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From "Jean-Daniel Cryans (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-546) Use Zookeeper in HBase
Date Thu, 27 Nov 2008 01:06:44 GMT

    [ https://issues.apache.org/jira/browse/HBASE-546?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12651206#action_12651206

Jean-Daniel Cryans commented on HBASE-546:

Had this chat with Nitay on IRC:

[19:42]	<nitay>	got your updates, gonna make the write do some retries and then throw
all the way up, which should shutdown the HMaster?
[19:43]	<nitay>	also, about the safe mode, so i saw that i removed that but i thought
that was safe b/c the client will just keep trying to read the root region location?
[19:43]	<jdcryans>	the retries should be done in RegionManager
[19:43]	<nitay>	jdcryans: oh ok i can move them there
[19:43]	<jdcryans>	yeah but the root region will be assigned first, but that doesn't
mean that the meta regions are all assigned
[19:44]	<jdcryans>	so the safe mode should really work like in hdfs
[19:44]	<nitay>	how does it work in hdfs?
[19:44]	<jdcryans>	you can't do anything until safe mode is off
[19:45]	<nitay>	ok, i can make that a direct rpc then instead of having it be injected
through the getRootRegion
[19:45]	<jdcryans>	It's one way to do it
[19:45]	<jdcryans>	another one would be to store that value in ZK
[19:45]	<nitay>	better suggestions?
[19:45]	<jdcryans>	so that we don't rely on the master for that info
[19:46]	<nitay>	ah ok so just some empty ephemeral file?
[19:46]	<jdcryans>	for example, during normal operations the master fails and a new
client tries to instantiate a HCM
[19:46]	<nitay>	when master comes back up its in safe mode again?
[19:47]	<jdcryans>	was thinking about exactly the same
[19:47]	<jdcryans>	but I don't know...
[19:47]	<jdcryans>	we should try it first in a busy cluster
[19:47]	<jdcryans>	for the mo, I would make it an ephemeral file
[19:48]	<jdcryans>	the less the client is coupled to HMaster the better
[19:48]	<nitay>	k
[19:48]	<nitay>	so the RM owned by HMaster would start up in safe mode
[19:49]	<jdcryans>	yep
[19:50]	<nitay>	ok, ephemeral it is, makes sense
[19:50]	<jdcryans>	I think it's a safe design
[19:51]	<nitay>	oh i also found another interesting thing
[19:52]	<nitay>	when we get a regionServerStartup
[19:53]	<nitay>	if the old data from that server tells us it was serving the root region
we clear it out by calling setRootRegionLocation(null)
[19:53]	<nitay>	right now that would NPE in ZKW
[19:54]	<jdcryans>	so that would be a delete?
[19:54]	<nitay>	so im making it so that if u pass in null it deletes the file

> Use Zookeeper in HBase
> ----------------------
>                 Key: HBASE-546
>                 URL: https://issues.apache.org/jira/browse/HBASE-546
>             Project: Hadoop HBase
>          Issue Type: Improvement
>            Reporter: Bryan Duxbury
>            Assignee: Jean-Daniel Cryans
>            Priority: Critical
>             Fix For: 0.20.0
>         Attachments: 0001-use-zookeeper-to-store-root-region-location.patch, DistributedLockInterface.java,
hbase-546-scripts-v2.patch, hbase-546-scripts.patch, zookeeper-config.patch
> Zookeeper =~ Chubby. This means that we could take advantage of a distributed lock manager
to coordinate things like failover masters, regionservers staying online when master is dead,
atomic region->regionserver assignments, etc. There are a lot of opportunities for improvements
here. Please add discussions of particular features in comments or sub-tasks.

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