Messages have sequence numbers

Principle

Messages contain a sequence number in their header that assist to process them in the correct order.

Description

Time is not a reliable indicator for the correct ordering of messages in distributed systems, so a publisher should include a sequence number unless the broker can support this out of the box.

Consequences

• Producers need to manage the state of a counter which ensures an atomic increase of the sequence number
• When multiple producers are deployed (e.g. multiple pods for load balancing) the state counter may need to be shared amongst them
• A header will need to be added to all message types that are produced containing the counter

Example

1. Insert COUNTER=1000 into Redis cache
2. On production of an message read COUNTER from Redis cache
3. Set message header SequenceNumber=1001
4. Publish message to broker

References