Message bus architecture

The message bus architecture is primarily designed to separate the logic of the message patterns (ie. subscriptions/publishers) from the underlying technology that delivers the messages themselves (either Kafka or a database). There is a nice diagram of the architecture on the design repo.

Using the different message buses

Each bus is designed to be bundled into an application separately. You must specify in your build which bus to use.

Using the Kafka message bus

For the Kafka message bus, add the following to your build.gradle:

    implementation project(":components:kafka-topic-admin")

    runtimeOnly project(":libs:messaging:kafka-topic-admin-impl")
    runtimeOnly project(":libs:messaging:kafka-message-bus-impl")

Using the DB message bus

Unfortunately, there is a lot of boiler plate in getting the DB message bus working. It is basically just getting the correct dependencies included in your build.gradle. Add the following to your build.gradle for the DB message bus:

    runtimeOnly project(":libs:messaging:messaging-impl")
    runtimeOnly project(":libs:messaging:db-message-bus-impl")
    runtimeOnly project(":libs:schema-registry:schema-registry-impl")

    runtimeOnly "org.apache.aries.spifly:org.apache.aries.spifly.dynamic.bundle:$ariesDynamicBundleVersion"
    runtimeOnly "com.sun.activation:javax.activation:$activationVersion"
    runtimeOnly "org.hsqldb:hsqldb:$hsqldbVersion"
    runtimeOnly "org.postgresql:postgresql:$postgresDriverVersion"

"In-memory" testing using the DB message bus

When testing libraries and components it is likely that you will want to forego Kafka for messaging and, instead, opt for an in-memory message bus. For this we have decided to use the DB message bus, using the HSQL in-memory database. This is very similar to the changes needed for using the db-message-bus for actual database messaging. The main difference is that you must add some dependencies and adjust your test and test.bndrun file.

Add the following to your build.gradle for testing on the DB message bus:

    integrationTestImplementation project(":testing:db-message-bus-testkit")

    integrationTestRuntimeOnly project(":libs:messaging:messaging-impl")
    integrationTestRuntimeOnly project(":libs:messaging:db-message-bus-impl")
    integrationTestRuntimeOnly project(":libs:schema-registry:schema-registry-impl")

    integrationTestRuntimeOnly "org.apache.aries.spifly:org.apache.aries.spifly.dynamic.bundle:$ariesDynamicBundleVersion"
    integrationTestRuntimeOnly "com.sun.activation:javax.activation:$activationVersion"
    integrationTestRuntimeOnly "org.hsqldb:hsqldb:$hsqldbVersion"

Here are the things you need to add to your test.bndrun for testing on the DB message bus:

    bnd.identity;id='net.corda.messaging-impl',\
    bnd.identity;id='net.corda.db-message-bus-impl',\
    bnd.identity;id='net.corda.db-orm-impl',\
    bnd.identity;id='net.corda.schema-registry-impl',\
    bnd.identity;id='net.bytebuddy.byte-buddy',\
    bnd.identity;id='org.hsqldb.hsqldb',\

And, finally, you must add DBSetup::class to your OSGi test class as a @ServiceExtension:

 @ExtendWith(ServiceExtension::class, DBSetup::class)
class SomeOSGiIntegrationTest {

This allows the in-memory database to be correctly set up with the necessary tables for messaging. Note: This automatic database setup only works for HSQL. PostgreSql is not supported.