Monday, July 29, 2013

Out of the Box Support for RESTful Transactions

The latest release of narayana includes a couple of great additions to the REST-AT module that represent a step change in the usability of restful transactions. The work has been contributed by transactions' team member Gytis Trikleris.

Mark has previously blogged about why RESTful transactions are useful and our implementation of the draft REST-AT specification is steadily finding real world uses (within BlackTie for example). However, although using REST-AT is easier than building upon a full web services stack containing support for WS-Atomic Transaction, there are still barriers to overcome such as making sure the REST-AT coordinator is managed and installed correctly, ensuring the coordinator endpoint is deployed and the responsibilities of the service writer to conform to the specification are certainly not trivial. Gytis' work addresses both deficiencies.

REST-AT Now Installs as a Wildfly Extension

Wildfly extensions provide a mechanism for extending the functionality provided by the Wildfly application server. Installing the REST-AT coordinator via an extension not only means that the coordinator will be available to all client applications but it will also benefit from the same management and configuration enjoyed by all subsystems. The application developer no longer needs to be concerned with deploying the coordinator, with managing versions of the coordinator, with managing conflicts with other applications that require REST-AT, nor does he need to ensure that any dependencies such as JAX-RS support or web services are available and compatible. The feature is available for use now in the latest version 8.0.0.Alpha3 of wildfly.

Simple Interface for Participating in REST-AT Transactions

Until now the service developer has needed to understand and comply with the requirements of the REST-AT specification including the responsibility of listening for transaction completion requests on HTTP endpoints. This forces the developer to design his service as a web service. The new mechanism means that any Java component can join a REST-AT transaction without this burden and consequently it becomes isolated from the details of using REST-AT. Writing an application that uses REST-AT means that services can take part in transactions that can potentially span multiple services and servers.
So how does it work in practice? The service writer simply registers his service to receive callback notifications when the global transaction is ready to complete. Specifically, you will need to depend on the RTS subsystem by including a dependency on it in your application manifest (add Dependencies: org.jboss.narayana.rts to MANIFEST.MF) and pull in the RTS library when building your services. For maven users you will need:

    <dependency>
        <groupId>org.jboss.narayana.rts</groupId>
        <artifactId>restat-integration</artifactId>
        <version>5.0.0.M3</version>
        <scope>provided</scope>
    </dependency>

The application developer interface to the RTS subsystem is

    String enlist(String applicationId,
                  String participantEnlistmentURL,
                  Participant participant);
    void registerDeserializer(String applicationId,
                  ParticipantDeserializer deserializer);
    void reportHeuristic(String participantId,
                  HeuristicType heuristicType);

The participantEnlistmentURL in the enlist method corresponds to a running REST transaction which the service acquires during normal interactions with service clients. To register for completion callbacks the service writer registers an interface using the enlist method and passes in an implementation of Participant:

    public interface Participant {
        Vote prepare();
        void commit() throws HeuristicException;
        void commitOnePhase();
        void rollback() throws HeuristicException;
    }

Now when a service client terminates a transaction the services' callback methods will be invoked (by a REST-AT coordinator which may or may not be running locally since these are distributed transactions). It is interesting to note that the wildfly application server is a modular container so subsystems and applications run in their own class loaders. In the event of failures a recovery system will need to recreate participant callback registrations in order to complete any pending transaction and therefore will no longer have access to the original class. The service writer must help the recovery system in this task via the registerDeserializer call to the RTS subsystem. The final method on the interface to RTS (reportHeuristic) is to allow services to independently abort or commit work before being asked to via the callback interface.

What's next

I think you will agree that this is a pretty simple, low overhead, interface that allows services to participate in global transactions and we encourage users to start experimenting with it. In forthcoming narayana snapshot releases we will be adding quickstarts and examples that showcase the functionality including how it is used in a J2SE environment. If you would like to try out the quickstarts now then go to our narayana quickstarts git repository and look for the service2 and recovery2 examples. We will also be providing more documentation of REST-AT features in the documentation repository.



In addition Gytis Trikleris will soon be integrating some more exciting work that he has been doing in this area: currently, if a service takes part in a REST-AT transaction which also starts JTA transactions then the two are not related, i.e. terminating the REST-AT transaction has no effect on the JTA transactions (and vice versa). Gytis has written a bridge that fixes this deficiency, "watch this space" as they say.
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