1	Remote Procedure Calls
2	RPC Introduced by Birrell & Nelson (1984) Remote Procedure Calls allow a program to make use of procedures executing on a remote machine If it doesn’t sound OO, it’s because it isn’t OO! RPCs are based on sockets, and therefore dispense us from using them directly Remote Procedures could in principle be written in a different language than the clients.
3	RPC Concepts Registration and identification of a procedure The procedure registers its name to the RPC server The client invokes the procedure by specifying the machine name, the procedure name, and by passing the parameters Encoding and decoding of procedure parameters Given the difference in address space, and possibly in internal type encoding schemes and programming language, there is a need to use an external data representation The client (and the server) will have to translate procedure parameters (encoding) and the return value (decoding) to and from that representation Alternatively: have one side translate into the other side’s representation, no common external representation (bad idea, why?)
4	RPC Concepts (cont.) Dealing with errors Client and server can fail independently (not the case with “regular” procedure calls). How often should remote procedure be invoked? Ideally: exactly once (hard to guarantee) At-most once At-least once (idempotent operations) Ordering relationships Does order of two RPCs to same server matter? If so, what should be that order (partial order semantics, total order semantics): Isis toolkit
5	Basic Communication Pattern Principle of RPC between a client and server program.
6	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
7	Passing Value Parameters (1) Steps involved in doing remote computation through RPC
8	Passing Value Parameters (2) Original message on the Pentium The message after receipt on the SPARC The message after being inverted. The little numbers in boxes indicate the address of each byte
9	RPC “old style” To use RPC in C under Unix… Each procedure belongs to a program The program is designated by a number and version The procedure itself has a number as well The rpcinfo command returns the list of procedures that are registered to the RPC server
10	RPC “old style” Example: (for the “portmapper” program) Program ver proto port 100000 2 tcp 111 portmapper 100000 2 udp 111 portmapper Obviously, each portmapper procedure has its own procedure number in addition.
11	RPC “old style” The intermediary representation “was” called eXternal Data Representation (XDR) XDR supports primitive data types and means to define more complex structures… Advantage: new computers (with new internal representations) need to convert to/from XDR only Disadvantage: requires typically two translations: from sender’s representation to XDR and then to receiver’s representation Alternative: Sender directly converts into receiver’s format (or vice versa). Disadvantages?
12	RPC “old style” Server-side: Register the procedure: registerrpc(858993459, 1, 1, my_proc, xdr_pair, xdr_int) Start listening to remote calls: svc_run() Client-side: Call the procedure: test = callrpc(“Eureka”, 858993459, 1, 1, xdr_pair, &mypair, xdr_int, &myreturn); Use the result! if (test == 0) printf(“return value: %d\n“, myreturn);
13	RPC “less old style” there has been since a way to hide XDR from the developer to some extent use of “stubs” and “skeletons” to do the encoding and decoding we’ll see more of what those are when we cover RMI… for the time being, check simple example on course website
14	Writing a Client and a Server The steps in writing a client and a server in DCE RPC.
15	Binding a Client to a Server Client-to-server binding in DCE.
16	RPC “XML style”: XML-RPC first released in 1998 uses HTTP for transport (or SOAP, CORBA, …) XML replaces XDR as intermediate data representation Resources: http://www.xmlrpc.com http://xmlrpc-c.sourceforge.net/
17	XML-RPC data types XML-RPC supports these data types: base64 (for binary data) boolean date.Time.iso8601 double int string struct (comparable to Hashtable) array
18	XML-RPC under the hood an XML-RPC client makes a remote procedure call using an HTTP POST request the following is a POST header example: POST /XMLRPC HTTP/1.0 Host: www.advogato.org Content-Type: text/xml Content-Length: 151
19	XML-RPC under the hood after the header, the actual RPC call is encoded using XML the method is composed of the program name followed by the method (note: no brackets for methods) <?xml version=“1.0”?> <methodCall> <methodName>test.square</methodName> <params> <param> <value><int>14</int></value> </param> </params> </methodCall>
20	XML-RPC under the hood the XML-RPC response from the server is a standard HTTP response, with : HTTP/1.0 200 OK … (some header info omitted) Content-Length: 157 Content-Type: text/xml <?xml version=“1.0”?> <methodResponse> <params> <param> <value><int>196</int></value> </param> </params> </methodResponse>
21	Java support for XML-RPC without using any extra library: you can always use the java.net.* facilities such as the URL and HttpUrlConnection classes to create XML-RPC requests and retrieve responses you need to translate parameters into XML-RPC and parse the resulting XML or you can use for example the Apache XML-RPC class libraries: java package org.apache.xmlrpc http://xml.apache.org/xmlrpc
22	Apache XML-RPC Library client-side: instantiate a client and specify the server address: XmlRpcClient client = new XmlRpcClient(“http://www.advogato.org”); store the remote procedure parameters in a Vector Vector params = new Vector(); params.addElement(new Integer(14)); execute the call and get the return value in an Object Integer result = (Integer) client.execute(“test.square”, params);
23	Apache XML-RPC Library client-side: parameters must be of a type compatible to XML-RPC: Boolean, byte[], Date, Double, Integer, String, Hashtable, Vector if there are no parameters, must still create an empty vector the networking and all the XML encoding and decoding is handled by the XML-RPC library
24	Apache XML-RPC Library server-side: use the simple built-in Web Server that only responds to XML-RPC requests: WebServer server = new WebServer(4444); create the remote object, which methods will be called remotely MyMathClass myMath = new MyMathClass(); - register the object with the server and give it a handler name: server.addHandler(“test”, myMath);
25	Asynchronous RPC (1) The interconnection between client and server in a traditional RPC The interaction using asynchronous RPC
26	Asynchronous RPC (2) A client and server interacting through two asynchronous RPCs
27	Sun-RPC vs. XML-RPC Sun-RPC Additional code gets generated (rpcgen) More efficient Binary messages: more compact, easier to generate Uses sockets directly Invocation of remote procedures using their names and parameters XML-RPC No code generators Message in human-readable form More complex protocol stack: XML over HTTP/SOAP over TCP Remote invocation somewhat generic
28	Final Thoughts on RPC makes networking easier: hides the transport layer no application-specific protocol to write makes networking slower: because of all-purpose encoding/decoding must either conform to very simple parameter types, or must do some work enforces/limits application programmer to certain communication patterns