RequestDispatcher

Java library for dispatching request to extrenal services over ZeroMQ sockets. The library targets use inside Java Servlet Containers, like Apache Tomcat, which expose a multithreaded environment.

It comes acompanied with ZmqWorker and ZmqProxy classes that allow the creation of simple remote services.

Features

• Non-blocking access to remote Request/Reply ZMQ services.
• Asynchronus dispatching of several requests before the replys are received.
• Single threaded callback execution (gatherResults())
• Forwarding of Exceptions
• One way requests (with callback = null)

Example: id server

In this example we create a remote service that creates globally unique ids.

    //
    //// SHARED OBJECTS
    //
    String idChannel = "ipc:///idChannel";      // ZMQ Endpoint for communication
    class idRequest implements Serializable {}  // request class for dispatching

    //
    //// SERVER SIDE
    //
    // Setup an idServer with a single thread listening on idChannel
    ZmqWorkerProxy proxy = new ZmqWorkerProxy(idChannel);
    proxy.add(1, new RequestHandler<idRequest, Long>() {
        long currentId = 0;

        @Override
        public Long handleRequest(idRequest request) throws Exception {
            return currentId++;
        }
    });
    proxy.startWorkers();

    //
    //// CLIENT SIDE
    //
    // We create a dispatcher object and tell it to send idRequests to the idChannel:
    Dispatcher dispatcher = new Dispatcher();
    dispatcher.registerService(idRequest.class, idChannel);

    // In the simplest case we do a synchronous call and wait for the result
    long id = (Long) dispatcher.executeSync(new idRequest());

    System.out.println("Sync id: " + id);
    // prints "Sync id: 0"


    // For asyncrhonus calls we define a callback object and pass it to the dispatcher
    dispatcher.execute(new idRequest(), new Callback<Long>() {
        @Override
        public void onSuccess(Long id) {
            System.out.println("Async id: " + id);
        }
    });
    // the request is now executed and is processing on the server.

    // The following call waits for the results of all requests
    // and executes the callback methods:
    dispatcher.gatherResults();
    // prints "Async id: 1"

    // Cleanup
    dispatcher.shutdown();
    proxy.shutdown();

Components

• Request/Reply interfaces. To be implemented by new services and instanciated for requests. The objects are serialized and sent over the wire.
• Abstract Callback Class. To be extended by callback anonymous inner classes (AIC), hat handle the replies.
• Dispatcher Class. Orchestrages serivces and callback execution in a synchronus event loop.
• ZMQ Network Service library. Handles socket communication and polling.

Control Flow

Acknowledgements

This project used by the metalcon.de project, which have provided the initial motivation to write this tool and contrinuted a lot of valuable feedback.