4 posts tagged this topic.
A mail guard that throws an exception when it blocks a message teaches the retry queue to keep attempting a send that will never be allowed to leave the process — the harder design keeps a policy block and a transport failure looking nothing alike, then proves a worker killed after the send is recorded but before acknowledgment leaves one copy, not two.
A Docker Compose healthcheck gate held the ingestion worker back until the simulator had already fired a full day of factory telemetry at nobody — MQTT's clean:false plus QoS 1 only replay into a session that already exists, and none did yet.
AIDA's outbound email job derives References and In-Reply-To by querying every prior email-bearing message on the ticket. On a retry, the message being sent already has its own Message-ID persisted from the failed first attempt — without an explicit exclusion, it would show up in its own References chain.
Every webhook integration works in the demo. Production loses events in the silence after you reply 200 — so I built OmniSync around one guarantee: once an event is acknowledged, it is never silently lost. At-least-once delivery, idempotent everything.