Remote Method Invocation RMI

1
Lesson 3

• Remote Method Invocation (RMI)
• Mixing RMI and sockets
• Rethinking out tic-tac-toe game

2
Distributed Objects

• Simple idea objects existing on one machine
  (server) may be accessed from another machine
  through regular method call.
• Eliminates need to marshal and unmarshal data
  sent over sockets
• Underlying socket code still exists, but is not
  programmed by user.

3
RMI vs. CORBA

• RMI is Java framework for creating distributed
  object applications Remote Method Invocation
• CORBA is alternative technology based on open
  standard Common Object Request Broker
  Architecture
• RMI is only for pure Java applications CORBA is
  language independent
• JNI makes this distinction a little less rigid
  since it allows Java to interact with other
  languages

4
Socket flow of events -- synchronous
Client
Server

• Get user input
• Decode user input
• Create server message
• Send message to server
• Await server response
• Receive server message
• Decode reply
• Send output to user

• Await client message
• Receive client message
• Decode client message
• Perform action
• Create client message
• Send to client

Method call on standalone object
5
Socket flow - asynchronous
Client1
Server
Client2

• Wait
• Receive client msg
• Decode client msg
• Perform action
• Create client message
• Send to client1
• Send to client2

• User input (UI)
• Decode UI
• Create srvr msg
• Send srvr msg
• Await srvr reply
• Receive server message
• Decode reply
• Output to user

• User input (UI)
• Decode UI
• Create srvr msg
• Send srvr msg
• Await srvr reply
• Receive server message
• Decode reply
• Output to user

6
RMI flow of events -- synchronous
Client
Server

• Instantiate object(s)
• Bind to registry

• Get/cache remote obj ref
• Get user input
• Decode user input
• Remote method call
• Decode return value
• Send output to user

7
RMI Cartoon1
8
RMI Cartoon2
9
Steps for RMI Application

• Implement both client and server on single
  machine to test
• Create two directories
• client
• server

10
RMI steps, server side

• Three files need to be created
• The implementation class (Foo.java)
• An interface listing the methods in the
  implementation class which you want to make
  remote (FooInterface.java)
• A server class, which creates one or more
  implementation objects and posts them to the
  registry (FooServer.java)

11
Creating the interface

• Interface file (e.g. StoreInterface.java)
• StoreInterface must extend java.rmi.Remote
• All methods in interface must throw
  java.rmi.RemoteException
• Implementation file (e.g Store.java)
• Store must extend java.rmi.server.UnicastRemoteObj
  ect
• Store must implement StoreInterface
• Program implementations. Be sure to throw
  RemoteException in remote methods
• Explicitly include a null construct that calls
  super()

12
Creating Stubs

• The stub or proxy code is generated automatically
  by using the rmic utility.
• Once the interface and implementation classes are
  complete, generate the stubs as follows
• rmic v1.2 Store
• This creates Store_Stub.java the java
  networking layer that transparently handles the
  underlying message passing.

13
Creating the server class

• Server class (e.g. StoreServer.java)
• Create a new object instance
• Call java.rmi.Naming.bind() to store the
  register the object with the naming service
• contains String name associated with bound
  object

14
Steps for RMI, cont.

• Create the client
• Change to the client dir and copy Store.class and
  Store_Stub.class (or you can import them but
  remember that these will ultimately be on
  different machines).
• Create StoreClient.java and import
• java.rmi.Naming java.rmi.RemoteException
  java.net.MalformedURLException
  java.rmi.NotBoundException

15
Steps for rmi, cont.

• Call Naming.lookup() to get remote object
  reference (be sure to cast to interface type).
• Be sure to handle imported exceptions
• Once you get remote object reference, handle as
  regular object (there are some subtle differences
  that well explore later).

16
Deploying the Application

• Start the rmiregistry
• rmiregistry (Unix)
• start rmiregistry (Windows)
• Start the StoreServer class
• java StoreServer (Unix)
• Run the client
• Thats it!

17
Additional Issues covered next time

• Objects which are not remote are copied (slow!)
• Stub and interface codes can be downloaded by
  client (typical for real distributed systems)
• Security issues in real system (see ch. 5 in Core
  Java 2)
• Subtleties with Object methods (clone, etc)
• Using callbacks with RMI
• Synchronization
• Registering multiple objects
• Bottom Line Dont be too fancy!

18
Examples

• tic-tac-toe reorganized as standalone back-end
  object
• single-threaded test of TTT object
• multithreaded test of TTT object using polling
• client-server TTT using RMI and polling
• client-server TTT using RMI over sockets
• client-server TTT using RMI callbacks.