1. Absolutely. clickhouse-keeper is distributed as a standalone static binary or .deb package or .rpm package. You can use it without clickhouse as ZooKeeper replacement. 2. It's not recommended to use slow storage devices for logs in any coordination system (zookeeper, clickhouse-keeper, etcd and so on). Good setup will be small fast SSD/EBS disk for fresh logs and old logs + snapshots offloaded to S3. In such setup the amount of PUT requests will be tiny and latency will be as good as possible.

Yes, you are right ClickHouse Keeper is based on NuRaft. We did a lot of modifications for this library, both for correctness and performance. Almost all of them (need to check) are contributed back to upstream ebay/NuRaft library.

Yes, for internal RAFT implementation boost.asio is used.

All ZooKeeper libraries are compatible with clickhouse-keeper. The most popular and mature is https://kazoo.readthedocs.io/en/latest/. We use it in our integration tests framework (with clickhouse-keeper) a lot.

Usage of multi-armed bandits for low-level optimizations looks fairly unusual, I cannot remember any other examples in open-source codebases.