Dead Letter Queue

https://blueprints.launchpad.net/zaqar/+spec/dead-letter-queue

Problem description

Currently, Zaqar can’t do anything if the message has been claimed many times and still can’t be processed successfully. That could be nice if Zaqar can provide the dead letter queue support so that user can provide an existing queue A for queue B’s dead letter queue. Dead letter queue storing these messages allows developers to look for common patterns and potential software problems.

Proposed change

1. Add three new reserved attributes for queue’s metadata: _max_claim_count and _dead_letter_queue.

_max_claim_count is the max number the message can be claimed. Generally, it means the message cannot be processed successfully. There is no default value for this attribute. If it’s not set, then that means this feature won’t be enabled for current queue.

_dead_letter_queue is the target the message will be moved to when the message can’t processed successfully after meet the max claim count. It’s not supported to add queue C as the dead letter queue for queue B where queue B has been set as a dead letter queue for queue A. It’s not the case we want to address in this spec. Technically, even though user can set a dead letter queue C for queue B(when queue B is a dead letter queue of queue A), because the moving is triggered by claim, so that means if user won’t explicitly claim messages in queue B, then nothing happened.

_dead_letter_queue_messages_ttl is the new TTL setting for messages when moved to DLQ. If it’s not set, current TTL will be kept.

2. Make sure the DLQ and current queue are in the same pool when doing validation to get better performance.

3. Add a new filed for message in database which can be used to save the claim count.

  1. Changes for claim create method

    4.1. get current claim count from the message, update the claim count by plus one.

    4.2. check if the claim count has exceeded the max claim count defined in queue’s metadata. If it doesn’t exceed the _max_claim_count, just do normal claim.

    4.3. If the claim count does exceed the _max_claim_count, move the message to the dead letter queue directly. Given the goal of DLQ, we’d like to keep the claim information of the message. So depends on the storage backend, the “move” could introduce a limitation. For example, for MongoDB backend, we may have to move the message from the source queue to DLQ by changing the queue name directly. Which means the two have to be created on same storage pool.

    After moved the message, Zaqar will do nothing against those messages in dead letter queue. User could/will use these message to debug why those messages can’t be processed successfully. In other words, allows developers to look for common patterns and potential software problems from those messages.

    4.4. messages moved to DLQ will be updated their TTL based on the setting _dead_letter_queue_messages_ttl. If it’s not set, currentl TTL of messages will be kept.

Drawbacks

Based on current design, we may have a limitation because of the technical trade off. When move the message from source queue to dead letter queue, it’d be nice to keep the claim info, but if we want to do that, we have to make sure the two queues are on same storage pool. The implementation could be different on different back ends.

Alternatives

None

Implementation

Assignee(s)

Primary assignee:
flwang <flwang@catalyst.net.nz>

Milestones

Target Milestone for completion:
Pike-3

Work Items

  1. Add _max_claim_count and _dead_letter_queue as reserved attributes in queue metadata
  2. Add validation for the new reserved attributes
  3. Add a new filed for message to count the claim, this needs to be done against maongoDB, swift and redis.
  4. Add change in claim create which will add the claim count for messages and check if the count has exceed the max claim count defined in queue’s metadata. If it has exceeded, move the message to the dead letter queue.
  5. Add release note for this feature
  6. Update API reference
  7. Add user/developer document for this feature
  8. Change unit, functional and tempest tests accordingly.

Dependencies

None

Note

This work is licensed under a Creative Commons Attribution 3.0 Unported License. http://creativecommons.org/licenses/by/3.0/legalcode

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