impala/docs/topics/impala_logging.xml

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

  <title>Using Impala Logging</title>
  <titlealts audience="PDF"><navtitle>Logging</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="Logs"/>
      <data name="Category" value="Troubleshooting"/>
      <data name="Category" value="Administrators"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      The Impala logs record information about:
    </p>

    <ul>
      <li>
        Any errors Impala encountered. If Impala experienced a serious error during startup, you must diagnose and
        troubleshoot that problem before you can do anything further with Impala.
      </li>

      <li>
        How Impala is configured.
      </li>

      <li>
        Jobs Impala has completed.
      </li>
    </ul>

    <note>
      <p>
        Formerly, the logs contained the query profile for each query, showing low-level details of how the work is
        distributed among nodes and how intermediate and final results are transmitted across the network. To save
        space, those query profiles are now stored in zlib-compressed files in
        <filepath>/var/log/impala/profiles</filepath>. You can access them through the Impala web user interface.
        For example, at <codeph>http://<varname>impalad-node-hostname</varname>:25000/queries</codeph>, each query
        is followed by a <codeph>Profile</codeph> link leading to a page showing extensive analytical data for the
        query execution.
      </p>

      <p rev="1.1.1">
        The auditing feature introduced in Impala 1.1.1 produces a separate set of audit log files when
        enabled. See <xref href="impala_auditing.xml#auditing"/> for details.
      </p>

      <p rev="2.9.0 IMPALA-4431">
        In <keyword keyref="impala29_full"/> and higher, you can control how many
        audit event log files are kept on each host through the
        <codeph>--max_audit_event_log_files</codeph> startup option for the
        <cmdname>impalad</cmdname> daemon, similar to the <codeph>--max_log_files</codeph>
        option for regular log files.
      </p>

      <p rev="2.2.0">
        The lineage feature introduced in Impala 2.2.0 produces a separate lineage log file when
        enabled. See <xref href="impala_lineage.xml#lineage"/> for details.
      </p>
    </note>

    <p outputclass="toc inpage"/>

  </conbody>

  <concept id="logs_details">

    <title>Locations and Names of Impala Log Files</title>

    <conbody>

      <ul>
        <li>
          By default, the log files are under the directory <filepath>/var/log/impala</filepath>.
          To change log file locations, modify the defaults file described in
          <xref href="impala_processes.xml#processes"/>.
        </li>

        <li>
          The significant files for the <codeph>impalad</codeph> process are <filepath>impalad.INFO</filepath>,
          <filepath>impalad.WARNING</filepath>, and <filepath>impalad.ERROR</filepath>. You might also see a file
          <filepath>impalad.FATAL</filepath>, although this is only present in rare conditions.
        </li>

        <li>
          The significant files for the <codeph>statestored</codeph> process are
          <filepath>statestored.INFO</filepath>, <filepath>statestored.WARNING</filepath>, and
          <filepath>statestored.ERROR</filepath>. You might also see a file <filepath>statestored.FATAL</filepath>,
          although this is only present in rare conditions.
        </li>

        <li rev="1.2">
          The significant files for the <codeph>catalogd</codeph> process are <filepath>catalogd.INFO</filepath>,
          <filepath>catalogd.WARNING</filepath>, and <filepath>catalogd.ERROR</filepath>. You might also see a file
          <filepath>catalogd.FATAL</filepath>, although this is only present in rare conditions.
        </li>

        <li>
          Examine the <codeph>.INFO</codeph> files to see configuration settings for the processes.
        </li>

        <li>
          Examine the <codeph>.WARNING</codeph> files to see all kinds of problem information, including such
          things as suboptimal settings and also serious runtime errors.
        </li>

        <li>
          Examine the <codeph>.ERROR</codeph> and/or <codeph>.FATAL</codeph> files to see only the most serious
          errors, if the processes crash, or queries fail to complete. These messages are also in the
          <codeph>.WARNING</codeph> file.
        </li>

        <li>
          A new set of log files is produced each time the associated daemon is restarted. These log files have
          long names including a timestamp. The <codeph>.INFO</codeph>, <codeph>.WARNING</codeph>, and
          <codeph>.ERROR</codeph> files are physically represented as symbolic links to the latest applicable log
          files.
        </li>

        <li>
          The init script for the <codeph>impala-server</codeph> service also produces a consolidated log file
          <codeph>/var/logs/impalad/impala-server.log</codeph>, with all the same information as the
          corresponding<codeph>.INFO</codeph>, <codeph>.WARNING</codeph>, and <codeph>.ERROR</codeph> files.
        </li>

        <li>
          The init script for the <codeph>impala-state-store</codeph> service also produces a consolidated log file
          <codeph>/var/logs/impalad/impala-state-store.log</codeph>, with all the same information as the
          corresponding<codeph>.INFO</codeph>, <codeph>.WARNING</codeph>, and <codeph>.ERROR</codeph> files.
        </li>
      </ul>

