Currently master relies on ZK to elect active master, monitor liveness and store almost all of its states, such as region states, table info, replication info and so on. And zk also plays as a channel for master-regionserver communication(such as in region assigning) and client-regionserver communication(such as replication state/behavior change).
But zk as a communication channel is fragile due to its one-time watch and asynchronous notification mechanism which together can leads to missed events(hence missed messages), for example the master must rely on the state transition logic's idempotence to maintain the region assigning state machine's correctness, actually almost all of the most tricky inconsistency issues can trace back their root cause to the fragility of zk as a communication channel.
Replace zk with paxos running within master processes have following benefits:
1. better master failover performance: all master, either the active or the standby ones, have the same latest states in memory(except lag ones but which can eventually catch up later on). whenever the active master dies, the newly elected active master can immediately play its role without such failover work as building its in-memory states by consulting meta-table and zk.
2. better state consistency: master's in-memory states are the only truth about the system,which can eliminate inconsistency from the very beginning. and though the states are contained by all masters, paxos guarantees they are identical at any time.
3. more direct and simple communication pattern: client changes state by sending requests to master, master and regionserver talk directly to each other by sending request and response...all don't bother to using a third-party storage like zk which can introduce more uncertainty, worse latency and more complexity.
4. zk can only be used as liveness monitoring for determining if a regionserver is dead, and later on we can eliminate zk totally when we build heartbeat between master and regionserver.
I know this might looks like a very crazy re-architect, but it deserves deep thinking and serious discussion for it, right?