Fangmin Lv created ZOOKEEPER-3783:
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Summary: Build ZooKeeper based on RocksDB
Key: ZOOKEEPER-3783
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3783
Project: ZooKeeper
Issue Type: New Feature
Components: server
Reporter: Fangmin Lv
Here are the things we have done so far with ZK on RocksDB project, it's in an
initial phase, we don't have time to explore more, but put here in case the
community is interested in and can help move this forward.
*[Ideas]*
When we were designing and working on this project, it was set into 3 phases:
*1. Moving ZK snapshot onto RocksDB to reducing snapshot disk IO*
It still keeps the in memory DataTree but uses RocksDB for it's snapshot to
leverage the benefit of incremental snapshot in RocksDB to reduce the write
amplification when having large snapshot but only small partial of hot znodes
being changed.
Since ZK has it’s own txn file, the WAL in RocksDB could be disabled to avoid
double writing.
*2. Removing in memory DataTree to get rid of memory bound*
Read data from RocksDB (maybe with some cache), but keep key structures like
children in memory, so ZK won't be bounded to the memory limit, and still have
good through for APIs like getChildren.
*3. Moving txn log into RocksDB with different column family*
Consider to move the log into the RocksDB to get rid of the ZK txn files, so
that we don't need to manage the txn files ourselves and probably the
backup/restore would be simpler.
*[Implementation for phase 1]*
The attachment is the initial version of for the 1st phase, it may not able to
apply to the latest master code since it was based on a very old commit, but
you can get some idea from this.
The idea is simple, when changes are made in DataTree, they're recorded and
applied to the RocksDB when processTxn in ZKDatabase.
When loading from disk, it will iterate through the RocksDB keys to restore the
in memory DataTree.
It also considered things for slowly rollout or rollback, so it can support
reading from snapshot file and write to RocksDB, or read from RocksDB and write
to file during runtime based on the flag.
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