Distributed Real-Time in the RTSJ

Andrew Borg

Presentation

RMI – A brief introduction

The RTSJ – A (very) brief introduction

The DRTSJ – The 3 Levels of Integration

RMI+RTSJ: Concerns, Issues and Solutions

Presentation

RMI – A brief introduction

The RTSJ – A brief introduction

The DRTSJ – Current Status

RMI+RTSJ: Concerns, Issues and Solutions

The RMI Specification

RMI is very minimal. Assumes a homogeneous environment of

JVMs.

There is no ORB concept (see later)

Goals Support seamless remote invocation on objects. Retain most of the Java language's object

semantics. Make differences between the distributed object

model and local Java object model apparent. Make writing reliable distributed applications as

simple as possible. Preserve the type-safety provided by the Java

runtime environment. Various reference semantics for remote objects; for

example live (nonpersistent) references, persistent references, and lazy activation.

Enforce the safe Java environment provided by security managers and class loaders.

A simple HelloPerson example

Client sends name to a server.

Server returns “hello ” +the name passed

The interface

public interface HelloPersonInterface extends java.rmi.Remote

{

public void sayHello() throws java.rmi.RemoteException;

}

The Implementation

public class HelloPerson implements HelloPersonInterface { public HelloPerson () {} public String sayHello(String S) { return new String("Hello World"+S); }}

Generate Stub Using rmic

… See later

Launching the Server

{Registry r= createRegistry(4321);HelloPersonImpl x = new HelloPersonImpl();Remote s =

UnicastRemoteObject.exportObject(x);r.bind("MyHelloServer",s);}

Invoking the Server

HelloServerInterface H = (HelloServerInterface) r.lookup("MyHelloServer");

System.out.println(H.sayHello("Andrew"));

The RMI ClassesThe RMI Classes

Server

Interface

ServerStub

The Stubpublic final class HelloServerImpl_Stub extends java.rmi.server.RemoteStub implements HelloServerInterface{ public java.lang.String sayHello(java.lang.String

$param_String_1)throws java.rmi.RemoteException

{ Object $result = ref.invoke(this, $method_sayHello_0, new java.lang.Object[] {$param_String_1}, 8370655165776887524L); return ((java.lang.String) $result);

}

Distributed vs. Centralised

Clients of remote objects interact with remote interfaces

Non-remote arguments to, and results from, an RMI call are passed by copy.

A remote object is passed by reference, not by copying the actual remote implementation.

Since the failure modes of invoking remote objects are inherently more complicated than the failure modes of invoking local objects, clients must deal with additional exceptions.

The DGC

Is part of the specification Uses a lease-mechanism with reference

counting Internally also keeps the VM alive when

remote objects exist

JRMP

Serialization Protocol HTTP Protocol

Output defines sub-protocol: single-opstreammultiplex

Presentation

RMI – A brief introduction

The RTSJ – A brief introduction

The DRTSJ – The 3 Levels of Integration

RMI+RTSJ: Concerns, Issues and Solutions

The Real Time Specification for Java (RTSJ)

The 7 enhanced areas over Java:Thread Scheduling and DispatchingMemory ManagementSynchronization and Resource SharingAsynchronous Event HandlingAsynchronous Transfer of ControlAsynchronous Thread TerminationPhysical Memory Access

JSR 50

… extends RMI in the RTSJ to provide support for predictability of end-to-end timeliness of trans-node activities.

Expert group has published a framework for the implementation of JSR 50, describing three levels of integration.

The 3 levels of Integration

Level 0: RTSJ + RMI

Level 1: RTSJ + Realtime RMI

Level 2: RTSJ + Realtime RMI + Distributed Threads

Two Requirements for RT-RMI

Requirement 1: Real-time InvocationEnd-to-End timeliness in the invocation

process

Requirement 2: Support for RT-RMISerialization, the DGC, etc.

Real-time Invocation Control in RT-RMIReal-time Invocation Control in RT-RMI

Application

RMI Framework

Unicast Classes:Define Both threading

and Network Logic

VM Network

RMI in JavaRMI in Java

Application

Network

RMI Framework

ThreadingConcerns

NetworkConcerns

RTVM

RT-RMI RT-RMI

The RMI ClassesThe RMI Classes

Exporting Server Parameters

Export methods defined in subclass of RealtimeRemoteServer:

public static RealtimeRemoteServer exportObject(

RealtimeRemote obj, int port,SchedulingParameters sp, NetworkParams np,Threadpool pool

)

Realtime-parameters passed up to RealtimeRemoteServer

Network parameters stay with subclass

Propagating Client Parameters

Assigning parameters to a RealtimeRemoteStub instance.

Implicitly inheriting or deriving the parameters from the client Schedulable object.

Explicitly specifying parameters for each invocation.

RT-RMI Server Thread Model

Single Acceptor Thread associate with each exported object

Acceptor is most eligible thread if CP Accepts call and deserialises RT parameters

Handler Thread Deserialises non RT parameters Makes Upcall on Object Serialises the result and sends back to the server

Threadpool of handler Threads

Memory Considerations in implementation

Problems for the application developer:Parameters exported are used in the Schedulables created.

RTSJ scoping rules must be respected in the upcall

Problems for the developer:Threadpools implementation in not trivialProper scoping should be used to free objects

after invocation

The DGC

If scoped memory removes GC, can it also be used to solve DGC?

What are the semantics involved?

In particular, what is a remote object?

Connections

Should be part of a spec? Attach connections to pools? Can we assume connection-oriented

protocols RSVP

Carrying out distribution

How would a correct RTSJ be moved to a distributed environment?

Asynchronicity

What are the semantics of AIEs across node boundaries?

2 levels to address

CORBA and DRTSJ

Is RTSJ + RTCORBA = DRTSJ?

Must RTCORBA be Java aware?

How would a mapping work?

Conclusion