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Raw Messaging Reliability

What this chapter answers — it separates what Core guarantees for delivery from what it does not guarantee, which then becomes the application's or Framework's responsibility (retry, deduplication, durable storage).

Core provides the delivery rules of the transport and socket pattern, but it does not create an application-level delivery guarantee. Retry, deduplication, durable storage, and business transactions are the application's or Framework's responsibility.

Queue And Backpressure

Once the HWM is reached, a send can return a backpressure result. Setting a blocking timeout does not mean delivery completed or that remote processing completed. Even after a send-ready notification, another sender may claim the queue first, so check the retry result.

PUB/SUB

A subscriber cannot receive a message published before it sets up the subscription and connection. A slow subscriber's processing is limited by the HWM and publisher options. Design a separate synchronization or resend path for an important event.

Request/Reply

A request timeout means the reply did not arrive within the time limit — it is not proof that the remote never processed the request. To retry a request that has side effects, use an application request id and a deduplication rule.

Connection Transitions

Transport state and route availability change during a reconnect. Success from zlink_connect() means the connection intent was accepted, not that sending is immediately possible. Use the poller, send-ready, and socket monitor to observe the actual state.