Custom Transport

This page summarizes an experience and use cases of implementing a new custom CXF transport.

Use Cases

Basically there are two main use cases for implementing a new CXF transport:

Providing a new physical protocol not yet supported by CXF (udp or ftp, for example). Some such cases can be solved by using integration with a corresponding Camel component, but if no such component is available or workable creating a new custom CXF transport should be considered.
Supporting tight integration with another framework (like JBI or Camel). In this case integration would be kept transparent for CXF applications - they would just speak directly with the target framework on the transport level. Here, the Transport implementation would be responsible for converting and transferring CXF exchange, messages and faults to target framework.

Presently the CXF distribution provides a transport implementation for the following protocols: HTTP(S), JBI, JMS and Local(inside one JVM). Camel additionally implements a CXF transport for Camel exchanges.

Architecture and Design

The transport functionality is based on two fundamental definitions: conduit and destination. Conduits are responsible for sending a message to recipients and destinations for receiving a message from the sender. In order to send a response, a destination needs its own back-channel conduit (in case of request-response communication). Conduits and destinations are created by a TransportFactory. CXF selects the correct TransportFactory based on the transport URL. SOAP is also considered a high level transport and has its own conduit and destination in CXF.
To send a message into a physical channel, the conduit should access the message context. Normal practice in this case would be to use a subclass of OutputStream extending CachedOutputStream. The custom stream will be fed the message and provided the possibility to access context in streaming or buffered form depending on the transport requirements. CachedOutputStream is configured to keep message in memory only up to a predefined size. If this size is exceeded, the message is swapped to disk.

A class diagram of TransportFactory, Conduit, Destination and OutputStream is shown below:

How it Works

Interaction between JAX-WS client and service using CXF transport is represented in the following figure:

Simplified Client Workflow:

Step1: The JAX-WS client invokes a service, in this manner for example:

        URL wsdlURL = this.getClass().getResource("/HelloWorld.wsdl");
        HelloWorldService service = new HelloWorldService(wsdlURL, SERVICE_NAME);        
        HelloWorld hw = service.getHelloWorldPort();       
        String result = hw.sayHi(TEST_REQUEST); }}

Step2: The CXF runtime selects the correct TransportFactory based on some criteria (described below)
Step3: The CXF runtime calls TransportFactory.getConduit() method to obtain the conduit
Step4: The CXF runtime invokes Conduit.prepare() and passes the outgoing message as an argument
Step5: Conduit sets own OutputStream (normally extended CachedOutputStream) as the outgoing message content
Step6: CXF runtime processes the outgoing message, calls the interceptor chain and invokes Conduit.close(Message) method for the outgoing message.
Step7: Finally, OutputStream.doClose() for the outgoing message is invoked
Step8: In the doClose() method, the OutputStream class has access to the marshalled outgoing message and exchange and will send this message to the service using the corresponding transport protocol
Step9: In case of one-way communication exchange will be closed. Skip to Step 14
Step10: In case of request-response communication, the conduit will wait for the service response in a synchronous or asynchronous manner
Step11: If a successful response is received, the conduit creates a new message, sets its context and puts it as In-Message in the exchange as an incoming message
Step12: If a fault is instead received, the Conduit creates a new Message, sets its context and places it as a fault message in exchange as in-fault message
Step13: Conduit notifies incomingObserver (that is ClientImpl object) about the response using incomingObserver.onMessage() call
Step14: Conduit.close(Message) method is invoked for incoming message. Normally the conduit implementation decreases the reference count with the service, potentially closing the network connection if the count is zero.
Step15: The JAX-WS client code receives the response in sync or async style

Simplified Service Workflow:

Step1: JAX-WS service is registered for example in this way:

	HelloWorldImpl serverImpl = new HelloWorldImpl();
	Endpoint.publish("udp://localhost:9000/hello", serverImpl);

