Flask-SignalBus

Flask-SignalBus adds to Flask-SQLAlchemy the capability to atomically send messages (signals) over a message bus.

Important note: Flask-SignalBus does work with Flask-SQLAlchemy 2.5, and does not work with Flask-SQLAlchemy 3.0 or later. And for this reason, SQLAlchemy 2.0 or later is not supported.

The Problem

In microservices, the temptation to do distributed transactions pops up all the time.

Distributed transaction:

any situation where a single event results in the mutation of two separate sources of data which cannot be committed atomically

One practical and popular solution is to pick one of the services to be the primary handler for some particular event. This service will handle the original event with a single commit, and then take responsibility for asynchronously communicating the secondary effects to other services via a message bus of some sort (RabbitMQ, Kafka, etc.).

Thus, the processing of each "distributed" event involves three steps:

1. As part of the original event transaction, one or more messages are recorded in the SQL database of the primary handler service (as rows in tables).
2. The messages are sent over the message bus.
3. Messages' corresponding table rows are deleted.

Flask-SignalBus automates this process and make is less error prone. It automatically sends the recorded messages after each transaction commit (steps 2 and 3). Also, when needed, the sending of the recorded messages can be triggered explicitly with a method call, or through the Flask command line interface.

You can read the docs here.