Asynchronous infrastructure.
WHAT A BROKER PROVIDES
Reliable delivery of messages between producers and consumers.
WHAT THE PATTERNS ARE
A queue, where one consumer processes each message Publish and subscribe, where every subscriber receives it
WHAT TO CHOOSE
A queue for work distribution Publish and subscribe for notifying several systems
WHAT DELIVERY GUARANTEES MEAN
- At most once: may be lost
- At least once: may be duplicated
- Exactly once: rarely achievable in practice
WHAT TO DESIGN FOR
At least once, meaning duplicates.
WHAT THAT REQUIRES
Processing that is safe to repeat.
WHAT ACKNOWLEDGEMENT DOES
Confirms a message was processed, so it is not redelivered.
WHEN TO ACKNOWLEDGE
After processing succeeds, not on receipt.
WHY
Acknowledging first loses the message if processing fails.
WHAT A DEAD LETTER DESTINATION IS FOR
Messages that repeatedly fail, so they do not block the queue.
WHAT TO MONITOR
Depth, age of the oldest message, and the dead letter destination.
WHAT TO PLAN
What happens when the broker is unavailable.
