Supporting parallel raft

[BusyJay]

Is your feature request related to a problem? Please describe.
Paper introduces a mechanism that allows applying raft logs out of order. Quotes from the paper:

. As the I/O depth increases, the performance gap between two protocols becomes wider. When I/O depth grows to 32, the
latency of Raft is approximately 2.5X times of the latency of ParallelRaft, with less than half of the IOPS ParallelRaft achieves.

This feature is also useful in TiKV.

1. TiKV already have memory locks for transaction writes, so the dependency between raft entries are already known.
2. If we support multi-range in the future, each range is independent by design, which can reduce the latency and even make it possible to scale vertically in single group.

[Fullstop000]

If we support multi-range in the future, each range is independent by design,

What does multi-range mean?

[BusyJay]

It means single raft group manages multiple ranges.

[Fullstop000]

It means single raft group manages multiple ranges.

Only set up a single raft group but serve the commands and replication for multiple regions?

[BusyJay]

Yes, exactly.

[breezewish]

https://zhuanlan.zhihu.com/p/228335745 may be helpful which provides a TLA+ formal verification, as well as a detailed explanation for the original algorithm.

[gengliqi]

https://zhuanlan.zhihu.com/p/228335745 may be helpful which provides a TLA+ formal verification, as well as a detailed explanation for the original algorithm.

I have seen it yesterday. What a coincidence！🤣