RPC flow. 4.3 Remote procedure calls IDL. RPC components. Procedure. Program. sum (j,k) int j,k; {return j+k;} i = sum (3,7); Local procedure call

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1 4.3 Remote procedure calls RPC flow Client process Server process Program i = sum (3,7); Procedure sum (j,k) int j,k; {return j+k; Client stub Program Return Call Unpack Pack result para s Invisible to user Call Procedure Unpack para s Return Pack result Server stub Local procedure call Kernel Kernel Client machine Server machine 19/06/ /06/01 2 RPC components Interface Defined in an Interface Definition Language (IDL) Similar to class definition, except the code for methods are left blank: they are implemented by the server Clients need to know only parameter types Client stub and server stub (also called skeleton) Make the remote call transparent, i.e., hide communication from users. Marshalling (serializing or flattening) and unmarshalling (de-serializing) of procedure ID and parameters Data format conversion between heterogeneous systems (Big/little endian, 2 s/1 s complement, ASCII, EBCDIC, etc.) IDL No existing language has all types in all other languages E.g., C cannot be IDL: char* in C cannot be translated uniquely to pointer to a character, or pointer to an array of characters. New I/O types, in, out, inout, would be useful for IDL. CORBA CDR, Sun XDR; DCOM, DCE, MiG also define their own IDLs. Java can function as IDL for Java programs (see next section) 19/06/ /06/01 4

2 Stub generator (=IDL compiler) Generates stubs by compiling the interface definition rmic (Java) rpcgen (Sun) MiG (Mach) uuigen (OSF DCE). The generated stubs are linked to the client and server programs Generation and use of stubs Interface Definition file IDL compiler Client stub Client prog Server prog Server stub Compile and link Include header.h Include CORBA can generate client and server stubs in different languages. Compile and link 19/06/01 5 Client binary Server binary 19/06/ Remote Method Invocation (RMI) Can pass any object as parameter; remote objects are passed by their reference (unique id) Java RMI uses Java itself as IDL. Interface definition example: import java.rmi.* public interface Find extends Remote { public String findline ( String word) throws RemoteException; 19/06/01 7 Server implementation public class FindServer extends UnicastRemoteObject implements Find { public FindServer ( String afile ) throws RemoteException { This is constructor; afile is a file name public String findline ( String word ) { This method finds if search keyword word is in afile public static void main ( String args[ ]) { try { System.setSecurityManager ( new RMISecurityManager () );.. catch {.. 19/06/01 8

3 Server machine Registry server Java registry Client machine Naming class methods Server machine Client machine Registry server Server applicat. 1. rmiregistry & 5. Return ref (stub) 2. Export service 3. Bind to registry Client application 4. Query by name 6. RMI 1. rmiregistry & 5. Return ref stub 2. Export service 3. Naming.rebind Server applicat. 3a 4a HTTP server Client application 4. Naming.lookup 6. RMI 4b Skeleton Stub Skeleton Stub 19/06/ /06/01 10 Registry process Type command manually: Unix: rmiregistry 2099 & 1099 is default Windows: start rmiregistry 2099 In Server s main method: FindServer server = new FindServer (phonebook); Naming.rebind ( :2099/FindServer, server ); Naming.rebind ( //hostname:2099/findserver, server ); Naming.rebind ( //:2099/FindServer, server ); 19/06/01 11 Registry query Client public class FindClient { String host = hostname:2099 ; String name = rmi:// + host + /FindServer ; Find obj = ( Find ) Naming.Lookup (name); String results = obj.findline ( argus[0] ); 19/06/01 12

4 Big picture Shared object-based middleware Java RMI CORBA File system-based middleware NFS Document-based middleware Web pages (hyperlink, URL, HTTP, HTML, XML) Coordination-based middleware Jini Linda File system-based middleware Transfer Models (a) upload/download model (b) remote access model 19/06/01 13 Document-based middleware CORBA (Common Object Request Broker Architecture) Browser s steps to download a web page 1. Asks DNS for IP address of web server 2. DNS replies with IP address 3. Browser makes connection 4. Sends request for specified page 5. Server sends file 6. TCP connection released 7. Browser displays text 8. Browser fetches, displays images ORB: Its major role is to implement request-reply protocol, maintaining requestids.

5 Object adapter CORBA repositories Carries out the following tasks: Creates unique remote object references for CORBA objects Dispatches RMI to appropriate server program Activates objects client implementation repository interface repository object adapter server skeleton Makes it possible to use an object that was not written for CORBA with CORBA systems Is a wrapper that handles registering such an object, generating object reference, and activating it. client program proxy for A ORB core or dynamic invocation Request Reply Communication modules ORB core or dynamic skeleton Servant A 19/06/ /06/01 18 Repositories Implementation repository Responsible for activating registered serves on demand and locating servers that are currently running. Each of its entries has the form (object adopter name, pathname to the file containing object implementation, hostname and port of server) Interface repository For each IDL interface registered, provides the names of the methods and their argument types. IIOP (Internet InterORB Protocol): Defines standard form for remote object references IOR format IDL interface type name Protocol and address details Object key interface repository identifier Interoperable Object Reference IIOP host domain name port number adapter name object name 19/06/ /06/01 20

6 CORBA services Naming service Event service Notification service Security service Every object is located on only one server doesn t scale well (e.g., object maintaining stock prices) The class concept doesn t exist in CORBA 19/06/01 21