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https://issues.apache.org/jira/browse/HBASE-10296?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13868800#comment-13868800
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Feng Honghua commented on HBASE-10296:
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bq.The google chubby paper goes into some detail about why they implemented a
Paxos Service and not a paxos library.
I believe google should have a paxos library, which is used in megastore and
spanner, right? And this fact is mentioned in a google paper *paxos made live*
:-)
Implementing paxos as a standalone/shared service or a library has their own
benefits and drawbacks.
A service: simple API and simple for app to use, can be shared by multiple
apps; but abuse by one app can negatively influence other apps using the same
paxos service (we ever encountered several times such cases before :-()
A library: more difficult for app to use, but have better isolation level(won't
be affected by possible abuse from other app), and have more primitives and
more flexibility.
> 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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