Message ordering guarantees in core NATS

[Marko19907]

I'm trying to understand the exact scope of ordering guarantees in core NATS, not JetStream.

Scenario

• 1 server

• 1 publisher (Client A), 1 subscriber (Client B)

• Client A uses a single connection and does:

  1. Publish("foo", m1)
  2. Publish("foo", m2)
  3. Publish("bar", m3)
  4. Publish("foo", m4)

• Client B (same process / single connection) has:

  • Subscribe("foo", ...)
  • Subscribe("bar", ...)

All subscriptions are created before any publish.

Is ordering guaranteed only per subject / subscription (i.e. m1, m2, m4 always in order on the foo subscription), or is there any notion of a global order across all subjects in a connection?

[jnmoyne]

For Core NATS the ordering is guaranteed per publisher (connection): if a publisher uses a single connection to publish m1, m2, m3 then everyone subscribing to the subject(s) will see the messages in that order (there could be more than one publisher at the same time so their publications could get interlaced, but you will always see m2 after m1 and m3 after m2). This is true no matter what subjects the messages are published on (i.e. m1 m2 and m3 can be on any subject, the server processes the messages in the order it receives them from the client connection).

[Marko19907]

Thanks, that helps. Just to double-check my understanding:

For a single publisher connection, publishing m1, m2, m3, m4 on arbitrary subjects, the server delivers them to each subscriber connection in that same order, so that's one FIFO stream across subjects, and then the client library just demuxes them to the appropriate subscription callbacks. Is that accurate?

[derekcollison]

Yes for a single publish connection order will always be preserved globally.