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https://issues.apache.org/jira/browse/HDFS-7858?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14341375#comment-14341375
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Arun Suresh commented on HDFS-7858:
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[~kihwal], [~bikassaha], I understand your concerns over the use of ZK. But
consider the following:
# Most DFSClients are cached (since the Filesystem objects are generally
cached). Thus long lived clients will probably not have more than 1-2
persistent connections to ZK.
# Short lived clients will first check if there is cached entry (possibly in
the home directory, something like ~/.lastNN) that contains the last accessed
active NN. The client will proceed to connect to that NN first (thereby
removing non-determinism from the current scheme).. and will most probably
succeed. It will contact ZK only if the connection was unsuccessful.. and we
can limit this to just a ping (not a watch registration) so the connection is
not persistent.
# A Client that has connected to a NN without the need for a ZK connection can
continue to NOT talk to ZK till
## the client dies and no ZK connection is ever made
## after waiting for a configurable time (maybe an hour).. after which it is
established that it is a ling lived client,
Do you think this might reduce the total number of connection to ZK at any
point of time ?
> Improve HA Namenode Failover detection on the client using Zookeeper
> --------------------------------------------------------------------
>
> Key: HDFS-7858
> URL: https://issues.apache.org/jira/browse/HDFS-7858
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Arun Suresh
> Assignee: Arun Suresh
>
> In an HA deployment, Clients are configured with the hostnames of both the
> Active and Standby Namenodes.Clients will first try one of the NNs
> (non-deterministically) and if its a standby NN, then it will respond to the
> client to retry the request on the other Namenode.
> If the client happens to talks to the Standby first, and the standby is
> undergoing some GC / is busy, then those clients might not get a response
> soon enough to try the other NN.
> Proposed Approach to solve this :
> 1) Since Zookeeper is already used as the failover controller, the clients
> could talk to ZK and find out which is the active namenode before contacting
> it.
> 2) Long-lived DFSClients would have a ZK watch configured which fires when
> there is a failover so they do not have to query ZK everytime to find out the
> active NN
> 2) Clients can also cache the last active NN in the user's home directory
> (~/.lastNN) so that short-lived clients can try that Namenode first before
> querying ZK
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