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https://issues.apache.org/jira/browse/IMPALA-7802?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16710704#comment-16710704
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Zoram Thanga commented on IMPALA-7802:
--------------------------------------
The documentation states that:
{quote}
Once a session is expired, you cannot issue any new query requests to it. The
session remains open, but the only operation you can perform is to close it.
{quote}
This basically says that an expired session serves no useful purpose to any one
- not to Impala as it consumes an fe_service_thread, and not to the client
because the only operation allowed on it is to close it.
I would like to change the session expiry code to always force-close expired
sessions from the server side by calling ImpalaServer::CloseSessionInternal()
or a modified version of it.
> 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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