Step2: The CXF runtime selects correct TransportFactory based on some criteria (described below)
Step3: The CXF runtime calls TransportFactory.getDestination() method to obtain the destination
Step4: As soon as the CXF runtime activates the endpoint (adds a listener, etc) the Destination.activate() method will be automatically invoked
Step5: The implementation of Destination.activate() normally opens network transport connections and listens to incoming requests
Step6: When a request comes, the destination creates a message, sets the content and notifies message observer (that is ChainInitializationObserver object) via incomingObserver.onMessage() about request. Normally an incoming connection is saved in a correlation map to be extracted for the appropriate response.
Step7: The business service implementation will be called with the request message. In case of one-way communication the exchange is now finished. In case of request-response, the business implementation either returns a response or throws a fault exception.
Step8: The CXF Runtime requests a back-channel conduit from the destination via Destination.getInbuiltBackChannel()
Step9: The Back-channel conduit's prepare() method will be called with a response message as argument
Step10: Back-channel conduit sets its own OutputStream as a message context
Step11: CXF runtime processes the response message, calls the interceptor chain and invokes Conduit.close(Message) for the response message.
Step12. Finally OutputStream.doClose() method for the response message is invoked
Step13: In doClose() method the OutputStream class has access to the marshalled response message and will send this message through the network as a response to the client. An appropriate incoming connection normally is extracted from a correlation map.

Registration of Transport Factory

There are two ways to register a transport factory: programmatically or via Spring configuration.
To register transport factory programmatically it is necessary to execute the following code:

     Bus bus = BusFactory.getThreadDefaultBus();
     DestinationFactoryManagerImpl dfm = bus.getExtension(DestinationFactoryManagerImpl.class);
     CustomTransportFactory customTransport = new CustomTransportFactory();
     dfm.registerDestinationFactory("http://cxf.apache.org/transports/TRANSPORT_PREFIX", customTransport);
     dfm.registerDestinationFactory("http://cxf.apache.org/transports/TRANSPORT_PREFIX/configuration", customTransport);

     ConduitInitiatorManager extension = bus.getExtension(ConduitInitiatorManager.class);
     extension.registerConduitInitiator("http://cxf.apache.org/transports/TRANSPORT_PREFIX", customTransport);
     extension.registerConduitInitiator("http://cxf.apache.org/transports/TRANSPORT_PREFIX/configuration", customTransport);

Where TRANSPORT_PREFIX is the protocol of the new transport (http, https, jms, udp).

For Spring configuration, the following could be used instead:

	<bean class="org.company.cxf.transport.CustomTransportFactory"
		lazy-init="false">
		<property name="transportIds">
			<list>
			<value>http http://cxf.apache.org/transports/TRANSPORT_PREFIX</value>
                	<value>http://cxf.apache.org/transports/TRANSPORT_PREFIX/configuration</value>
			</list>
		</property>
	</bean>

To define a new transport endpoint in the WSDL document follow these two steps:
a) Set the soap:binding transport attribute to the transport URL value (http://cxf.apache.org/transports/TRANSPORT_PREFIX)
b) The Port address element should be bound to namespace equal to the transport URL in the WSDL XML

Sample:

<wsdl:definitions 
    xmlns:transport="http://cxf.apache.org/transports/TRANSPORT_PREFIX" …> …
…
<wsdl:binding name="GreeterPortBinding" type="tns: GreeterPortType">
        <soap:binding style="document" transport="http://cxf.apache.org/transports/TRANSPORT_PREFIX"/>
…
<wsdl:service name="GreeterService">
       <wsdl:port binding="tns:GreeterPortBinding" name="GreeterPort">
           <transport:address location="LOCATION_URL">
…

Conduit and Destination Lifecycle

The conduit and destination lifecycle can be started by the TransportFactory during every client or service creation. The TransportFactory can either create a conduit and destination for each request or cache them based on service endpoint information.

Concurrency Aspects

Conduit and destination objects can by concurrently accessed by multiple threads. Implementations should care about concurrent correlations maps and/or synchronization primitives.

References

CXF transport implementations: package org.apache.cxf.transport.*

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