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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4942) Add retry cache support in Namenode
Date Fri, 12 Jul 2013 22:45:50 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4942?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13707492#comment-13707492
] 

Konstantin Shvachko commented on HDFS-4942:
-------------------------------------------

I was thinking about it and I feel that adding the retry-related fields and flags in the RPC
layer is not the best way. Mostly because the retry logic is intended for a few HDFS methods
only, while the new field and flag will be serialized and de-serialized by everybody including
DataNodes, Balancers, MapReduce and Yarn.

I think a better way would be to use <clientName + callId> as a key to index the retry
cache entries. This will 
- constrain changes to HDFS only
- avoid incompatible RPC changes that effect sub-projects
- limit serialization overhead to only the methods involved in the retry.

This will require making clientName unique as many recently advocated for.
Would that sound reasonable?
                
> Add retry cache support in Namenode
> -----------------------------------
>
>                 Key: HDFS-4942
>                 URL: https://issues.apache.org/jira/browse/HDFS-4942
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: ha, namenode
>            Reporter: Suresh Srinivas
>            Assignee: Suresh Srinivas
>         Attachments: HDFSRetryCache.pdf
>
>
> In current HA mechanism with FailoverProxyProvider and non HA setups with RetryProxy
retry a request from the RPC layer. If the retried request has already been processed at the
namenode, the subsequent attempts fail for non-idempotent operations such as  create, append,
delete, rename etc. This will cause application failures during HA failover, network issues
etc.
> This jira proposes adding retry cache at the namenode to handle these failures. More
details in the comments.

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