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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 Sat, 08 Feb 2014 01:51:19 GMT

    [ https://issues.apache.org/jira/browse/HBASE-10296?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13895363#comment-13895363

Feng Honghua commented on HBASE-10296:

bq.There has been at least 3 proposals so far for a master + assignment rewrite in HBASE-5487,
and all want to get rid of zk and fix assignment.
Agree, but all those proposals still use third-party storage(from zk to auxiliary system table)
outside of master processes/machines for persisting data such as assign status information,
# the new active master needs to read those data from outside third-party storage before serving
as active master after the previous master dies, hence with suboptimal master failover performance.
# the same data/information still be maintained in two different locations: master memory
and outside third-party storage, hence with potential consistency issues

bq.What I was trying to understand is about the deployment...with the incremental approach,
we might even implement RAFT quorum inside region server processes, so that we gradually get
rid of the master role as well, and have only 1 type of server, where (2n+1) of them would
act like masters (while still serving data).
Now I can understand what you meant:-). If we take incremental approach then 3 zk , 3 master
and N regionservers, yes it's a suboptimal setup:-(.
If we implement all functionalities that zk provides for HBase such as data replicating, master
election, liveness monitor and watch/notify and  eliminate zk totally, the deployment of a
HBase is (3 master + N regionserver)
Though it's workable eventually to concurrently run master and regionserver roles within a
single server, I'm not a fan of this deployment:
# master and regionserver roles can affect each other, it's hard to debug/diagnose when issue
# master and regionserver are both memory-consuming, for servers concurrently running both
roles we need to balance the memory usage, and for servers running only regionserver role
we need regionserver memory/heap configuration different from running both roles to take full
advantage of the available memory

> 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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