RPC and RMI. 2501ICT Nathan

Transcription

1 RPC and RMI 2501ICT Nathan

2 Contents Client/Server revisited RPC Architecture XDR RMI Principles and Operation Case Studies Copyright René Hexel. 2

3 Client/Server Revisited Server Accepts commands over Network connections Executes code according to commands No direct control over this code Client Communicates with server Sends commands Awaits results Copyright René Hexel. 3

4 Remote Methods/Procedures Extension to the Client/Server concept Client-Programmable Interface Similar to Function/Method calls Method Invocations over Network RPC / RMI Remote Procedure Calls Remote Method Invocations Implementation: Middleware Copyright René Hexel. 4

5 Middleware Programming Model above Basic Networking Layer Applications RPC / RMI Request/Reply Protocol External Data Represent. Operating System Copyright René Hexel. 5

6 Interfaces Modules run as separate Tasks Similar to Process/Thread Model Data exchange via Messages Carry Parameters and Return Values Integration Direct Objective-C Distributed Objects, RMI for C++ Interface Definition Languages (IDLs) Copyright René Hexel. 6

7 Interface Definition Languages Uniform Interface Across Programming Languages CORBA CORBA IDL Java Java RMI Sun XDR IDL for RPC Copyright René Hexel. 7

8 RPC Originally designed for NFS Networking File System Extension Function Call Interface Clients can call Procedures across Network Connections Service Interface Provides Function and Data Prototypes Request/Response Protocol Copyright René Hexel. 8

9 RPC Architecture client process server process Request client program client stub procedure Communication module Reply server stub procedure Communication module dispatcher service procedure Copyright René Hexel. 9

10 Stub Procedures Proxy between local and remote procedures Server Skeletons that receive calls from dispatcher and pass them on to the Server Application Client Stubs that can be called using local (statically compiled) code Copyright René Hexel. 10

11 Sun XDR Example const MAXLEN = 80; typedef Person { char name[maxlen]; char address[maxlen]; int year_of_birth; }; program PERSONLIST { version VERSION { void ADDPERSON(Person)=1; int GETYEAROFBIRTH(Person)=2; }=2; }=9999; Copyright René Hexel. 11

12 XDR Properties No Symbolic Interface Names Program, Version, and Procedure Numbers Only Single Input Parameter Compound Structure to encapsulate multiple parameters! Binding via Portmapper Globally register RPCs Unique signatures required! Copyright René Hexel. 12

13 Distributed Objects Extension of the local OO Model Design Issues Suitable Semantics for Remote Invocations? How can RMI semantics be made similar to local method call semantics? Let s Compare Local Object Model vs. Distributed Object Model Copyright René Hexel. 13

14 Local Object Access Access via Object References Target: Object whose Method is invoked Interfaces Signatures of a Set of Methods Does not specify actual Implementation Actions and Exceptions Normal and abnormal Method terminations Garbage Collection Freeing unused Space Copyright René Hexel. 14

15 Distributed Object Access Remote Object References Provide Access to Remote Objects Remote Interfaces Set of Methods to be invoked Actions and Exceptions Synchronous or Asynchronous termination Copyright René Hexel. 15

16 RMI Architecture Objects distributed over Network Location is transparent Client/Server applications represented by objects Peer-to-Peer architecture Roles of Client or Server only specific for a particular method call Copyright René Hexel. 16

17 Remote Method Illustration A remote invocation B local C invocation local E invocation local remote invocation F invocation D Copyright René Hexel. 17

18 Remote Object Interface remote object Data remote interface { m1 Impl. m2 m3 of methods m4 m5 m6 Copyright René Hexel. 18

19 Distributed Objects API Language Specific Framework for Distributed Objects Applications must Locate remote Objects Communicate with remote Objects Invoke remote methods Handle remote exceptions Copyright René Hexel. 19

20 Distributed Objects API Messages Transport Parameters and Results Interface Specifies Services provided by an Object Describes Messages an Object can Respond to Instance Variables Only accessed and changed through invoking Methods Requirement for Access Methods! Copyright René Hexel. 20

21 Object Proxy Local Proxy for Remote Objects Local object that represents an instance of a remote object Transparent integration into Programs Methods are invoked on the proxy pretending the server object is a local object NSDistantObject class Copyright René Hexel. 21

