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https://issues.apache.org/jira/browse/HDFS-13923?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16621327#comment-16621327
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Chao Sun commented on HDFS-13923:
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bq. Here you are talking, I believe, about a way of telling the client
dynamically not to use ObserverNode. Do you?
Yes that's right.
bq. I don't see how configuration parameter can help with that. If the client
is cached, then whatever value of a config variable stored in it during the
initial instantiation will still be there, even if you updated the config file.
If using the default {{FileSystem}} cache key, then yes, the same cached client
will be used. But if we also change the cache key class in {{FileSystem}} and
make it consider this configuration flag, then a different {{FileSystem}}
instance will be used when the configuration flag changes, and so a different
client with different behavior will be used.
> Add a configuration to turn on/off observer reads
> -------------------------------------------------
>
> Key: HDFS-13923
> URL: https://issues.apache.org/jira/browse/HDFS-13923
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Reporter: Chao Sun
> Priority: Major
>
> In some situations having a config to turn on/off observer reads dynamically
> may be useful. For instance, some applications may be sensitive for data
> freshness and want to always reach directly to active NN. In a more complex
> scenario, services such as Presto may want to apply observer reads for
> different types of queries. In this case, simply change
> {{dfs.client.failover.proxy.provider.<nameservice>}} may not be enough, since
> with FileSystem cache (which is usually turned on) will ignore the change and
> still use the same FileSystem object.
> Here I'm proposing to add a flag in {{HdfsClientConfigKeys}}, such as
> {{dfs.client.observer.reads.enabled}}, that can be used to dynamically turn
> on/off observer reads. The FileSystem cache key should also take account of
> this flag in its {{hashCode}} and {{equals}} impl, so that different
> FileSystem objects will be used depending on the flag.
>
> cc [~shv], [~xkrogen], [~vagarychen], [~zero45] for discussion.
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