Ability to publish to a kv style topic only if empty

[ripienaar]

Feature Request

Use Case:

Today we have Nats-Expected-Last-Subject-Sequence that limits publishing to a stream if the previous message had a specific sequence

Along the same lines it would be useful to ensure only the first client can write a message when the subject is empty.

We discussed maybe (ab)using Nats-Expected-Last-Subject-Sequence:0 but decided not to, so maybe Nats-Expected-Empty-Subject:1

A sample use case could be for example registering a schema that relates to a specific topic, the first publisher to this topic should register the subject but that's true only ever once.

This setting should probably only be accepted in max_msgs_per_subject > 0 stream

[derekcollison]

Thinking more about this I think the original suggestion should work.

Nats-Expected-Last-Subject-Sequence: 0

[ripienaar]

It needs to mean a subject we have never seen or one that we have seen and purged out.

But yeah I think that’s quite a obvious way to do it

[derekcollison]

Working on it now.