22 RMI Process RMI In a Nutshell Server makes available its remote objects Client invokes methods on a local proxy Request is marshalled and sent onto server Request is unmarshalled on server Remote object executes method for the request and returns result to client Transparent to Program Copyright René Hexel. 22

23 RMI Schematic object A client proxy for B Request server skeleton & dispatcher for B s class remote object B Reply Remote Communication Communication Remote reference reference module module module module Copyright René Hexel. 23

24 RMI Transport Layer Uses TCP (UDP also possible) Transport Layer Functions Locate RMI server Establish socket connection to server Pass connection back to client Remote Reference Layer Add remote object to a table of reachable objects Monitor connection status Copyright René Hexel. 24

25 Parameter Marshalling Encoding Parameters Required for Network Transport Remote Interface Parameter Call By Reference: byref qualifier Call By Copy: bycopy qualifier Efficiency in: parameter is only sent to the server out: parameter is returned from the server inout: parameter is sent in both directions Copyright René Hexel. 25

26 Registration/Naming Service Allows Finding a Server by Name Intermediary between Object Client and Server NSConnection class Server Side Registration [NSConnection registername: myname]; Client Side Access [NSConnection rootproxyforconnectionwithregisteredname: myname host: * ]; Copyright René Hexel. 26

27 Distributed Object Server NSConnection *c = [NSConnection defaultconnection]; [c setrootobject: myobject]; if ([c Name] == NO) { /* Handle the error */ } Copyright René Hexel. 27

28 Distributed Object Client id proxy = [NSConnection Name * ]; if (proxy == nil) { /* Handle the error */ } [proxy somemethod]; // invokes method on server Copyright René Hexel. 28

29 Asynchronous Calls Client won t wait for Server for methods defined as oneway void More flexible Server operates independently of client Useful to support collaborative functionality e.g. chat programs, BBS s, etc. What if return value is needed? Explicit synchronisation required Callbacks Copyright René Hexel. 29

30 Callbacks Allow Server to call Client method Client acts as an RMI server Implicitly: registering an object name Explicitly: object references (byref parameters) Peer-to-Peer Relationship No dedicated clients and servers Each peer is both a client and a server Copyright René Hexel. 30

31 CORBA Common Object Request Broker Architecture Machine and Language-Independent Distributed OO Architecture Supports RMI CORBA IDL Compiled for different programming languages Copyright René Hexel. 31

32 CORBA IDL Example struct Person { string name; string address; long year_of_birth; }; interface PersonList { readonly attribute string listname; void addperson(in Person p); void getperson(in string name, out Person p); long getyearofbirth(); } Copyright René Hexel. 32

33 The CORBA Architecture ORBs Object Request Brokers Help a Client to invoke a method on an object GIOP Locate/activate object, communicate client s request General Inter-Orb Protocol Standardised communication Protocol IIOP IP Implementation of GIOP Copyright René Hexel. 33

34 CORBA RMI Multi-Language RMI System More Versatile Object Model Client s don t need to be objects CORBA Naming Service Allows services to register (bind) objects Pseudo-Objects init, connect, conversion objects Copyright René Hexel. 34

35 CORBA Components client implementation repository interface repository object adapter server skeleton client program proxy for A ORB core Request Reply ORB core Servant A or dynamic invocation or dynamic skeleton Copyright René Hexel. 35

36 CORBA Language Support CORBA is cross-platform No native implementation Language bindings through toolkits Toolkit examples AdORB: CORBA API for Objective-C ORBit: CORBA API for C ORBit C++: CORBA API for C++ Copyright René Hexel. 36

37 References Objective-C Distributed Objects ogrammingmanual/manual_7.html istrobjects/index.html ORBit Tutorial: ORBit C++: C++ RMI Copyright René Hexel. 37

38 Appendix Java RMI

39 Java RMI Java-Specific Framework for Distributed Objects Applications must Locate remote Objects Communicate with remote Objects Provide Mechanisms for loading/passing Bytecode of an Object Copyright René Hexel. 39

40 Generating Proxies Involves rmic Java RMI Compiler Procedure for generating proxies Interface and implementation Compiled normally, using javac Generate stubs Use rmic on implementation class file, e.g. rmic myremoteclassimpl will generate myremoteclassimpl_stub.class myremoteclassimp_skel.class Copyright René Hexel. 40

41 Java RMI Programming The RMI Development Steps are: Identify remote objects and their interfaces Provide implementation for distributed objects Compile sources Use rmic to create stubs Start a registry Web server with access to the server classes Start the applications Copyright René Hexel. 41

