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From "Todd Lipcon (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2709) HA: Appropriately handle error conditions in EditLogTailer
Date Fri, 30 Dec 2011 04:03:30 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2709?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13177540#comment-13177540
] 

Todd Lipcon commented on HDFS-2709:
-----------------------------------

A few thoughts on the overall approach:
- Rather than modify EditLogFileInputStream to take a startTxId, why not do the "skipping"
(what you call {{setInitialPosition}}) from the caller? ie modify {{FSEditLogLoader}} to skip
the transactions that have already been replayed? The skipping code doesn't seem specific
to the input stream itself.
- I'm not convinced why we need to have the {{partialLoadOk}} flag in {{FSEditLogLoader}}.
IMO if the log is truncated, it's still an error as far as the loader is concerned - we just
want to let the caller continue from where the error occured. The only trick is how to go
about getting the last successfully loaded txid out of the FSEditLogLoader in the error case
-- I guess a member variable and a getter would work there? Do you think this ends up messier
than the way you've done it?
- Can we add some non-HA tests that exercise FileJournalManager/FSEditLogLoader's ability
to start mid-stream? Not sure if that's feasible.
                
> HA: Appropriately handle error conditions in EditLogTailer
> ----------------------------------------------------------
>
>                 Key: HDFS-2709
>                 URL: https://issues.apache.org/jira/browse/HDFS-2709
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: ha, name-node
>    Affects Versions: HA branch (HDFS-1623)
>            Reporter: Todd Lipcon
>            Assignee: Aaron T. Myers
>            Priority: Critical
>         Attachments: HDFS-2709-HDFS-1623.patch, HDFS-2709-HDFS-1623.patch, HDFS-2709-HDFS-1623.patch
>
>
> Currently if the edit log tailer experiences an error replaying edits in the middle of
a file, it will go back to retrying from the beginning of the file on the next tailing iteration.
This is incorrect since many of the edits will have already been replayed, and not all edits
are idempotent.
> Instead, we either need to (a) support reading from the middle of a finalized file (ie
skip those edits already applied), or (b) abort the standby if it hits an error while tailing.
If "a" isn't simple, let's do "b" for now and come back to 'a' later since this is a rare
circumstance and better to abort than be incorrect.

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