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https://issues.apache.org/jira/browse/HBASE-10296?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13868787#comment-13868787
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Feng Honghua commented on HBASE-10296:
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bq.but that ZK path is used to find the hbase master even if it moves round a
cluster -what would happen there?
Typically we adopt master-based paxos in practice, so naturally the master
process hosting the master paxos replica is the active master. the active
master is elected by paxos protocal, not by zk. and each standby master knows
who is the current active master. when the active master moves around(for
instance when active master dies or its lease timeout), the client or app who
attempts to talk with the old active master will fail in two ways: fail to
connect if active master dies, or fail by knowing it's now not the active
master and the current new active master info. for the former the client/app
will try randomly other alive master instance and that master will accept its
request if it's the new active master, or tell it the current active master
info if it's not the current active master. for the latter it can now talk to
the active master...and like how to access a zk, client/app should know the
master assemble addresses to access a HBase cluster. (assuming you're saying
finding the active master, correct me if I'm wrong)
> Replace ZK with a paxos running within master processes to provide better
> master failover performance and state consistency
> ---------------------------------------------------------------------------------------------------------------------------
>
> Key: HBASE-10296
> URL: https://issues.apache.org/jira/browse/HBASE-10296
> Project: HBase
> Issue Type: Brainstorming
> Components: master, Region Assignment, regionserver
> Reporter: Feng Honghua
>
> 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?
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