impala/docs/topics/impala_timeouts.xml

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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept rev="1.2.1" id="timeouts">

  <title>Setting Timeout Periods for Daemons, Queries, and Sessions</title>

  <titlealts audience="PDF">

    <navtitle>Setting Timeouts</navtitle>

  </titlealts>

  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="Administrators"/>
      <data name="Category" value="Scheduling"/>
      <data name="Category" value="Scalability"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      Depending on how busy your <keyword keyref="distro"/> cluster is, you might increase or decrease various timeout
      values. Increase timeouts if Impala is cancelling operations prematurely, when the system
      is responding slower than usual but the operations are still successful if given extra
      time. Decrease timeouts if operations are idle or hanging for long periods, and the idle
      or hung operations are consuming resources and reducing concurrency.
    </p>

    <p outputclass="toc inpage"/>

  </conbody>

  <concept id="statestore_timeout">

    <title>Increasing the Statestore Timeout</title>

    <conbody>

      <p rev="IMP-1210">
        If you have an extensive Impala schema, for example with hundreds of databases, tens of
        thousands of tables, and so on, you might encounter timeout errors during startup as the
        Impala catalog service broadcasts metadata to all the Impala nodes using the statestore
        service. To avoid such timeout errors on startup, increase the statestore timeout value
        from its default of 10 seconds. Specify the timeout value using the
        <codeph>-statestore_subscriber_timeout_seconds</codeph> option for the statestore
        service, using the configuration instructions in
        <xref href="impala_config_options.xml#config_options"/>. The symptom of this problem is
        messages in the <codeph>impalad</codeph> log such as:
      </p>

<codeblock>Connection with state-store lost
Trying to re-register with state-store</codeblock>

      <p>
        See <xref href="impala_scalability.xml#statestore_scalability"/> for more details about
        statestore operation and settings on clusters with a large number of Impala-related
        objects such as tables and partitions.
      </p>

    </conbody>

  </concept>

  <concept id="impalad_timeout">

    <title>Setting the Idle Query and Idle Session Timeouts for impalad</title>

    <conbody>

      <p>
        To keep long-running queries or idle sessions from tying up cluster resources, you can
        set timeout intervals for both individual queries, and entire sessions.
      </p>

      <note conref="../shared/impala_common.xml#common/timeout_clock_blurb"/>

      <p>
        Specify the following startup options for the <cmdname>impalad</cmdname> daemon:
      </p>

      <ul>
        <li>
          <p>
            The <codeph>--idle_query_timeout</codeph> option specifies the time in seconds after
            which an idle query is cancelled. This could be a query whose results were all fetched
            but was never closed, or one whose results were partially fetched and then the client
            program stopped requesting further results. This condition is most likely to occur in
            a client program using the JDBC or ODBC interfaces, rather than in the interactive
            <cmdname>impala-shell</cmdname> interpreter. Once the query is cancelled, the client
            program cannot retrieve any further results.
          </p>

          <p rev="2.0.0">
            You can reduce the idle query timeout by using the <codeph>QUERY_TIMEOUT_S</codeph>
            query option. Any non-zero value specified for the <codeph>--idle_query_timeout</codeph> startup
            option serves as an upper limit for the <codeph>QUERY_TIMEOUT_S</codeph> query option.
            A zero value for <codeph>--idle_query_timeout</codeph> disables query timeouts.
            See <xref href="impala_query_timeout_s.xml#query_timeout_s"/> for details.
          </p>
        </li>

        <li>
          <p>
            The <codeph>--idle_session_timeout</codeph> option specifies the time in seconds after
            which an idle session is expired. A session is idle when no activity is occurring for
            any of the queries in that session, and the session has not started any new queries.
            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. The default value
            of 0 means that sessions never expire.
          </p>
        </li>
      </ul>

      <p>
        For instructions on changing <cmdname>impalad</cmdname> startup options, see
        <xref href="impala_config_options.xml#config_options"/>.
      </p>

      <note>
        <p rev="IMPALA-5108">
          Impala checks periodically for idle sessions and queries
          to cancel. The actual idle time before cancellation might be up to 50% greater than
          the specified configuration setting. For example, if the timeout setting was 60, the
          session or query might be cancelled after being idle between 60 and 90 seconds.
        </p>
      </note>

    </conbody>

  </concept>
  <concept id="concept_rfy_jl1_rx">
    <title>Setting Timeout and Retries for Thrift Connections to the Backend
      Client</title>
    <conbody>
      <p>Impala connections to the backend client are subject to failure in
        cases when the network is momentarily overloaded. To avoid failed
        queries due to transient network problems, you can configure the number
        of Thrift connection retries using the following option: </p>
      <ul id="ul_bj3_ql1_rx">
        <li>The <codeph>--backend_client_connection_num_retries</codeph> option
          specifies the number of times Impala will try connecting to the
          backend client after the first connection attempt fails. By default,
            <cmdname>impalad</cmdname> will attempt three re-connections before
          it returns a failure. </li>
      </ul>
      <p>You can configure timeouts for sending and receiving data from the
        backend client. Therefore, if for some reason a query hangs, instead of
        waiting indefinitely for a response, Impala will terminate the
        connection after a configurable timeout.</p>
      <ul id="ul_vm2_2v1_rx">
        <li>The <codeph>--backend_client_rpc_timeout_ms</codeph> option can be
          used to specify the number of milliseconds Impala should wait for a
          response from the backend client before it terminates the connection
          and signals a failure. The default value for this property is 300000
          milliseconds, or 5 minutes. </li>
      </ul>
    </conbody>
  </concept>

  <concept id="cancel_query">

    <title>Cancelling a Query</title>

    <conbody>

      <p>
        Sometimes, an Impala query might run for an unexpectedly long time, tying up resources
        in the cluster. You can cancel the query explicitly, independent of the timeout period,
        by going into the web UI for the <cmdname>impalad</cmdname> host (on port 25000 by
        default), and using the link on the <codeph>/queries</codeph> tab to cancel the running
        query. For example, press <codeph>^C</codeph> in <cmdname>impala-shell</cmdname>.
      </p>

    </conbody>

  </concept>

</concept>