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https://issues.apache.org/jira/browse/HDFS-13522?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16834424#comment-16834424
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Surendra Singh Lilhore commented on HDFS-13522:
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{quote}For implementing a queue similar to FairCallQueue, Handlers have to
understand which downstream namenode a call is meant for and also whether its a
read/write. These aspects of a RPC can be inspected only once the call is
handled at the ClientProtocol implementation layer which callqueue won't have
any visibility into.
{quote}
{{org.apache.hadoop.ipc.Server.Call.isCallCoordinated()}}, this API tells
if it is read or write call and this is available in handlers. Based on this
API we can add call to respective queue. Currently namespace info is not
available in handlers.
{quote}Since we haven't yet finalized how to solve these, in the interim we
have taken a approach as shown in ShortTerm-Routers+Observer.png.
{quote}
You mean, we can give state same as namenode to router (ACTIVE, OBSERVER) and
utilize the functionality of {{ObserverReadProxyProvider}} or going to change
client to support read-only/Write-only router ?
> Support observer node from Router-Based Federation
> --------------------------------------------------
>
> Key: HDFS-13522
> URL: https://issues.apache.org/jira/browse/HDFS-13522
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: federation, namenode
> Reporter: Erik Krogen
> Assignee: Chao Sun
> Priority: Major
> Attachments: HDFS-13522.001.patch, Router+Observer RPC clogging.png,
> ShortTerm-Routers+Observer.png
>
>
> Changes will need to occur to the router to support the new observer node.
> One such change will be to make the router understand the observer state,
> e.g. {{FederationNamenodeServiceState}}.
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