Transactional Outbox is a library that provides a simple way to implement the Transactional Outbox Pattern in your application, developed by Blueground.

API Docs: https://bluegroundltd.github.io/transactional-outbox/

• Installation
• Usage
  • Creating an Outbox library instance
  • Creating a new outbox handler
  • Creating a new outbox entry
  • Monitoring the outbox entries
  • Shutting down
• Publishing
• Maintainers

Transactional Outbox is published on mavenCentral. In order to use it just add the following dependency:

implementation("io.github.bluegroundltd:transactional-outbox-core:2.3.2")

When you have an operation that you want to be executed asynchronously you can start by configuring the Outbox. If you're using Spring for example, you can configure the library as shown below using the provided builder.

@Configuration
class OutboxConfiguration(
  private val clock: Clock,
  private val outboxHandlers: Set<OutboxHandler>,
  private val postgresLockDao: PostgresLockDao,
  private val outboxStore: OutboxStore,
) {

  @Bean
  fun transactionalOutbox(): TransactionalOutbox {
    val monitorLocksProvider = OutboxLocksProvider(postgresLockDao, MONITOR_APPLICATION_SPECIFIC_ID)
    val cleanupLocksProvider = OutboxLocksProvider(postgresLockDao, CLEANUP_APPLICATION_SPECIFIC_ID)

    return TransactionalOutboxBuilder
      .make(clock)
      .withHandlers(outboxHandlers)
      // Ideally you want your thread pool < db connection pool size or similar
      .withThreadPoolSize(5) 
      // Override the default Thread priority with caution
      // If you are facing performance issues, you should
      // adjust the thread pool size first.
      // .withThreadPriority(Thread.NORM_PRIORITY)
      .withMonitorLocksProvider(monitorLocksProvider)
      .withCleanupLocksProvider(cleanupLocksProvider)
      .withStore(outboxStore)
      .build()
  }
}

private class OutboxLocksProviderImpl(
  private val postgresLockDao: PostgresLockDao,
  private val id: Long
) : OutboxLocksProvider {
  override fun acquire() {
    postgresLockDao.acquire(id)
  }

  override fun release() {
    postgresLockDao.release(id).getOrThrow()
  }
}

Then you can create a new OutboxHandler that will be responsible for processing the Outbox entries.
Below you can see a barebones handler, but there's also a utility handler, which uses JSON (de)serialization and reduces the outbox handlers boilerplate code. Refer to SimpleOutboxHandler in our docs page.

enum class MyOutboxType: OutboxType {
  GOOGLE_CALENDAR_CREATE, OTHER_TYPE;

  override fun getType(): String {
    return name
  }
}

class GoogleCalendarCreationOutboxHandler(
  private val userService: UserService,
  private val objectMapper: ObjectMapper
) : OutboxHandler {

  companion object {
    private const val MAX_RETRIES = 5
  }

  override fun getSupportedType() = MyOutboxType.GOOGLE_CALENDAR_CREATE

  override fun serialize(payload: OutboxPayload): String = objectMapper.writeValueAsString(payload)

  override fun getNextExecutionTime(currentRetries: Long): Instant = Instant.now()
    .plusSeconds(NEXT_EXECUTION_TIME_OFFSET_IN_SECONDS[currentRetries.toInt()])

  override fun hasReachedMaxRetries(retries: Long) = retries >= MAX_RETRIES

  override fun handle(payload: String) {
    val deserializedPayload = deserialize(payload)
    userService.createCalendarAndUpdateUser(
      deserializedPayload.userId,
      deserializedPayload.calendarName,
      deserializedPayload.emailsToShareWith
    )
  }

  override fun handleFailure(payload: String) =
    Log.warn("$LOGGER_PREFIX Google calendar creation failed after $MAX_RETRIES retries. Payload: $payload")

  private fun deserialize(payload: String) =
    objectMapper.readValue(payload, GoogleCalendarCreationOutboxPayload::class.java)
}

