How to implement mailbox and manually acknowledge received messages without Message reference?

[zaynetro]

Hi! I am building a mailbox that client can query for new messages.

In my current implementation client provides last seen offset. I delete all messages with offset less then client provided. Then I fetch all messages with bigger offset from the database. This way if client fails to receive an HTTP response it could retry the request with the old offset.

How could I implement this with NATS?

I see a couple of options:

Option A: Get stream, get persistent consumer, manually inform NATS about last consumer sequence (client provided), fetch the messages.

This is a direct translation from my current implementation.

1. A stream per client (easy)
2. A consumer per client (easy)
3. Delete messages older than client provided sequence / messages that client acked (easy: WorkQueue policy)
4. Clean up consumer after a period of inactivity (easy: inactive_threshold config option)
5. Delete stream after consumer has been deleted (??)
6. Manually acknowledge a consumer sequence (??)

(5) is nice to have but not necessary. (6) is crucial, on the other hand, and I can't see how to do that without having a Message reference.

Option B: Get stream, create ephemeral consumer that starts from client provided sequence, fetch the messages.

1. A stream per client (easy)
2. An ephemeral consumer per client (easy)
   • Starts from client provided offset (easy)
3. Delete messages older than client provided sequence / messages that client acked (??)

(3) With WorkQueue policy and ephemeral consumer all messages will be deleted once consumer unsubscribes but only acked should be.

Option C: Connect directly to NATS from client's device (skip the server completely).

This is theoretically possible but I wouldn't want to do that.

P.S I am coming from Kafka world...

[zaynetro]

Apparently, the simplest way would be to run NATS client on the device itself without a server in the middle.