# Transactional Events (MySQL to Kafka)

This example shows how to use the SQL Subscriber from the [SQL Pub/Sub](https://github.com/ThreeDotsLabs/watermill-sql). 

## Background

When producing domain events, you may stumble on a dilemma: should you first persist the aggregate to the storage and
then publish a domain event or the other way around? Whatever order you choose, one of the operations can fail and
you will end up with inconsistent state.

For more detailed description, see [When an SQL database makes a great Pub/Sub](https://threedots.tech/post/when-sql-database-makes-great-pub-sub/).

## Solution

This example presents a solution to this problem: saving domain events in transaction with the aggregate in the same
database and publishing it asynchronously.

The SQL subscriber listens for new records on a MySQL table. Each new record will result in a new event published
on the Kafka topic. Kafka Publisher is used just as an example and any other publisher can be used instead.

The example uses `DefaultMySQLSchema` as the schema adapter, but you can define your own table definition and queries.
See [SQL Pub/Sub documentation](https://watermill.io/pubsubs/sql) for details.

## Requirements

To run this example you will need Docker and docker-compose installed. See installation guide at https://docs.docker.com/compose/install/

## Running

```bash
docker-compose up
```

Observe the log output. You will notice new events, generated by the example.

In another terminal, run the following command to consume events produced on the Kafka topic.

```bash
docker-compose exec server mill kafka consume -b kafka:9092 -t events
```