Order, duplicates and snapshots

These rules come from SOM's conformance rules for consumers and from how any at-least-once bus behaves. They hold however you receive messages: over the HTTPS pull API or from the queue with AWS credentials.

What the bus guarantees

• Every message is valid. Consumers never see a message the gateway refused.
• Order per correlation_id, in the order the bus published them. None across different correlation_ids.
• At least once. The same message_id can arrive again after your own failure, a replay or a redrive.
• The producer's message, unchanged. A reader of the queue with AWS credentials gets its exact bytes; the pull API returns the same envelope, parsed, as JSON.

What your consumer must do

Rule	Why	Test
Choose the payload schema by message_type only	Never by topic, producer or the payload's shape	–
Ignore message_types you don't recognise, without failing	SOM 1.x can add families, and some types have no 1.0 schema yet	T4
Ignore extensions you don't recognise	Vendors add their own, namespaced com.<vendor>.	T4
Never branch on som_version	A 1.1.0 message with the same content means the same thing	T5
Tolerate unknown payload fields and enum values from 1.x producers	SOM's compatibility policy lets 1.x producers add them	T5
Be idempotent on message_id	Redelivery, replay and redrive send messages again	T2
Keep the highest sequence_number per story; drop older snapshots	A redriven or replayed snapshot can arrive after a newer one	T3
Apply a snapshot as a whole replacement, not a merge	A snapshot is the whole story. What's missing from it is gone	T1
Leave KILLED, SPIKED, ARCHIVED and ORPHAN stories out of live views	They aren't live stories	T1

The tests are on Test your consumer.

Why the highest snapshot wins

The bus refuses a snapshot whose sequence_number doesn't increase, so on the way in, snapshots only move forward. On the way out, a message you failed and had redriven from your dead-letter queue lands behind newer messages of its story, and a replay delivers messages you've already applied. Your consumer is the last line: compare sequence_number with the one you hold for that story_id, and drop anything lower or equal.

How the bus treats newer producers

The bus is strict on the envelope and generous on the payload: a message from a newer 1.x producer may carry payload fields and enum values 1.0 doesn't know, and is delivered to you as sent. See Forward compatibility.

Next: Dead-letter queues and replay.