Acn Outbox Starter

acn-outbox-starter is a Spring Boot starter that implements the outbox pattern to ensure reliable event processing in distributed systems. It is agnostic to the specific event handling logic, leaving the responsibility to the client code to manage the actual record operation as well as handling the events that have been recorded in the outbox.

Table of Contents

• Installation
  • Prerequisites
• Usage
  • Implementing OutboxItemHandlerProvider
  • Using OutboxEventRecorder
• Features
• Configuration
• Observability
  • Metrics Exposed
  • Sample Prometheus Queries
• Contributing
• License
• Contact

Installation

To use acn-outbox-starter, add the following dependency to your pom.xml:

<dependency>
    <groupId>it.gov.acn</groupId>
    <artifactId>acn-outbox-starter</artifactId>
    <version>?.?.?</version>
</dependency>

Prerequisites

• Database: Ensure your database has the required transactional_outbox table:

CREATE TABLE IF NOT EXISTS transactional_outbox (
    id UUID PRIMARY KEY,
    event_type VARCHAR(255) NOT NULL,
    creation_date TIMESTAMP WITH TIME ZONE NOT NULL,
    last_attempt_date TIMESTAMP WITH TIME ZONE,
    completion_date TIMESTAMP WITH TIME ZONE,
    attempts INT NOT NULL,
    event TEXT NOT NULL,
    last_error TEXT
);

• Shedlock: This starter requires Shedlock for handling locks, so ensure the Shedlock table is created as prescribed by Shedlock.

Usage

There are only two things the client app needs to do:

1. Implement the OutboxItemHandlerProvider interface to handle the outbox items. The outbox items are those that are retrieved at each outbox run, according to the configuration provided.
2. Use the OutboxEventRecorder bean to record events into the outbox.

Implementing OutboxItemHandlerProvider

Create a class that implements the OutboxItemHandlerProvider interface and provides the logic for handling the outbox items.

@Component
public class MyOutboxItemHandlerProvider implements OutboxItemHandlerProvider {
    @Override
    public void handle(OutboxItem outboxItem) {
        // Implement your message sending logic here
    }
}

Important: Ensure that any exceptions encountered during event handling are thrown or rethrown so that the outbox knows the sending didn't succeed and should be retried in future runs.

Using OutboxEventRecorder

Inject the OutboxEventRecorder bean and use it to record events into the outbox.

@Service
public class MyBusinessService {

  private final OutboxEventRecorder outboxEventRecorder;

  @Autowired
  public MyBusinessService(OutboxEventRecorder outboxEventRecorder) {
    this.outboxEventRecorder = outboxEventRecorder;
  }

  @Transactional
  public void createSomething() {
    MyType something;
    // Perform your business logic here
    // ...
    // At the appropriate moment, record the event in the outbox
    outboxEventRecorder.recordEvent(something, "SomethingCreatedEvent");
  }
}

Features

• Implements the outbox pattern for reliable event processing.
• Supports custom event handling through OutboxItemHandlerProvider.
• Configurable scheduling for event processing with retry logic.
• Integrates with Shedlock for distributed lock management.

Configuration

Configure the outbox scheduler and other properties in your application.yml:

acn:
  outbox:
    scheduler:
      enabled: true
      table-name: transactional_outbox
      fixed-delay: 60000 # milliseconds
      max-attempts: 3
      backoff-base: 5    # minutes

Configuration Properties

Property	Description	Default Value
acn.outbox.scheduler.enabled	Enable or disable the outbox scheduler.	false
acn.outbox.scheduler.table-name	The name of the database table used for the outbox.	transactional_outbox
acn.outbox.scheduler.fixed-delay	Fixed delay in milliseconds between the end of the last invocation and the start of the next.	60000 (60 seconds)
acn.outbox.scheduler.max-attempts	The maximum number of attempts to process an item from the outbox, after which it will no longer be processed.	3
acn.outbox.scheduler.backoff-base	The base value for the backoff calculation in minutes. See the example below for how backoff is calculated.	5 (minutes)

Backoff Calculation Example

With backoff-base=5 and max-attempts=4:

• First attempt: as soon as the scheduler runs.
• Second attempt: 5 minutes after the first failed attempt.
• Third attempt: 25 minutes after the second failed attempt.
• Fourth attempt: 125 minutes after the third failed attempt.

Observability

To provide observability into the outbox processing, acn-outbox-starter integrates with Prometheus through custom metrics. This allows for monitoring key performance indicators and operational metrics related to the outbox.

Metrics Exposed

The following custom metrics are exposed:

• outbox.observation_start: A gauge indicating the start time of the observation. This metric helps in understanding when the metrics collection period started.
• outbox.last_observation: A gauge showing the last time any metric was collected. This metric is useful for tracking the freshness of the metrics data.
• outbox.queued: A counter tracking the number of events enqueued in the outbox during the current period.
• outbox.successes: A counter tracking the number of events that have been successfully processed during the current period.
• outbox.failures: A counter tracking the number of events that have failed to be processed during the current period.
• outbox.dlq: A counter tracking the number of events moved to the Dead Letter Queue (DLQ) during the current period.

Sample Prometheus Queries

# Absolute Number of Queued Messages
outbox_queued_total

# Absolute Number of Successful Messages
outbox_successes_total

# Absolute Number of Failed Messages
outbox_failures_total

# Absolute Number of DLQ Messages
outbox_dlq_total

# Rate Queries (rate of change over the past 5 minutes)

# Rate of Queued Messages
rate(outbox_queued_total[5m])

# Rate of Successful Messages
rate(outbox_successes_total[5m])

# Rate of Failed Messages
rate(outbox_failures_total[5m])

# Rate of DLQ Messages
rate(outbox_dlq_total[5m])

# Percentage of Successful Messages Over 5m Time
rate(outbox_successes_total[5m]) / (rate(outbox_successes_total[5m]) + rate(outbox_failures_total[5m])) * 100

# Percentage of Failed Messages Over 5m Time
rate(outbox_failures_total[5m]) / (rate(outbox_successes_total[5m]) + rate(outbox_failures_total[5m])) * 100

# Success / Enqueued ratio
rate(outbox_successes_total[5m]) / rate(outbox_queued_total[5m]) * 100

# Success / Failure ratio
rate(outbox_successes_total[5m]) / rate(outbox_failures_total[5m]) * 100

Contributing

Contributions are welcome! Please fork the repository and submit a pull request for review.

License

This project is licensed under the MIT License - see the LICENSE file for details.

Contact

For any inquiries or support, please don't contact me.