# Narayana LRA Participant Support

## Introduction

The LRA (short for Long Running Action) participant extension is useful in microservice based designs where different services can benefit from a relaxed notion of distributed consistency.

The idea is for multiple services to perform different computations/actions in concert, whilst retaining the option to compensate for any actions performed during the computation. This kind of loose coupling of services bridges the gap between strong consistency models such as JTA/XA and "home-grown" ad hoc consistency solutions.

The model is based on the Eclipse MicroProfile LRA specification. The approach is for the developer to annotate a business method with a Java annotation (`@LRA`). When such a method is called, an LRA context is created (if one is not already present) which is passed along with subsequent Jakarta REST invocations until a method is reached which also contains an `@LRA` annotation with an attribute that indicates that the LRA should be closed or cancelled. The default is for the LRA to be closed in the same method that started the LRA (which itself may have propagated the context during method execution). The Jakarta REST resource indicates that it wishes to participate in the interaction by, minimally, marking one of the methods with an `@Compensate` annotation. If the context is later cancelled, then this `@Compensate` action is guaranteed to be called even in the presence of failures and is the trigger for the resource to compensate for any activities it performed in the context of the LRA. This guarantee enables services to operate reliably with the assurance of eventual consistency (when all compensation activities have ran to completion). The participant can ask to be reliably notified when the LRA it is participating in is closed by marking one of the methods with an `@Complete` annotation. In this way cancelling an LRA causes all participants to be notified via their Compensate callback and closing an LRA causes all participants to be notified via their Complete callback (if they have one). Other annotations for controlling participants are documented in the MicroProfile LRA API v1.0 javadoc.

## Configuration

Once you have your Quarkus Maven project configured you can add the `narayana-lra` extension by running the following command in your project base directory:

CLI

    quarkus extension add narayana-lra,resteasy-jackson,resteasy-client-jackson

Maven

    ./mvnw quarkus:add-extension -Dextensions='narayana-lra,resteasy-jackson,resteasy-client-jackson'

Gradle

    ./gradlew addExtension --extensions='narayana-lra,resteasy-jackson,resteasy-client-jackson'

This will add the following to your build file:

pom.xml

    <dependency>
        <groupId>io.quarkus</groupId>
        <artifactId>quarkus-narayana-lra</artifactId>
    </dependency>
    <dependency>
        <groupId>io.quarkus</groupId>
        <artifactId>quarkus-resteasy-jackson</artifactId>
    </dependency>
    <dependency>
        <groupId>io.quarkus</groupId>
        <artifactId>quarkus-resteasy-client-jackson</artifactId>
    </dependency>

build.gradle

    implementation("io.quarkus:quarkus-narayana-lra")
    implementation("io.quarkus:quarkus-resteasy-jackson")
    implementation("io.quarkus:quarkus-resteasy-client-jackson")

| `quarkus-narayana-lra` needs to be complemented with a server Jakarta REST implementation and a REST Client implementation in order to work. This means that users should also have either `quarkus-resteasy-jackson` and `quarkus-resteasy-client-jackson` or `quarkus-rest-jackson` and `quarkus-rest-client-jackson` dependencies in their application.

If there is a running coordinator then this is all you need in order to create new LRAs and to enlist participants with them.

The LRA extension can be configured by updating an `application.properties` file in the `src/main/resources` directory. The only LRA specific property is `quarkus.lra.coordinator-url=<url>` which specifies the HTTP endpoint of an external coordinator, for example:

    quarkus.lra.coordinator-url=http://localhost:8080/lra-coordinator

For a Narayana coordinator the path component of the url is normally `lra-coordinator`. Coordinators can be obtained from https://quay.io/repository/jbosstm/lra-coordinator or you can build your own coordinator using a maven pom that includes the appropriate dependencies. A Quarkus quickstart will be provided to show how to do this, or you can take a look at one of the Narayana quickstarts. Another option would be to run it managed inside a WildFly application server.