42 Java Remote Interfaces Remote Objects Any non-private object accepting method calls Needs interface defining their exported behaviour Defining Remote Interfaces Extends java.rmi.remote Each method throws java.rmi.remoteexception Copyright René Hexel. 42

43 Java Remote Interface Example import java.rmi.remote; import java.rmi.remoteexception; public interface MyRemote extends Remote { public Object getxxxx() throws RemoteException; } Copyright René Hexel. 43

44 Java RMI Clients Are just like any other Java Programs! Can have access to a running Server Using Naming e.g. testserver = (xmplserver) Naming.lookup( rmi://somehost/xmplserver ); Copyright René Hexel. 44

45 Java RMI and Threading Server Processes Threads are generated and controlled automatically Multiple Threads for multiple connections Client Processes Threads are created upon service requests Synchronisation Not handled automatically! Copyright René Hexel. 45

46 Synchronising RMI Threads Critical Regions are Global Server Methods Global Server Data Need to be protected To avoid inconsistencies To prevent race conditions Use Synchronisations constructs Semaphores, synchronized, Copyright René Hexel. 46

47 Java RMI Programming The RMI Development Steps are: Identify remote objects and their interfaces Provide implementation for distributed objects Compile sources Use rmic to create stubs Start a registry Web server with access to the server classes Start the applications Copyright René Hexel. 47

48 Java Remote Interfaces Remote Objects Any non-private object accepting method calls Needs interface defining their exported behaviour Defining Remote Interfaces Extends java.rmi.remote Each method throws java.rmi.remoteexception Copyright René Hexel. 48

49 Implementing Java Remote Interfaces Classes need to Declare the remote interface they implement Provide a constructor Provide implementations for all remote methods in the interface Remote Object Implementations Usually export themselves to RMI Optionally register with the naming service Copyright René Hexel. 49

50 Remote Object Activation Problems Addressed by ROA: What happens if remote object crashes? Need to be restarted from scratch! All instance data lost Remote objects are resource intensive! But usually only needed on occasion Only activate as needed Lazy Activation Addresses these efficiency issues Copyright René Hexel. 50

51 Java Lazy Activation New API in Java 2 Create remote objects that instantiate only when needed (conserve resources) No need for manual start: Extend java.rmi.activation.activatable: public myclass(activationid id, MarshalledObject data) throws RemoteException Register actionable object w/ activation daemon Start the RMI activation daemon rmid Copyright René Hexel. 51

52 Java Activatable Objects Requirements An activation ID class variable ActivationID id; A Serializable data variable of any type, e.g. Date mymarshalledobject; rmid can re-instantiate an object by its ID Client cannot call default constructor Has no idea about activation ID or marshalled object Create another constructor for client Copyright René Hexel. 52

53 Java Object Activation Server Codebase URL: String codebase = ; Create activation group descriptor for server: ActivationGroupDesc lag = new ActivationGroupDescriptor(null, null); Register activation group: ActivationGroupID lagid = ActivationGroup.getSystem().registerGroup(lAg); Copyright René Hexel. 53

54 Java Object Activation Server (2) Create actual activation group: ActivationGroup.createGroup(lAgID, lag, 0); Create MarshalledObject used to initialise the server object: Date linitobject = new Date(); MarshalledObject lmo = new MarshalledObject(lInitObject); Copyright René Hexel. 54

55 Java Object Activation Server (3) Construct Activation Descriptor ActivationDesc lad = new ActivationDesc(lAgID, ExampleDateServ, codebase, lmo); Register with activation daemon: ExampleDateServ ldateserver = (ExampleDateServ)Activatable.register(lAD); Bind instance to a name: Naming.rebind( DateServer, ldateserver); Copyright René Hexel. 55

56 Java Object Activation Setup Compile javac *.java Generate Stubs and Skeletons rmic ExampleDateServ Start RMI registry and Daemon rmiregistry rmid Copyright René Hexel. 56

57 Running Java Server and Client Start Server Application java -Djava.security.policy=all.policy ExampleDateServ Start Client Application java -Djava.security.policy=all.policy ExampleDateClient Start Client Applet appletviewer Test.html Copyright René Hexel. 57

58 CORBA in Java RMI-IIOP Access Java RMI interfaces via CORBA Use -iiop option on rmic Java to IDL Use -idl option on rmic IDL to Java idlj compiler Copyright René Hexel. 58