Dear Community,
Would like to share some design ideas around the above topic. Feedback is welcomed!
Current State
- in "pull mode" [1] we have the same guarantees as the multisite syncing mechanism (guarantee against HW/SW failures). On top of that, if writing the event to RADOS fails, this trickle back as sync failure, which means that the master zone will try to sync the pubsub zone
- in "push mode" [2] we send the notification from the ops context that triggered the notification. The original operation is blocked until we get a reply from the endpoint. As part of the configuration for the endpoint, we also configure the "ack level", indicating whether we block until we get a reply from the endpoint or not.
Since the operation response is not sent back to the client until the endpoint acks, this method guarantees against any failure in the radosgw (at the cost of adding latency to the operation).
This, however, does not guarantee delivery if the endpoint is down or disconnected. The endpoint we interact with (rabbitmq, kafka) , usually have built in redundancy mechanism, but this does not cover the case where there is a network disconnect between our gateways and these systems.
In some cases we can get a nack from the endpoint, indicating that our message would never reach the endpoint. But we can only log these cases:
- we cannot fail the operation that triggered us, because we send the notification only after the actual operation (e.g. "put object") was done (=no atomicity)
- no retry mechanism (in theory, we can add one)
Next Phase Requirements
We would like to add delivery guarantee to "push mode" for endpoint failures. For that we would use a message queue with the following features:
- rados backed, so it would survive HW/SW failures
- blocking only on local read/writes (so it introduces smaller latency than over-the-wire endpoint acks)
- has reserve/commit semantics, so we can "reserve" before the operation (e.g. "put object") was done, and fail it if we cannot reserve a slot on the queue, and commit the notification to the queue only after the operation was successful (and unreserve if the operation failed)
- we would have a retry mechanism based on the queue, which means that if a notification was successfully pushed into the queue, we can assume it would (eventually) be successfully delivered to the endpoint
Proposed Solution
- use the cls_queue [3] (cls_queue is not omap based, hence, no builtin iops limitations)
- add reserve/commit functionality (probably store that info in the queue head)
- a dedicated thread(s) should be reading requests from the queue, sending the notifications to the endpoints, and waiting for the replies (if needed) - this should be done via coroutines
- acked requests are removed from the queue, nacked or timed-out requests should be retried (at least for a while)
- both mechanism would coexist, as this would be configurable per topic
- as a stretch goal, we may add a "best effort queue". This would be similar to the cls_queue solution, but won't address radosgw failures (as the queue would be in-memory), only endpoint failures/disconnects
- for now, this mechanism won't be supported for pushing events from the pubsub zone (="pull+push mode"), but might be added if users would find it useful
Yuval