Finally, you can create a new OutboxEntry and save it to the Outbox, called inside a higher level transaction:

fun addGoogleCalendarOutboxItem(user: User) {
  val payload = GoogleCalendarCreationOutboxPayload(
    user.getId(),
    user.getFullname(),
    user.getEmail()
  )
  outbox.add(MyOutboxType.GOOGLE_CALENDAR_CREATE, payload)
}

You can monitor the Outbox entries by using the TransactionalOutbox::monitor method using a data store poller. For example, if you're using Spring you can use the @Scheduled annotation:

@Component
class OutboxMonitor(
  private val transactionalOutbox: TransactionalOutbox
) {

  @Scheduled(fixedDelay = 1000)
  fun monitor() {
    transactionalOutbox.monitor()
  }
}

You can shut down the Outbox by using the TransactionalOutbox::shutdown method.

shutdown() blocks new tasks from being processed and waits up to a specified period of time for all tasks to be completed. You can configure this timeout with the TransactionalOutboxBuilder. The default shutdownTimeout is set to 5 seconds. If that time expires, the execution is stopped immediately. Any tasks that did not start execution will have their corresponding item's status set to PENDING.

If you're using Spring you can use the @PreDestroy annotation:

@Component
class OutboxMonitor(
    private val transactionalOutbox: TransactionalOutbox
) {

  @Scheduled(fixedDelay = 1000)
  fun monitor() {
    transactionalOutbox.monitor()
  }

  @PreDestroy
  fun shutdown() {
    transactionalOutbox.shutdown()
  }

}

If you're working on a new version of the core module, and you want this version to be available to other project before publishing it, you can do so in two ways

• Alternative 1: Install the new version to Maven

$ mvn install:install-file \ 
   -Dfile=core/build/libs/core-x.y.z.jar \
   -DgroupId=io.github.bluegroundltd \ 
   -DartifactId=transactional-outbox-core \
   -Dversion=x.y.z \
   -Dpackaging=jar \
   -DgeneratePom=true

then depend on the x.y.z version as usual.

implementation("io.github.bluegroundltd:transactional-outbox-core:x.y.z")

• Alternative 2: Change your dependencies to directly reference the jar file

implementation(files("../../../transactional-outbox/core/build/libs/core-x.y.z.jar"))

• Alternative 3: You can publish a snapshot version of the library and make it available to maven snapshot repository.
  1. Update the version in gradle.properties to a snapshot version, e.g. 2.2.0-SNAPSHOT
  2. Publish it using the instructions here: [Publish via your workstation](###Publish via your workstation)
  3. Snapshot will be published to the maven snapshot repository and you can use it in your project by adding the following to your build.gradle file:

    repositories {
      maven {
        url = uri("https://s01.oss.sonatype.org/content/repositories/snapshots/")
        credentials {
          username = <your_username> or <your_gpr_user>
          password = <your_password> or <your_gpr_token>
        }
      }
    }
    dependencies {
        implementation("io.github.bluegroundltd:transactional-outbox-core:2.2.0-SNAPSHOT")
    }

Firstly, bump version in gradle.properties of core module, commit and push a PR. Once it gets merged, follow one of the two options below.

Now, you can either:

1. publish via GitHub, or
2. locally from your machine

Using this method has the benefit of not having to provide any secrets whatsoever.
Simply, push a git tag after a PR is merged, which will trigger the release.yml pipeline.
Said pipeline, will publish the artifact.

Please note that this will be automated in future work.

To tag, please follow a format like v0.4.0, that is prefixed with v and the version number of the artifact you're about to release.

• Having built the artifact
• Execute the following to upload artifact:

$ ./gradlew publishAllPublicationsToMavenCentral \
            --no-configuration-cache \
            -Psigning.secretKeyRingFile=<keyring_file_path> \
            -Psigning.password=<keyring_password> \
            -Psigning.keyId=<keyring_id> \
            -PmavenCentralUsername=<nexus_username> \ 
            -PmavenCentralPassword=<nexus_password>

Note that if you maintain the properties above (-P) in your local gradle.properties file, you can omit them from the command.

The core maintainer of this project are:

• Chris Aslanoglou
• Apostolos Kiraleos
• Thanasis Polydoros
• Grigoris Balaskas