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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-10296) Replace ZK with a consensus lib(paxos,zab or raft) running within master processes to provide better master failover performance and state consistency
Date Wed, 29 Jan 2014 05:16:10 GMT

    [ https://issues.apache.org/jira/browse/HBASE-10296?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13885014#comment-13885014
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stack commented on HBASE-10296:
-------------------------------

Sweet [~liyin].  I was going to write that 1. would be fun but looks like it'd be a bit of
work going by this list http://raftconsensus.github.io/.  There does not seem to be a complete
easy-to-integrate implementation as yet, not unless, we go JNI it.  Varying group membership
and compacting the logs we'd have to contrib.  I suppose we'd write the logs to the local
filesystem if we want edits persisted.

On 2. above [~fenghh], We'd have no callback/notification mechanism so it will take a little
more effort replacing the zk-based mechanism.  We'd have to add being able to pass state messages
for say when a region has opened on a regionserver or is closing...
On 3. and 4., agree.

On undoing the notion of a 'master', we'd also be simplifying hbase packaging and deploy.
 There would be no more need to set aside machines for this special role.  Lets keep chatting
on this one.

> Replace ZK with a consensus lib(paxos,zab or raft) 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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