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From "Aaron T. Myers (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2709) HA: Appropriately handle error conditions in EditLogTailer
Date Fri, 30 Dec 2011 19:46:30 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2709?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13177735#comment-13177735

Aaron T. Myers commented on HDFS-2709:

bq. 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.

What I did seems cleaner to me. We're necessarily changing the code which selects streams
to allow a request for a starting txid in the middle of an ELF, so why should that return
an ELFIS which starts at a lower txid?

bq. 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?

I considered that. I also considered throwing a custom {{Exception}} which includes the last
successfully-loaded txid. Both of those seemed more messy than the way I did it, but I could
probably be convinced otherwise.

Note that the way I did it in this patch does not preclude the {{EditLogTailer}} from detecting
that not all expected transactions were loaded. The {{EditLogTailer}} already knows both how
many are available from the files in the shared dir, and how many transactions were in fact
loaded. This would allow one to implement Eli's suggestion of "retry a read failure X times
and then exit," though this patch does not currently do that.

bq. Can we add some non-HA tests that exercise FileJournalManager/FSEditLogLoader's ability
to start mid-stream? Not sure if that's feasible.

I'm not quite sure what you mean by this. The way the code is currently structured, the code
for continuing from the middle of an ELF will only be reached in an HA context. That's the
point of the {{partialLoadOk}} option, which is only passed as true when HA is enabled.
> 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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