      <p>
        Impala stores information using the <codeph>glog_v</codeph> logging system. You will see some messages
        referring to C++ file names. Logging is affected by:
      </p>

      <ul>
        <li>
          The <codeph>GLOG_v</codeph> environment variable specifies which types of messages are logged. See
          <xref href="#log_levels"/> for details.
        </li>

        <li>
          The <codeph>--logbuflevel</codeph> startup flag for the <cmdname>impalad</cmdname> daemon specifies how
          often the log information is written to disk. The default is 0, meaning that the log is immediately
          flushed to disk when Impala outputs an important messages such as a warning or an error, but less
          important messages such as informational ones are buffered in memory rather than being flushed to disk
          immediately.
        </li>
      </ul>

    </conbody>

  </concept>

  <concept id="logs_managing">

    <title>Managing Impala Logs</title>
  <prolog>
    <metadata>
      <data name="Category" value="Administrators"/>
    </metadata>
  </prolog>

    <conbody>

      <p>
        Review Impala log files on each host, when you have traced an issue back to a specific system.
      </p>

    </conbody>

  </concept>

  <concept id="logs_rotate">

    <title>Rotating Impala Logs</title>
  <prolog>
    <metadata>
      <data name="Category" value="Disk Storage"/>
    </metadata>
  </prolog>

    <conbody>

      <p>
        Impala periodically switches the physical files representing the current log files, after which it is safe
        to remove the old files if they are no longer needed.
      </p>

      <p>
        Impala can automatically remove older unneeded log files, a feature
        known as <term>log rotation</term>.
      </p>

      <p>
        In Impala 2.2 and higher, the <codeph>--max_log_files</codeph>
        configuration option specifies how many log files to keep at each
        severity level (<codeph>INFO</codeph>, <codeph>WARNING</codeph>,
          <codeph>ERROR</codeph>, and <codeph>FATAL</codeph>). You can specify
        an appropriate setting for each Impala-related daemon
          (<cmdname>impalad</cmdname>, <cmdname>statestored</cmdname>, and
          <cmdname>catalogd</cmdname>).
        <ul>
          <li>
            A value of 0 preserves all log files, in which case you would set
            up set up manual log rotation using your Linux tool or technique of
            choice.
          </li>
          <li>
            A value of 1 preserves only the very latest log file.
          </li>
          <li>
            The default value is 10.
          </li>
        </ul>
      </p>
      <p>
        Impala checks to see if any old logs need to be removed based on the
        interval specified in the <codeph>--logbufsecs</codeph> setting, every 5
        seconds by default.
      </p>
      <p>
        For some log levels, Impala logs are first temporarily buffered in
        memory and only written to disk periodically. The
          <codeph>--logbufsecs</codeph> setting controls the maximum time that
        log messages are buffered for. For example, with the default value of 5
        seconds, there may be up to a 5 second delay before a logged message
        shows up in the log file.
      </p>
      <p>
        It is not recommended that you set <codeph>--logbufsecs</codeph> to 0
        as the setting makes the Impala daemon to spin in the thread that tries
        to delete old log files.
      </p>

    </conbody>

  </concept>

  <concept id="logs_debug">

    <title>Reviewing Impala Logs</title>

    <conbody>

      <p>
        By default, the Impala log is stored at <codeph>/var/logs/impalad/</codeph>. The most comprehensive log,
        showing informational, warning, and error messages, is in the file name <filepath>impalad.INFO</filepath>.
        View log file contents by using the web interface or by examining the contents of the log file. (When you
        examine the logs through the file system, you can troubleshoot problems by reading the
        <filepath>impalad.WARNING</filepath> and/or <filepath>impalad.ERROR</filepath> files, which contain the
        subsets of messages indicating potential problems.)
      </p>

      <p>
        On a machine named <codeph>impala.example.com</codeph> with default settings, you could view the Impala
        logs on that machine by using a browser to access <codeph>http://impala.example.com:25000/logs</codeph>.
      </p>

      <note>
        <p>
          The web interface limits the amount of logging information displayed. To view every log entry, access the
          log files directly through the file system.
        </p>
      </note>

      <p>
        You can view the contents of the <codeph>impalad.INFO</codeph> log file in the file system. With the
        default configuration settings, the start of the log file appears as follows:
      </p>

