— The ZooKeeper server cannot be migrated from version 3.4 to 3.3, then back to 3.4, without user intervention.
Upgrading from 3.3 to 3.4 is supported, as is downgrading from 3.4 to 3.3. However, moving from 3.4 to 3.3 and back to 3.4 will fail. 3.4 is checking the datadir for acceptedEpoch and currentEpoch files and comparing these against the snapshot and log files contained in the same directory. These epoch files are new in 3.4.
As a result: 1) Upgrading from 3.3 to 3.4 is fine - the *Epoch files don't exist, and the server creates them. 2) Downgrading from 3.4 to 3.3 is also fine as version 3.3 ignores the *Epoch files. 3) Going from 3.4 to 3.3 then back to 3.4 fails because 3.4 sees invalid *Epoch files in the datadir; 3.3 will have ignored them, applying changes to the snapshot and log files without updating the *Epoch files.
Anticipated Resolution: See workaround
Workaround: Delete the *Epoch files if this situation occurs — the version 3.4 server will recreate them as in case 1) above.