Web Application Messaging Protocol (WAMP v2) support for Java
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readme.md

jawampa

  • is a library that brings support for the Web Application Messaging Protocol [WAMP] to Java.
  • provides WAMPv2 client side functionality as well as server side functionality and supports all currently defined WAMPv2 roles (caller, callee, publisher, subscriber, broker, dealer).
  • provides a pluggable transport layer. Connection providers and servers which use different networking mechanisms and low-level libraries can be built and plugged into jawampa.
  • exposes the client-side user-interface through RxJava Observables, which enable powerful compositions of different asynchronous operations and provide an easy solution for delegating data handling between different threads.
    Observables are also used in places where only single return values are expected and Futures would be sufficient - However the common use of Observables provides less dependencies and allows to schedule continuations for all kinds of asynchronous operations in a consistent way.
  • is compatible with Java6. However the examples in this document use Java8 syntax for convenience.

Install

Declare the following dependency for the base library:

<dependency>
    <groupId>ws.wamp.jawampa</groupId>
    <artifactId>jawampa-core</artifactId>
    <version>0.4.2</version>
</dependency>

However as the core library of jawampa does not provide a transport layer users should typically use a jawampa transport provider library (e.g. jawampa-netty - see subdirectory) as a depency.
This will automatically also add a dependency on jawampa-core.

WAMP client API (WampClient)

The client-side API is exposed through the WampClient object.
WampClients must be created through WampClientBuilder objects, which allow to configure the created clients.

There are 3 mandatory parameters that have to be set through the builder:

  • A connector provider which describes the framework which will be used for establishing a connection to the WAMP router. An example is the NettyWampClientConnectorProvider which is described in the documentation of the jawampa-netty subproject.
  • The URI that describes the address of the WAMP router
  • The realm that the client should join on the router

Additionally there exist some optional parameters, which for example allow to activate automatic reconnects between the client and the router or allow to configure how the client should behave in case of communication errors.

Example:

final WampClient client;
try {
    // Create a builder and configure the client
    WampClientBuilder builder = new WampClientBuilder();
    builder.withConnectorProvider(connectorProvider)
           .withUri("ws://localhost:8080/wamprouter")
           .withRealm("examplerealm")
           .withInfiniteReconnects()
           .withReconnectInterval(5, TimeUnit.SECONDS);
    // Create a client through the builder. This will not immediatly start
    // a connection attempt
    client = builder.build();
} catch (WampError e) {
    // Catch exceptions that will be thrown in case of invalid configuration
    System.out.println(e);
    return;
}

The WampClient object provides the RxJava Observable statusChanged() that notifies the user about the current status of the session between the client and the router, which can be either DisconnectedState, ConnectingState or ConnectedState. The application can monitor this Observable to detect when other steps should be performed (e.g. subscribe to topics or register functions after connect).

The onNext() status notification method of the Subscriber will be called on the WampClients thread. However it can easily be delegated to a Scheduler or EventLoop of the host application be using the Observable.observerOn() member function.

statusChanged() returns a BehaviorObservable, therefore it will immediatly send a notification about the current state to subscribers on subscribe and not only in case of state changes.

Example:

client.statusChanged()
      .observeOn(applicationScheduler)
      .subscribe((WampClient.State newState) -> {
        if (newState instanceof WampClient.ConnectedState) {
          // Client got connected to the remote router
          // and the session was established
        } else if (newState instanceof WampClient.DisconnectedState) {
          // Client got disconnected from the remoute router
          // or the last possible connect attempt failed
        } else if (newState instanceof WampClient.ConnectingState) {
          // Client starts connecting to the remote router
        }});

In order to start the connection between a client and a router the clients open() member function has to be called. This will lead to the first connection attempt and a state change from DisconnectedState to ConnectingState.

When the client is no longer needed is must be closed with the close() member function. This will shutdown the connection to the remote router and stop all reconnect attempts. After a WampClient was closed it can not be reopened again. Instead of this a new instance of the WampClient should be created if necessary.

The close process is also asynchronous. Therefore a call to close() does not guarantee an immediate close of the client. However the close() call returns an Observable which can be used to wait until the client was successfully closed.

Example for a typical session lifecycle:

WampClient client = builder.build();
client.statusChanged().subscribe(...);
client.open();

// ...
// use the client here
// ...

// Wait synchronously for the client to close
// On environments like UI thread asynchronous waiting should be preferred
client.close().toBlocking().last();

Performing procedure calls

Remote procedure calls can be performed through the various call member functions of the WampClient.

All overloaded version of call require the name of the procedure that should be called (and which must be a valid WAMP Uri) as the first parameter. All versions of call() return an Observable which is used to transfer the result of the function call in an asynchronous fashion to the caller. It is a hot observable, which means the call will be made indepently of whether someone subscribes to it or not. However the result will be cached in the Observable, which means that also late subscribers will be able to retrieve the result.

