Implementation sketch for a reliable event bus with both task queuing and pub/sub functionality, originally based on this restbase doc.
A popular and well supported option is to use websockets or long polling. Fall-back to polling is common in websocket libraries.
Goal: deliver each message at least once while avoiding duplicate processing.
Kafka splits a topic into many partitions.
- Each partition's *commit* offset is stored in zookeeper
- Reading from a partition is separate from committing the offset. Client keeps track of read offset
- Offset can be *committed* once acks are received by service clients (auto.commit.enable = false)
- Clients rebalance partitions on join; need more (~2x?) partitions than clients
Error / timeout handling
- Each message has an associated timeout
- configured per queue
- can be increased by client up to 12 hours (AWS) in poll request or per message ('heartbeat')
- If no ack is received within this timeout window:
- message is placed in internal queue & handed out again *once*; messages are only committed up to the failed message
- if retry succeeds, all is well
- else: place message(s) into retry queue, ack primary queue
- dequeue messages from retry queue as well, keep counter of retries
- once max number of retries reached, place message in separate dead-letter queue for inspection (ideally with some debugging info)
Goal: Deliver each message to all consumers.
- use one consumer group id per pub/sub client
- each consumer group maintains its own commit offset, so speed independent
- error handling as in queuing
- rebalance periodically based on committed offsets in each partition?