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From "Feng Honghua (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 Fri, 07 Feb 2014 10:03:24 GMT

    [ https://issues.apache.org/jira/browse/HBASE-10296?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13894362#comment-13894362

Feng Honghua commented on HBASE-10296:

bq.The only thing is that, this would require multi master setup unless we keep the logs in
hdfs and be able to use a single node RAFT quorum right?
Do you mean master back-compatibility issue? 

No master back-compatibility when replacing zk with embedded consensus lib, nor between different
incremental phases.

My point when proposing increment approach is that the data/functionalities provided by zk
have different urgency level to be moved to inside master processes, data such as region assignment
info has top urgency level to be moved inside master processes, configuration-like data has
less urgent level(which needs additional watch/notify feature), liveness monitoring functionality
has the least urgent level(which needs additional heart-beat feature)...we can remarkably
improve master failover performance and eliminate inconsistency by replicating region assignment
info among master processes' memory, the foremost concern/goal of this jira.

To eliminate ZK as a whole to reduce deploying and machine number/roles, as [~stack] said
"...simplifying hbase packaging and deploy. There would be no more need to set aside machines
for this special role", we also need to implement liveness monitoring(for regionservers) and
watch/notify features within master processes...

Above features are independent and can be implemented in an incremental fashion, that's what
I meant by 'incremental', but certainly we can implemented them as a whole.

Not sure I understand your question correctly, and wonder whether I answer your question,
any further clarification is welcome :-)

> 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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