## Handling failures

When an LRA is told to finish, i.e. when a method annotated with `@LRA(end = true, …​)` is invoked, the coordinator will instruct all services involved in the interaction to finish. If a service is unavailable (or still finishing) then the coordinator will retry periodically. It is the user’s responsibility to restart failed services on the same endpoint that they used when they first joined the LRA, or to tell the coordinator that they wish to be notified on new endpoints. An LRA is not deemed finished until all participants have acknowledged that they have finished.

The coordinator is responsible for reliably creating and ending LRAs and for managing participant enlistment, and it therefore must be available (for example if it or the network fail then something in the environment is responsible for restarting the coordinator or for repairing the network, respectively). To fulfill this task the coordinator must have access to durable storage for its logs (via a filesystem or in a database). At the time of writing, managing coordinators is the responsibility of the user. An "out-of-the-box" solution will be forthcoming.

## Examples

The following is a simple example of how to start an LRA and how to receive a notification when the LRA is later cancelled (the `@Compensate` annotated method is called) or closed (`@Complete` is called):

    @Path("/")
    @ApplicationScoped
    public class SimpleLRAParticipant
    {
        @LRA(LRA.Type.REQUIRES_NEW) // a new LRA is created on method entry
        @Path("/work")
        @PUT
        public void doInNewLongRunningAction(@HeaderParam(LRA_HTTP_CONTEXT_HEADER) URI lraId)
        {
            /*
             * Perform business actions in the context of the LRA identified by the
             * value in the injected Jakarta REST header. This LRA was started just before
             * the method was entered (REQUIRES_NEW) and will be closed when the
             * method finishes at which point the completeWork method below will be
             * invoked.
             */
        }

        @org.eclipse.microprofile.lra.annotation.Complete
        @Path("/complete")
        @PUT
        public Response completeWork(@HeaderParam(LRA_HTTP_CONTEXT_HEADER) URI lraId,
                                     String userData)
        {
            /*
             * Free up resources allocated in the context of the LRA identified by the
             * value in the injected Jakarta REST header.
             *
             * Since there is no @Status method in this class, completeWork MUST be
             * idempotent and MUST return the status.
             */
             return Response.ok(ParticipantStatus.Completed.name()).build();
        }

        @org.eclipse.microprofile.lra.annotation.Compensate
        @Path("/compensate")
        @PUT
        public Response compensateWork(@HeaderParam(LRA_HTTP_CONTEXT_HEADER) URI lraId,
                                       String userData)
        {
            /*
             * The LRA identified by the value in the injected Jakarta REST header was
             * cancelled so the business logic should compensate for any actions
             * that have been performed while running in its context.
             *
             * Since there is no @Status method in this class, compensateWork MUST be
             * idempotent and MUST return the status
             */
             return Response.ok(ParticipantStatus.Compensated.name()).build();
        }
    }

The example also shows that when an LRA is present its identifier can be obtained by reading the request headers via the `@HeaderParam` Jakarta REST annotation type.

And here’s an example of how to start an LRA in one resource method and close it in a different resource method using the `end` element of the `LRA` annotation. It also shows how to configure the LRA to be automatically cancelled if the business method returns the particular HTTP status codes identified in the `cancelOn` and `cancelOnFamily` elements:

      @LRA(value = LRA.Type.REQUIRED, // if there is no incoming context a new one is created
           cancelOn = {
               Response.Status.INTERNAL_SERVER_ERROR // cancel on a 500 code
           },
           cancelOnFamily = {
               Response.Status.Family.CLIENT_ERROR // cancel on any 4xx code
           },
           end = false) // the LRA will continue to run when the method finishes
      @Path("/book")
      @POST
      public Response bookTrip(...) { ... }

      @LRA(value = LRA.Type.MANDATORY, // requires an active context before method can be executed
           end = true) // end the LRA started by the bookTrip method
      @Path("/confirm")
      @PUT
      public Booking confirmTrip(Booking booking) throws BookingException { ... }

The `end = false` element on the bookTrip method forces the LRA to continue running when the method finishes and the `end = true` element on the confirmTrip method forces the LRA (started by the bookTrip method) to be closed when the method finishes. Note that this end element can be placed on any Jakarta REST resource (ie one service can start the LRA whilst a different service ends it). There are many more examples in the Microprofile LRA specification document and in the Microprofile LRA TCK.