Clarification: relationship between SLIM pubsub session topics and publish topic argument

[therealaditigupta]

Questions

1. If a session is created with a topic, why does publish(session, …, topic) require specifying the topic again?
2. Is a session intended to support multiple topics? If yes:
   • Can I publish to a different topic on the same session?
   • Are there guarantees/semantics around how participants are subscribed to topics within a session (e.g., must they be invited per-topic, or does session membership imply topic access)?
3. If a session is single-topic by design, would it make sense for publish to:
   • Omit the topic parameter (derive it from the session), or
   • Accept an optional topic but validate it matches the session’s topic and error otherwise?

Why I’m asking

From the current surface area it looks like session and topic are tightly tied, so repeating the topic feels like an opportunity for mismatch/bugs.

If sessions are multi-topic, it would be helpful to have docs that explain how to:

• Declare/add topics to a session
• Invite/authorize participants per-topic
• Route/subscribe clients to the right topic(s)
• Understand delivery guarantees across topics within the same session

[therealaditigupta]

Related to this, I see this open PR: #561 that shows multiple sessions being open on the same topic. Does that mean all sessions are privy to all messages on that topic? To achieve true pubsub without invites, could multiple agents act as a "moderator" and join a topic via different sessions and receive all group messages?

[sancyx]

The relation between session & topic depends on the exact session configuration.

There are two types of sessions: FireAndForget, Streaming. You can read more about these here. Basically FireAndForget can be used mostly for One to one (anycast) communication while Streaming is for group communication. Also these can be used with or without MLS and FireAndForget can be set as sticky, meaning all messages will go the first discovered endpoint in case there are multiple apps subscribed with same channel name.

In case you set sticky = true for a FireAndForget session or you enable MLS (enablining MLS will also force the session to be sticky) then the relation between session & topic is 1 -> 1. In this case you can only publish to one topic which is the one you specified when creating a Streaming session or in case of FireAndForget as you don't have to specify the topic name at configuration time, will be first topic you publish to.

In case you don't use MLS and sticky session you can publish to different topics using the same session.

[therealaditigupta]

So to clarify, multi-topic sessions only exist for FireAndForget session types, not Streaming?

[sancyx]

yes