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https://issues.apache.org/jira/browse/IMPALA-7802?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16718239#comment-16718239
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Zoram Thanga commented on IMPALA-7802:
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Thanks for the comment, [~tarmstrong]. You brought up a good point regarding
what the client experience has to be, when the session has expired. I think we
will have to maintain the current behavior, but without tying up FE service
threads. That is, expired sessions should not consume any resources on the
Impala server besides the session states.
I am thinking about moving expired sessions to a separate list (a death row)
where they remain until explicitly cancelled and/or closed by the client.
Perhaps we can have a single thread handle cancellation and closing of expired
sessions.
> Implement support for closing idle sessions
> -------------------------------------------
>
> Key: IMPALA-7802
> URL: https://issues.apache.org/jira/browse/IMPALA-7802
> Project: IMPALA
> Issue Type: Improvement
> Components: Clients
> Affects Versions: Impala 3.0, Impala 2.12.0
> Reporter: Michael Ho
> Assignee: Zoram Thanga
> Priority: Critical
> Labels: supportability
>
> Currently, the query option {{idle_session_timeout}} specifies a timeout in
> seconds after which all running queries of that idle session will be
> cancelled and no new queries can be issued to it. However, the idle session
> will remain open and it needs to be closed explicitly. Please see the
> [documentation|https://www.cloudera.com/documentation/enterprise/latest/topics/impala_idle_session_timeout.html]
> for details.
> This behavior may be undesirable as each session still consumes an Impala
> frontend service thread. The number of frontend service threads is bound by
> the flag {{fe_service_threads}}. So, in a multi-tenant environment, an Impala
> server can have a lot of idle sessions but they still consume against the
> quota of {{fe_service_threads}}. If the number of sessions established
> reaches {{fe_service_threads}}, all new session creations will block until
> some of the existing sessions exit. There may be no time bound on when these
> zombie idle sessions will be closed and it's at the mercy of the client
> implementation to close them. In some sense, leaving many idle sessions open
> is a way to launch a denial of service attack on Impala.
> To fix this situation, we should have an option to forcefully close a session
> when it's considered idle so it won't unnecessarily consume the limited
> number of frontend service threads. cc'ing [~zoram]
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