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https://issues.apache.org/jira/browse/HDFS-13522?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16462885#comment-16462885
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Íñigo Goiri commented on HDFS-13522:
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bq. One thing I'm not certain is that observer reads can be enabled/disabled on
the client side (e.g., via StandbyReadProxyProvider and some config key). How
this can be reflected in the RouterRpcClient which sits on the server side?
{{RouterRpcClient}} uses the raw {{ClientProtocol}}.
The core of the code is in {{RouterRpcClient#invokeMethod}} and it has a check
for {{StandbyException}} and it allows a failover internally.
This method also gets the namenodes to invoke this methods in order based on
the current status.
The change would be in {{getNamenodesForNameservice(nsId)}}, this method should
recognize if the operation is READ/WRITE (we can piggyback this information
from the {{RouterRpcServer}} and if it's a WRITE, keep the current order and if
it's a READ allow the OBSERVER NNs to go first.
Not sure the policies that are being design right now but we can plug them
here. Examples I can think: random, observer first...
> 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
> Priority: Major
>
> 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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