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From "Feng Honghua (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-9469) Synchronous replication
Date Tue, 21 Jan 2014 05:44:21 GMT

    [ https://issues.apache.org/jira/browse/HBASE-9469?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13877226#comment-13877226
] 

Feng Honghua commented on HBASE-9469:
-------------------------------------

after some rethought, I think this feature has less value than seemingly: if we response write
to caller until the push to peer succeeds, both master/peer clusters are required to be available,
this means from an overall perspective we just trade-off write availability for read availability
 but without overall availability improvement. 
on the other hand, such 'synchronous replication' in functionality is almost equal to letting
client write to both clusters, and with worse performance: client can issue write concurrently
to both clusters while synchronous replication serializes the writes to the two clusters,
and additional memory overhead resulted from duplicate kvs in peer cluster(replication sink
plays the role as a proxy client which receives the push from master cluster and then write
the local cluster as a hbase client, the kvs duplicate in peer cluster from an overall view)

> Synchronous replication
> -----------------------
>
>                 Key: HBASE-9469
>                 URL: https://issues.apache.org/jira/browse/HBASE-9469
>             Project: HBase
>          Issue Type: New Feature
>            Reporter: Feng Honghua
>            Assignee: Feng Honghua
>
> Scenario: 
> A/B clusters with master-master replication, client writes to A cluster and A pushes
all writes to B cluster, and when A cluster is down, client switches writing to B cluster.
> But the client's write switch is unsafe due to the replication between A/B is asynchronous:
a delete to B cluster which aims to delete a put written earlier can fail due to that put
is written to A cluster and isn't successfully pushed to B before A is down. It can be worse
if this delete is collected(flush and then major compact occurs) before A cluster is up and
that put is eventually pushed to B, the put won't ever be deleted.
> Can we provide per-table/per-peer synchronous replication which ships the according hlog
entry of write before responsing write success to client? By this we can guarantee the client
that all write requests for which he got success response when he wrote to A cluster must
already have been in B cluster as well.



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