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https://issues.apache.org/jira/browse/HDFS-13779?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16581593#comment-16581593
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Erik Krogen commented on HDFS-13779:
------------------------------------
Thanks for looking [~vagarychen]!
For #1, the {{currentIndex}} only increases, then you take the modulo vs. the
current list size to get the actual index. So in the case you've discussed with
{{currentIndex = 2}} (not 3, since it is 0-indexed), before we have (caret
indicating current index):
{code}
1 2 3 4 5
^
{code}
Then we try 3 and 4, they both throw standby so we remove them, so the
remaining list is ({{currentIndex = 2}} still):
{code}
1 2 5
^
{code}
Next we try 5, is says RETRY, so we increment to get {{currentIndex = 3}}, then
we try 1 ({{3 % 3 == 0}}) as expected.
I went with {{currentIndex}} increasing and taking modulo, rather than always
maintaining an actual index, since the size of the list may change and
otherwise we would have to potentially update {{currentIndex}} every time the
list size changes (to avoid AIOOBE). Modulus gives us AIOOBE avoidance for free.
For #2, good point. For #3 / #4, yes, it needs cleanup, just a WIP for now.
> Implement performFailover logic for ObserverReadProxyProvider.
> --------------------------------------------------------------
>
> Key: HDFS-13779
> URL: https://issues.apache.org/jira/browse/HDFS-13779
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: hdfs-client
> Reporter: Konstantin Shvachko
> Assignee: Erik Krogen
> Priority: Major
> Attachments: HDFS-13779-HDFS-12943.WIP00.patch
>
>
> Currently {{ObserverReadProxyProvider}} inherits {{performFailover()}} method
> from {{ConfiguredFailoverProxyProvider}}, which simply increments the index
> and switches over to another NameNode. The logic for ORPP should be smart
> enough to choose another observer, otherwise it can switch to a SBN, where
> reads are disallowed, or to an ANN, which defeats the purpose of reads from
> standby.
> This was discussed in HDFS-12976.
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