In Kafka Streams we have two restoration callbacks:
- RestoreCallback (BatchingRestoreCallback): specified per-store via registration to specify the logic of applying a batch of records read from the changelog to the store. Used for both updating standby tasks and restoring active tasks.
- RestoreListener: specified per-instance via `setRestoreListener`, to specify the logic for `onRestoreStart / onRestoreEnd / onBatchRestored`.
As we can see these two callbacks are for quite different purposes, however today we allow user's to register a per-store RestoreCallback which is also implementing the RestoreListener. Such weird mixing is actually motivated by Streams internal usage to enable / disable bulk loading inside RocksDB. For user's however this is less meaningful to specify a callback to be a listener since the `onRestoreStart / End` has the storeName passed in, so that users can just define different listening logic if needed for different stores.
On the other hand, this mixing of two callbacks enforces Streams to check internally if the passed in per-store callback is also implementing listener, and if yes trigger their calls, which increases the complexity. Besides, toggle rocksDB for bulk loading requires us to open / close / reopen / reclose 4 times during the restoration which could also be costly.
Given that we have KIP-441 in place, I think we should consider different ways other than toggle bulk loading during restoration for Streams (e.g. using different threads for restoration).
The proposal for this ticket is to completely decouple the listener from callback – i.e. we would not presume users passing in a callback function that implements both RestoreCallback and RestoreListener, and also for RocksDB we replace the bulk loading mechanism with other ways of optimization: https://rockset.com/blog/optimizing-bulk-load-in-rocksdb/