<codeblock>[user@example impalad]$ pwd
/var/log/impalad
[user@example impalad]$ more impalad.INFO
Log file created at: 2013/01/07 08:42:12
Running on machine: impala.example.com
Log line format: [IWEF]mmdd hh:mm:ss.uuuuuu threadid file:line] msg
I0107 08:42:12.292155 14876 daemon.cc:34] impalad version 0.4 RELEASE (build 9d7fadca0461ab40b9e9df8cdb47107ec6b27cff)
Built on Fri, 21 Dec 2012 12:55:19 PST
I0107 08:42:12.292484 14876 daemon.cc:35] Using hostname: impala.example.com
I0107 08:42:12.292706 14876 logging.cc:76] Flags (see also /varz are on debug webserver):
--dump_ir=false
--module_output=
--be_port=22000
--classpath=
--hostname=impala.example.com</codeblock>

      <note>
        The preceding example shows only a small part of the log file. Impala log files are often several megabytes
        in size.
      </note>

    </conbody>

  </concept>

  <concept id="log_format">

    <title>Understanding Impala Log Contents</title>

    <conbody>

      <p>
        The logs store information about Impala startup options. This information appears once for each time Impala
        is started and may include:
      </p>

      <ul>
        <li>
          Machine name.
        </li>

        <li>
          Impala version number.
        </li>

        <li>
          Flags used to start Impala.
        </li>

        <li>
          CPU information.
        </li>

        <li>
          The number of available disks.
        </li>
      </ul>

      <p>
        There is information about each job Impala has run. Because each Impala job creates an additional set of
        data about queries, the amount of job specific data may be very large. Logs may contained detailed
        information on jobs. These detailed log entries may include:
      </p>

      <ul>
        <li>
          The composition of the query.
        </li>

        <li>
          The degree of data locality.
        </li>

        <li>
          Statistics on data throughput and response times.
        </li>
      </ul>

    </conbody>

  </concept>

  <concept id="log_levels">

    <title>Setting Logging Levels</title>

    <conbody>

      <p>
        Impala uses the GLOG system, which supports three logging levels. You can adjust logging levels
        by exporting variable settings. To change logging settings manually, use a command
        similar to the following on each node before starting <codeph>impalad</codeph>:
      </p>

<codeblock>export GLOG_v=1</codeblock>

      <note>
        For performance reasons, do not enable the most verbose logging level of 3 unless there is
        no other alternative for troubleshooting.
      </note>

      <p>
        For more information on how to configure GLOG, including how to set variable logging levels for different
        system components, see
        <xref keyref="glog.html">documentation for the glog project on github</xref>.
      </p>

      <section id="loglevels_details">

        <title>Understanding What is Logged at Different Logging Levels</title>

        <p>
          As logging levels increase, the categories of information logged are cumulative. For example, GLOG_v=2
          records everything GLOG_v=1 records, as well as additional information.
        </p>

        <p>
          Increasing logging levels imposes performance overhead and increases log size. Where practical, use
          GLOG_v=1 for most cases: this level has minimal performance impact but still captures useful
          troubleshooting information.
        </p>

        <p>
          Additional information logged at each level is as follows:
        </p>

        <ul>
          <li>
            GLOG_v=1 - The default level. Logs information about each connection and query that is initiated to an
            <codeph>impalad</codeph> instance, including runtime profiles.
          </li>

          <li>
            GLOG_v=2 - Everything from the previous level plus information for each RPC initiated. This level also
            records query execution progress information, including details on each file that is read.
          </li>

          <li>
            GLOG_v=3 - Everything from the previous level plus logging of every row that is read. This level is
            only applicable for the most serious troubleshooting and tuning scenarios, because it can produce
            exceptionally large and detailed log files, potentially leading to its own set of performance and
            capacity problems.
          </li>
        </ul>

      </section>

    </conbody>

  </concept>

  <concept id="redaction" rev="2.2.0">

    <title>Redacting Sensitive Information from Impala Log Files</title>
    <prolog>
      <metadata>
        <data name="Category" value="Redaction"/>
      </metadata>
    </prolog>

    <conbody>

      <p>
        <indexterm audience="hidden">redaction</indexterm>
        <term>Log redaction</term> is a security feature that prevents sensitive information from being displayed in
        locations used by administrators for monitoring and troubleshooting, such as log files and the Impala debug web
        user interface. You configure regular expressions that match sensitive types of information processed by your
        system, such as credit card numbers or tax IDs, and literals matching these patterns are obfuscated wherever
        they would normally be recorded in log files or displayed in administration or debugging user interfaces.
      </p>

      <p>
        In a security context, the log redaction feature is complementary to the Sentry authorization framework.
        Sentry prevents unauthorized users from being able to directly access table data. Redaction prevents
        administrators or support personnel from seeing the smaller amounts of sensitive or personally identifying
        information (PII) that might appear in queries issued by those authorized users.
      </p>

      <p>
        See <xref keyref="sg_redaction"/> for details about how to enable this feature and set
        up the regular expressions to detect and redact sensitive information within SQL statement text.
      </p>

    </conbody>

  </concept>

</concept>