So this is a very basic example of Pub Sub that shows how conventions can work to build relatively simple binding structures, mimicking more modern topic binding implementations in the cloud.

The main conventions are:

1. Every publisher has a single topic exchange for Outbound messages
2. Every subscriber has a single topic exchange for Inbound messages
3. A subscription between services is represented in effect by a binding between the publisher exchange and the subscriber exchange.
   1. This binding defines the routing key as #, which means give me all of your stuff
4. A subscriber subscribes to event types from an publisher
   1. Every subscription to an event is represented by the existence of an inbound queue for that event.
   2. Every subscription to an event is represented by the binding between the inbound event queue and the subscriber exchange
   3. Every binding between the inbound event type queue and the subscriber exchange has a routing key denoting the event type

Out of scope:

• Dead-lettering
  • This is easily set up via the API but out of scope for clarity
• Retries
  • This is a harder task, and would require a worker or client-side implementation
• Handlers
  • Too many opinionated frameworks that ship with their own handlers for queues. Best to leave this library to be a reference for a convention based approach
• Outbox pattern
  • Not even going there, that's an entirely different kettle of fish

1. Make
2. Docker and Docker Compose
3. yarn

yarn
make up
make start

yarn will obviously install all the gubbins you need
make up will start a rabbitmq instance with the management console on the default ports - 5672 for rabbit itself, and 15672 for the ui
make start will spin up several applications - a users service, a facebook service and a pinterest service.

The user service is the publisher, and exposes a simple API endpoint to create and consume invitations. The facebook and pinterest services subscribe to a subset of event types published by the user service.

To send an invitation:

curl --location --request POST 'http:https://localhost:5000/invitations' \
--header 'Content-Type: application/json' \
--data-raw '{
    "company_id": "4aaec88a-c130-4e94-87b8-eb6cb391cca6",
    "user_id": "36bb3076-852c-4042-b5ec-bbfd33ca6e93"
}'

There are no uniqueness checks at all, so this will always publish a new event.

Just one, for now. You can run make ui to open the RabbitMQ management console.

🎉