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Arpit Agarwal commented on HDFS-7858: ------------------------------------- Thanks for updating the patch Arun. The MultiException approach looks like a good alternative to refactoring RetryPolicy. A few comments: # I didn't understand the call to {{super.performFailover}} in {{RequestHedgingProxyProvider#getProxy}}. # The documentation in HDFSHighAvailabilityWithQJM.md and HDFSHighAvailabilityWithNFS.md should be updated as it states _The only implementation which currently ships with Hadoop is the ConfiguredFailoverProxyProvider_. Okay to do this in a separate Jira. # Agree with Jing's suggestion to use a {{CompletionService}}. Also we should file a task to make {{RequestHedgingProxyProvider}} the default eventually. Nitpicks: # {{getDelayMillis}} javadoc is wrong. # {{successfullproxy}} should be {{successfulproxy}}. # _new LinkedList<RetryAction>_ - explicit type argument redundant. # _static interface ProxyFactory_ - static is redundant. > Improve HA Namenode Failover detection on the client > ---------------------------------------------------- > > Key: HDFS-7858 > URL: https://issues.apache.org/jira/browse/HDFS-7858 > Project: Hadoop HDFS > Issue Type: Improvement > Reporter: Arun Suresh > Assignee: Arun Suresh > Labels: BB2015-05-TBR > Attachments: HDFS-7858.1.patch, HDFS-7858.2.patch, HDFS-7858.2.patch, > HDFS-7858.3.patch, HDFS-7858.4.patch, HDFS-7858.5.patch, HDFS-7858.6.patch > > > 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 -- This message was sent by Atlassian JIRA (v6.3.4#6332)