Distributed Real-Time in the RTSJ

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Distributed Real-Time in the RTSJ

• Andrew Borg

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Presentation

• RMI A brief introduction
• The RTSJ A (very) brief introduction
• The DRTSJ The 3 Levels of Integration
• RMIRTSJ Concerns, Issues and Solutions

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Presentation

• RMI A brief introduction
• The RTSJ A brief introduction
• The DRTSJ Current Status
• RMIRTSJ Concerns, Issues and Solutions

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The RMI Specification

• RMI is very minimal.
• Assumes a homogeneous environment of JVMs.
• There is no ORB concept (see later)

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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.

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A simple HelloPerson example

• Client sends name to a server.
• Server returns hello the name passed

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The interface

• public interface HelloPersonInterface extends
  java.rmi.Remote
• public void sayHello() throws
  java.rmi.RemoteException

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The Implementation

• public class HelloPerson implements
  HelloPersonInterface
• public HelloPerson ()
• public String sayHello(String S)
• return new String("Hello World"S)

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Generate Stub Using rmic

• See later

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Launching the Server

• Registry r createRegistry(4321)
• HelloPersonImpl x new HelloPersonImpl()
• Remote s UnicastRemoteObject.exportObject(x)
• r.bind("MyHelloServer",s)

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Invoking the Server

• HelloServerInterface H (HelloServerInterface)
  r.lookup("MyHelloServer")
• System.out.println(H.sayHello("Andrew"))

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The RMI Classes
Interface
ServerStub
Server
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The Stub

• public final class HelloServerImpl_Stub
• extends java.rmi.server.RemoteStub
• implements HelloServerInterface
• public java.lang.String sayHello(java.lang.Str
  ing 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)

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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.

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The DGC

• Is part of the specification
• Uses a lease-mechanism with reference counting
• Internally also keeps the VM alive when remote
  objects exist

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JRMP

• Serialization Protocol
• HTTP Protocol
• Output defines sub-protocol
• single-op
• stream
• multiplex

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Presentation

• RMI A brief introduction
• The RTSJ A brief introduction
• The DRTSJ The 3 Levels of Integration
• RMIRTSJ Concerns, Issues and Solutions

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The Real Time Specification for Java (RTSJ)

• The 7 enhanced areas over Java
• Thread Scheduling and Dispatching
• Memory Management
• Synchronization and Resource Sharing
• Asynchronous Event Handling
• Asynchronous Transfer of Control
• Asynchronous Thread Termination
• Physical Memory Access

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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.

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The 3 levels of Integration

• Level 0 RTSJ RMI
• Level 1 RTSJ Realtime RMI
• Level 2 RTSJ Realtime RMI Distributed
  Threads

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Two Requirements for RT-RMI

• Requirement 1 Real-time Invocation
• End-to-End timeliness in the invocation process
• Requirement 2 Support for RT-RMI
• Serialization, the DGC, etc.

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Real-time Invocation Control in RT-RMI
RMI in Java
RT-RMI
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The RMI Classes
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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

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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.

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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

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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 trivial
• Proper scoping should be used to free objects
  after invocation

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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?

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Connections

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

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Carrying out distribution

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

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Asynchronicity

• What are the semantics of AIEs across node
  boundaries?
• 2 levels to address

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CORBA and DRTSJ

• Is RTSJ RTCORBA DRTSJ?
• Must RTCORBA be Java aware?
• How would a mapping work?

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Conclusion