Transactional Outbox Library

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/

Table of Contents

• 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

Installation

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

implementation("io.github.bluegroundltd:transactional-outbox-core:1.0.0")

Usage

Creating an Outbox library instance

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 locksProvider = OutboxLocksProvider(postgresLockDao, APPLICATION_SPECIFIC_ID)

    return TransactionalOutboxBuilder
      .make(clock)
      .withHandlers(outboxHandlers)
      .withLocksProvider(locksProvider)
      .withStore(outboxStore)
      .build()
  }
}

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

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

Creating a new Outbox Handler

Then you can create a new OutboxHandler that will be responsible for processing the Outbox entries:

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)
}

Creating a new Outbox Entry

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)
}

Monitoring the Outbox Entries

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()
  }
}

Shutting down

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()
  }

}

Local Development

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"))

Publishing

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

Publish via GitHub

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.

Tag formatting

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.

Publish via your workstation

• 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.

Maintainers

The core maintainer of this project are:

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