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Currently, Keeper applies the operation only after the commit.
To support real-time digest check of operation results, we need to preprocess the operation at least on the Leader node so each Raft log can be replicated with it afterwards.
This is only the first step for adding support for digests.

Multiple operations can be batched for replication so we cannot calculate the digest by analysing possible changes a single operation will make, we need to process those operations in some way without committing them.

Additionally, the goal was to minimise load on the leader, a possible solution would be to process the requests twice on the leader, first time applying it to some in-memory store, and second time to the real storage but that would require some complex interaction with NuRaft because KeeperStorage would need to be aware of the current role of the node it's running on.

Because of the reasons above, I implemented preprocessing for all the operation in a such way that they minimise the commit itself.
In short, preprocessing transforms the ZooKeeper request in set of deltas.
Those deltas have 2 roles:

• efficient way of getting the latest state of some ZooKeeper node with minimal copying
• efficient way of committing the request by just applying those same deltas to our storage

Deltas are kept entirely in-memory, so applying them should be fast enough, while the data copying is minimal.

Those deltas also play nicely with the NuRaft interface (precommit -> transform into deltas, rollback -> simply delete deltas). If some node becomes a leader, it already has everything preprocessed and it can instantly continue processing the requests without any buildup beforehand.

Things left to do:

• Session management should support preprocessing (ZooKeeper's SessionTracker) - should be okay because fetching the SessionID is a blocking operation, i.e. nothing else can be done until the new session is committed
• Add support for rollback

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