Message Queue

One of more popular Messaging Patterns
Async communication on an infrastructural level
The Queue itself is just a data structure/container and usually needs a Broker (Queue Manager)
This is different from distributed Task Queue , Message Queue compliment Task queues.

Message Broker (Queue Manager)

Sort of middleware that can handle things like routing, validate, store, handle backpressure, deliver stuff etc.

Ack Acknowledges a message was completely handled
Nak Signals that the message will not be processed now and processing can move onto the next message, NAK’d message will be retried
- https://docs.nats.io/using-nats/developer/develop_jetstream#acknowledging-messages

User submits a request via the web application
Backend handles that message by adding a message to the message queue.
Consumer gets message, makes HTTP call to another web service to actually submit the request.
From there, the polling logic takes over
Subsequent messages on the queue each represent a polling attempt to retrieve the results.
If job has no results, consumer places message back on the queue
- We can delay the next polling attempt (configurable).
- Delay logic can use TTL and dead letter definitions etc.

Bail out

Failed messages will be written to a DLQ
Now you can inspect items in the DLQ and after making any required changes you can now re-process the message from the DLQ
Almost always needs manual intervention or code to handle stuff written to DLQ