CS696: Doc 16, Some RMI System Architecture

	CS 696 Emerging Technologies: Distributed Objects

Spring Semester, 1998
Some RMI System Architecture
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© 1998, All Rights Reserved, SDSU & Roger Whitney
San Diego State University -- This page last updated 21-Apr-98

Contents of Doc 16, Some RMI System Architecture

References

References

RMI Specification

JDK 1.2 beta Documentation

Doc 16, Some RMI System Architecture Slide # 2

Architectural Overview

Doc 16, Some RMI System Architecture Slide # 3

Stub Forwards invocation requests to server objects

Initiating a call to the remote object

Marshaling arguments to a marshal stream

Informing the remote reference layer that the call should be invoke

Unmarshaling the return value or exception from a marshal stream

Informing the remote reference layer that the call is complete
Skeleton Server-side entity that contains a method which dispatches calls to the actual remote object implementation

Unmarshaling arguments from the marshal stream

Making the up-call to the actual remote object implementation

Marshaling the return value of the call or an exception onto the marshal stream

Doc 16, Some RMI System Architecture Slide # 4

Remote Reference Layer

Carries out the semantics of the invocation

For example the remote reference layer handles such items as:

Whether the server is a single object or is a replicated object

Whether the server is always running or is only run when called on

Doc 16, Some RMI System Architecture Slide # 5

Transport Layer

Setting up connections to remote address spaces

Managing connections

Monitoring connection liveness

Listening for incoming calls

Maintaining a table of remote objects that reside in the address space

Setting up a connection for an incoming call

Located the dispatcher for the target of the remote call and passing the connection to this dispatcher

Doc 16, Some RMI System Architecture Slide # 6

The transport layer uses the four basic abstractions

endpoint

Abstraction used to denote an address space or java virtual machine

channel

Abstraction for a conduit between two address spaces

Manages connections between the local address space and the remote address space

connection

Abstraction for transferring data

Allows bi-directional multiplexing of multiple requests over one physical connection for applets

transport

Manages channels

Accepts incoming connections

Doc 16, Some RMI System Architecture Slide # 7

Threads in Remotes

Let R be a remote object

Let C1 and C2 be two clients that call methods on R at the "same" time,

They may or may not be calling the same method

If C1 and C2 are in the same virtual machines then the some calls to will be handled in different threads on R and some calls will be handled in the same thread

If C1 and C2 are in different virtual machines then the calls to will be handled in different threads

This means on object R will be accessed from different threads at the "same" time

The last case is the only guarantee RMI makes about mapping remote object invocations to threads

Doc 16, Some RMI System Architecture Slide # 8

Bootstrapping the Client

It is possible to download all the classes and interfaces needed by a client application when you "run" the client

This requires the use of an http server

This is a very trick process the first time you do it

Much of the traffic on the RMI mailing list centers on getting this correct

The use of www-rohan make the process even harder

The bootstrap program can not directly reference the name for any client classes, otherwise the classes will be loaded from the local machine

Doc 16, Some RMI System Architecture Slide # 9

RMI Though Firewalls

If an RMI request from a client hits a fire wall it will try to route through http to a http server on port 80

I have not done this

See the RMI Specification for more details

Doc 16, Some RMI System Architecture Slide # 10

RMI and JDK 1.2

Remote Method Invocation (RMI) has several new enhancements.

Remote Object Activation

Persistent references to remote objects
Automatic object activation via these references

Custom Socket Types

A remote object can specify the custom socket type that RMI will use for remote calls to that object.

RMI over a secure transport (such as SSL) can be supported using custom socket types.

Minor API Enhancements

Unexporting a remote object

Obtaining the stub for an object implementation

Obtaining a local object implementation from a stub

Exporting an object on a specific port.

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