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Tor

Tor (Transactional Outbox Router) it's a CDC (Change Data Capture) service that reads events from an outbox table using the MySQL binlog and writes them on Kafka preserving order.

It avoids reprocessing the same binlog entries on restart, persisting state on Redis.

Tor is inspired by Debezium Outbox Event Router and airbnb/SpinalTap. It is designed to be lightweight, simple, modular and easy to install as a container.

Use case

You have a service that persists state changes on a database (MySQL) and you want other services that do not share the same process/memory to be notified of the state change so they can react accordingly. This pattern is known as Pub/Sub, where notifications are called messages or events, those who publish them are called Publishers or Dispatchers, and those who consume the messages are known as Subscribers or Consumers.

Guarantees:

  • Message publishing must be eventually consistent with respect to the state change itself, and no change is lost.
  • Messages must be delivered in the same order that the state changes occurred. Delivering messages to a log-based stream broker (e.g., Kafka), ensures subscribers to process them in order and it avoids concurrency issues (by partition).
  • Messages may be duplicated and processing by subscribers must be idempotent.

Examples:

  • You want to build a microservice system that communicates in an event-driven (asynchronous) manner, and you want a reliable and consistent communication system that avoids out-of-order messages and message loss problems due to dual-writes .
  • You want to implement CQRS (or simply cache and denormalization) by building read-models asynchronously and consistently.
  • You want to decouple your monolithic code so that you communicate through a message-broker instead of through procedure calls, databases, or RPCs.

If you want to go deep on the topic we recommend reading this article by Martin Kleppmann or his amazing book Designing Data-Intensive Applications .

Modules

Tor is composed of several modules so it can be extensible and make dependencies footprint minimal.

  • router: contains the core of Tor. It is based on github.com/go-mysql-org/go-mysql.
  • adapters: contains the adapters with which router can be built to run a tor app.
    • kafka: an event dispatcher for Kafka.
    • redis: a state handler for Redis.
  • example: contains examples of tor apps.
    • tor: an example instance of router app using kafka and redis adapters.
    • api-server: an example api-server implementing a business logic, persisting state and producing events.
  • .devenv: developing and running examples locally. It uses Go Workspaces, so every change applied to a module is reflected automatically without the need of using replace or pseudo-versions.

Run example

Set up the system:

make up

Invoke api-server API:

curl --request PUT 'localhost:8080/29fd8c5b-6b44-48b6-98a3-c1c601eaae26'

curl --request PUT 'localhost:8080/5c0e9be6-ea69-452d-9824-7f55b544f2e3'

curl --request DELETE 'localhost:8080/29fd8c5b-6b44-48b6-98a3-c1c601eaae26'

Use plumber or similar to see events written in the Kafka topic:

plumber read kafka --address=localhost:9093 --topics=outbox_topic -f