  • If the procedure call succeeds the subscribers onNext method will be called with the result and followed by an onCompleted call.
  • If the remote procedure call fails then the subscribers onError() method will be called with the occurred error as a parameter.

The different overloads of call() allow to provide the arguments to the procedure in different fashions as well as to retrieve the return value in a different fashion:

The most explicit signature of call is
Observable<Reply> call(String procedure, ArrayNode arguments, ObjectNode argumentsKw)
It allows to pass positional arguments as well as keyword arguments to the WAMP procedure and will return a structure which will as well contains fields for the positional and keyword arguments of the call result.
The arguments and return values use the ArrayNode and ObjectNode data types from the Jackson JSON library which describe an array or object of arbitrary other types.

If only positional arguments are required for the call the simplified variant
Observable<Reply> call(String procedure, Object... args)
can be used which allows to pass the positional arguments as a varargs array. It will also automatically use Jacksons object mapping capabilities to convert all Java POJO keyword arguments in their JsonNode form and create an argument array from that. This means you can directly use any kind of Java objects as function parameters as long as they can be properly serialized and deserialized by Jackson. For more complex data structures you might need to use annotations to instruct the serializer.

The last variant of call() provides some further convenience and has the following signature: <T> Observable<T> call(String procedure, Class<T> returnValueClass, Object... args).
It can be used when the procedure provides none or a single positional return value. Then you can specify the type of the expected return value in the second arguments and call will automatically try to map the first result argument of the procedure call into the required type. This will also be done through Jackson object mapping.

With this simplification you can call remote procedures and listen for return values in the following way (with Java8):

Observable<String> result = client.call("myservice.concat", String.class, "Hello nr ", 123);
// Subscribe for the result
// onNext will be called in case of success with a String value
// onError will be called in case of errors with a Throwable
result.observeOn(applicationScheduler)
      .subscribe((txt) -> System.out.println(txt),
                 (err) -> System.err.println(err));

Providing remote procedures to other clients

With WAMP all clients that are connected to a router are able to provide procedures that can be used by any other client.

jawampa exposes this functionality though the registerProcedure() member function. registerProcedure will return an Observable which will be used to receive incoming function calls. Each incoming request to the registered procedure name will be pushed to the Subscribers onNext method in form of a Request class. The application can retrieve and process requests on any thread through observeOn and can send responses to the request with the Request classes member functions. The procedure will only be registered at the router after subscribe was called and will be unregistered at the router if the subscription was unsubscribed.

If an error occurs during the registration of the procedure at the router the Subscribers onError method will be called to notify about the error. If the session disconnects (or is disconnected) then the subscription will simply be completed with onCompleted.

Example for providing a procedure which echos the first integer argument:

// Provide a procedure
Observable proc = client.registerProcedure("echo.first").subscribe(
    request -> {
        if (request.arguments() == null || request.arguments().size() != 1
         || !request.arguments().get(0).canConvertToLong())
        {
            try {
                request.replyError(new ApplicationError(ApplicationError.INVALID_PARAMETER));
            } catch (ApplicationError e) { }
        } else {
            long a = request.arguments().get(0).asLong();
            request.reply(a);
        }
    },
    e -> System.err.println(e));

// Unregister the procedure
proc.unsubscribe();

Publishing events

The publish() function of the WampClient can be used to publish an event with a topic (that must be a valid WAMP Uri) towards the router and thereby to all other connected WAMP clients.

Similar to call() the publish() function provides various overloads that allow to use differnt formats for passing the event arguments.

The publish() function returns an Observable<Long>. Just like the Observable returned from call() this is also a hot observable and does not need to be subscribed to perform the publishing. Subscribers that subscribe on the Observable will either receive a single onNext() call which delivers the publicationId or an onError() call when the publishing fails. This can for example be the case when there is no connection to the router.

Example for publishing an event:

client.publish("example.event", "Hello ", "from nr ", 28)
      .subscribe(
        publicationId -> { /* Event was published */ },
        err -> { /* Error during publication */});

Subscribing to events

To subscribe on events that are published from other clients on the router the makeSubscription() function of the WampClient can be used. makeSubscription will require the topic which the client is interested in and will return an Observable that can be used to perform the subscription. The subscription on the topic at the router will only happen after subscribe() was called on the Observable. After that, for each received event the onNext() function of the Subscriber will be called to deliver the event in form of a PubSubData struct that contains the positional as well as the keyword arguments. If the subscription can not be performed at the router because of an error onError() will be called to deliver that error. If the connection is closed or get's closed the subscription will be completed with onCompleted().

