC function number (xdrproc_t) xdr_void, // argument type (caddr_t) argp, // argument value (xdrproc_t) xdr_long, // return value type (caddr_t) &clnt_res, // store in this memory area TIMEOUT); if (cli_call != RPC_SUCCESS) { return (NULL); } return (&clnt_res);

20. Sun RPC API
client_call
create_client
And more …

21. XML RPC

22. XML RPC Over HTTP POST 1998, many language bindings

23. XML RPC - request POST /RPC2 HTTP/1.1
User-Agent: Frontier/5.1.2 (WinNT)
Host: betty.userland.com
Content-Type: text/xml
Content-length: 181
<?xml version="1.0"?>
<methodCall>
<methodName>examples.getStateName</methodName>
<params>
<param>
<value><int>41</int></value>
</param>
</params>
</methodCall>

24. XML RPC – success response
HTTP/ OK
Connection: close
Content-Length: 158
Content-Type: text/xml
Date: Fri, 17 Jul :55:08 GMT
Server: UserLand Frontier/5.1.2-WinNT
<?xml version="1.0"?>
<methodResponse>
<params>
<param>
<value><string>South Dakota</string></value>
</param>
</params>
</methodResponse>

25. XML RPC – failure response
HTTP/ OK
Connection: close
Content-Length: 426
Content-Type: text/xml Date: Fri, 17 Jul :55:02 GMT
Server: UserLand Frontier/5.1.2-WinNT
<?xml version="1.0"?>
<methodResponse>
<fault> <value>
<struct>
<member>
<name>faultCode</name>
<value><int>4</int></value>
</member>
<name>faultString</name>
<value><string>Too many parameters. </string> </value>
</struct>
</value> </fault>
</methodResponse>

26. XML RPC vs XML: bandwidth 
<struct>
<member>
<name>ROOM_ID</name>
<value> <int>1</int> </value>
</member>
<name>CODE</name>
<value> <string>MR-101</string> </value>
<name>NAME</name>
<value> <string>Math Room</string> </value>
<name>CAPACITY</name>
<value> <int>30</int> </value>
</struct>
<room> <ROOM_ID>1</ROOM_ID> <CODE>MR-101</CODE> <NAME>Math Room</NAME> <CAPACITY>30</CAPACITY> </room>
class Room {
int id=1;
String code="MR-101";
String name="Maths room";
int capacity=30; };

27. Remote method invocation

28. Proxy objects Equivalent of stub, but for client objects
Goal: make RMI transparent to clients
Proxy objects behave like a local object to the invoker
Instead of executing, proxy objects serialize arguments and forward the call + arguments to the remote object
Human writes class interface with method names
RMI compiler generates proxy classes, skeleton classes, and server dispatcher

29. Remote Method Invocation
Client Process
Proxy
Object B
Object A
Proxy object is a hollow container of Method names.
Remote Reference Module translates between local and remote object references.
Remote Reference Module
Comm. Module
Server Process
Comm. Module
Remote Reference Module
Dispatcher sends the request to Skeleton Object
Skeleton deserializes parameters, sends it to the object, & marshals the results for return
Dispatcher
Object B
Skeleton
for B’s Class
MIDDLEWARE

30. More www.cs.bham.ac.uk/~bxb/courses/02_03_DS/Lecture05_Slides.pdf
www2.hh.se/staff/vero/concurrent/lectures/lecture6.pdf
Implementing Remote Procedure Calls, Birrell et al. 1984