Hint: For most applications it makes sense to perform subscriptions after they got connected to the router in the statusChanged() handler.

There exists also an overload of the makeSubscription function which can be used in the case that the client is only interested in the first positional argument of the event. It allows to specify the class type of the event data and will automatically try to transform the received event into this data type through Jacksons object mapping capabilities. If the client is interested in no parameter Void.class can be used get an Observable<Void> which will notifiy the subscriber when an event without parameters received. If the received event data can not be converted into the desired format the subscription will be cancelled and an error will be delivered through the onError() function.

Example for subscribing to an event:

// Subscribe to an event
Observable<String> eventSubscription =
client.makeSubscription("example.event", String.class)
      .subscribe((s) -> { /* String event received */ },
                 (e) -> { /* Error during subscription or object mapping */ });

// Unsubscribe from the event
eventSubscription.unsubscribe();

WAMP server API (WampRouter)

jawampa provides a WAMP router that can be bundled into your application in order to avoid installing, configuring and running an external router. By instantiating a WampClient that provides an API as well as a WampRouter in an application a classical server architecture can be mimiced, where the server listens for connections as well as provides an API.

The WampRouter class implements the whole routing and realm logic that is described in the WAMPv2 basic profile. It can be only be created through the WampRouterBuilder class, which allows to configure a router before instantiating it. In the current version of jawampa the realms that the router shall expose have to be configured through it.

Example for configuring and instantiating a router:

WampRouterBuilder routerBuilder = new WampRouterBuilder();
WampRouter router;
try {
    routerBuilder.addRealm("realm1");
    routerBuilder.addRealm("realm2");
    router = routerBuilder.build();
} catch (ApplicationError e) {
    e.printStackTrace();
    return;
}

The router will be directly up-and-running after it was built. However it won't listen to any connections yet and therefore won't do anything up to this point.

In order for the router to work servers must be set up that accept connections, register them at the WampRouter and then push messages towards it.

The WampRouter provides an implementation of the IWampConnectionAcceptor interface which can be used to register a new connection at the client. It can be queried through the WampRouter.connectionAcceptor() getter.

Registering a new connection at the router is a 2-stage process:

  • At first the connection provider must query an instance of a IWampConnectionListener from the router by calling connectionAcceptor.createNewConnectionListener();. This will be the interface to which the new connection should push messages after it was fully established and registered.
    The returned interface does not yet occupy any non-garbage-collectible resources in the router. Therefore it is not harmful to ignore the return value if the connection provider determines that the connection can not be properly established.
  • In a second step the new connection must be registered at the router by calling connectionAcceptor.acceptNewConnection(connection, connectionListener); and thereby providing the sending interface of type IWampConnection to the router.

The new connection may only send message to the listener once both steps have been finished. Sending messages earlier causes undefined behavior.

The WampRouter will use the provided IWampConnection interface in order to send messages through connections. The connection must guarantee the following contract to the router:

  • The router must be able to call methods on the interface as long as it has not called close(...) on it.
    If the connection is already closed or in an errorenous state implementations of the interface should answer sendMessage(...) calls by rejecting the provided promise.
  • The router will always call close(...) on the interface, even if the connection was closed by the remote side before.
  • The connection must guarantee that it calls no method on the retrieved IWampConnectionListener interface after it has acknowledged the close call by fulfilling the provided future. The router will take the acknowledgement of the close(...) call as a sign that all resources owned by the connection have been released.

An example implementation of a server that pushes messages towards the router which is based on the Netty framework can be found in the jawampa-netty subproject.

Closing a router

To close a router the close() member function has to be called:

router.close.toBlocking().last();

This will gracefully close all WAMP sessions established between the router and clients and will also close the underlying transport channels. If new connections are made to the router after close() it will reject those by closing them.

Just like the close() call on the WampClient closing a WampRouter is also an asnychronous process and the the call will return an Observable that signals when the router is fully shutdown.

In order to allow the router to listen on a port, accept incoming connections one or more servers have to be started which use the router as their final request handler.

Restrictions

jawampa is very young and in a work-in-progress state.
Therefore the following restrictions apply:

  • jawampa does not properly support the transmission of binary values as required in the WAMP specification. jawampa will use Jackson to transform data from binary to JSON which will use a base64 encoding, but will not prepend the data with a leading 0 byte.
  • jawampa only supports the WAMPv2 basic profile and some selected parts of the advanced profile. Many advanced profile features are not implemented.
  • jawampa only supports websocket connections between WAMP clients and routers.
  • The roles of the client and router are properly transmitted but not taken into account for all other actions. E.g. it won't be verified whether a remote peer actually provides the needed functionality or not. The assumption is that all peers implement all of the roles that apply for them.