kea/doc/guide/logging.xml

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  <chapter id="logging">
    <title>Logging</title>

    <section>
      <title>Logging configuration</title>

      <para>

        The logging system in Kea is configured through the
        Logging module. All  modules will look at the
        configuration in Logging to see what should be logged and
        to where.

<!-- TODO: what is context of Logging module for readers of this guide? -->

      </para>

      <section>
        <title>Loggers</title>

        <para>

          Within Kea, a message is logged through a component
          called a "logger". Different parts of log messages
          through different loggers, and each logger can be configured
          independently of one another.

        </para>

        <para>

          In the Logging module, you can specify the configuration
          for zero or more loggers; any that are not specified will
          take appropriate default values.

        </para>

        <para>

          The three most important elements of a logger configuration
          are the <option>name</option> (the component that is
          generating the messages), the <option>severity</option>
          (what to log), and the <option>output_options</option>
          (where to log).

        </para>

        <section>
          <title>name (string)</title>

          <para>
          Each logger in the system has a name, the name being that
          of the component using it to log messages. For instance,
          if you want to configure logging for the Dhcp4 module,
          you add an entry for a logger named <quote>Dhcp4</quote>. This
          configuration will then be used by the loggers in the
          Dhcp4 module, and all the libraries used by it.
              </para>

<!-- TODO: later we will have a way to know names of all modules

Right now you can only see what their names are if they are running
(a simple 'help' without anything else in bindctl for instance).

 -->
          <para>

          If you want to specify logging for one specific library
          within the module, you set the name to
          <replaceable>module.library</replaceable>.  For example, the
          logger used by the nameserver address store component
          has the full name of <quote>Dhcp4.dhcpsrv</quote>. If
          there is no entry in Logging for a particular library,
          it will use the configuration given for the module.

        </para>

       <para>

          To illustrate this, suppose you want the dhcpsrv library
          to log messages of severity DEBUG, and the rest of the
          Dhcp4 code to log messages of severity INFO. To achieve
          this you specify two loggers, one with the name
          <quote>Dhcp4</quote> and severity INFO, and one with
          the name <quote>Dhcp4.dhcpsrv</quote> with severity
          DEBUG. As there are no entries for other libraries,
          they will use the configuration for the module
          (<quote>Dhcp4</quote>), so giving the desired behavior.

        </para>

        <para>

          One special case is that of a module name of <quote>*</quote>
          (asterisks), which is interpreted as <emphasis>any</emphasis>
          module. You can set global logging options by using this,
          including setting the logging configuration for a library
          that is used by multiple modules (e.g. <quote>*.config</quote>
          specifies the configuration library code in whatever
          module is using it).

        </para>

        <para>

          If there are multiple logger specifications in the
          configuration that might match a particular logger, the
          specification with the more specific logger name takes
          precedence. For example, if there are entries for
          both <quote>*</quote> and <quote>Dhcp4</quote>, the
          Dhcp4 module &mdash; and all libraries it uses &mdash;
          will log messages according to the configuration in the
          second entry (<quote>Dhcp4</quote>). All other modules
          will use the configuration of the first entry
          (<quote>*</quote>).

        </para>

        <para>

          One final note about the naming. When specifying the
          module name within a logger, use the name of the module
          as specified in JSON configuration, e.g.
          <quote>Dhcp4</quote> for the Dhcp4 module,
          <quote>Dhcp6</quote> for the Dhcp6 module, etc. When
          the message is logged, the message will include the name
          of the logger generating the message, but with the module
          name replaced by the name of the process implementing
          the module (so for example, a message generated by the
          <quote>Dhcp4</quote> logger will appear in the output
          with a logger name of <quote>kea-dhcp4</quote>).

        </para>

        </section>

        <section>
          <title>severity (string)</title>

        <para>

          This specifies the category of messages logged.
          Each message is logged with an associated severity which
          may be one of the following (in descending order of
          severity):
        </para>

        <itemizedlist>
          <listitem>
            <simpara> FATAL </simpara>
          </listitem>

          <listitem>
            <simpara> ERROR </simpara>
          </listitem>

          <listitem>
            <simpara> WARN </simpara>
          </listitem>

          <listitem>
            <simpara> INFO </simpara>
          </listitem>

          <listitem>
            <simpara> DEBUG </simpara>
          </listitem>
        </itemizedlist>

        <para>

          When the severity of a logger is set to one of these
          values, it will only log messages of that severity, and
          the severities above it. The severity may also be set to
          NONE, in which case all messages from that logger are
          inhibited.

<!-- TODO: worded wrong? If I set to INFO, why would it show DEBUG which is literally below in that list? -->

        </para>

        </section>

        <section>
          <title>output_options (list)</title>

        <para>

          Each logger can have zero or more
          <option>output_options</option>. These specify where log
          messages are sent to. These are explained in detail below.

        </para>

        <para>

          The other options for a logger are:

        </para>

        </section>

        <section>
          <title>debuglevel (integer)</title>

        <para>

          When a logger's severity is set to DEBUG, this value
          specifies what debug messages should be printed. It ranges
          from 0 (least verbose) to 99 (most verbose).
        </para>


<!-- TODO: complete this sentence:

          The general classification of debug message types is

TODO; there's a ticket to determine these levels, see #1074

 -->

        <para>

          If severity for the logger is not DEBUG, this value is ignored.

        </para>

        </section>

        <section>
          <title>additive (true or false)</title>

        <para>

          If this is true, the <option>output_options</option> from
          the parent will be used. For example, if there are two
          loggers configured; <quote>Dhcp4</quote> and
          <quote>Dhcp4.dhcpsrv</quote>, and <option>additive</option>
          is true in the second, it will write the log messages
          not only to the destinations specified for
          <quote>Dhcp4.dhcpsrv</quote>, but also to the destinations
          as specified in the <option>output_options</option> in
          the logger named <quote>Dhcp4</quote>.

      </para>

      </section>

      </section>

      <section>
        <title>Output Options</title>

        <para>

          The main settings for an output option are the
          <option>destination</option> and a value called
          <option>output</option>, the meaning of which depends on
          the destination that is set.

        </para>

        <section>
          <title>destination (string)</title>

          <para>

            The destination is the type of output. It can be one of:

          </para>

          <itemizedlist>

            <listitem>
              <simpara> console </simpara>
          </listitem>

            <listitem>
              <simpara> file </simpara>
          </listitem>

            <listitem>
              <simpara> syslog </simpara>
            </listitem>

          </itemizedlist>

        </section>

        <section>
          <title>output (string)</title>

        <para>
          This value determines the type of output. There are several
          special values allowed here: <command>stdout</command> (messages
          are printed on standard output), <command>stderr</command>
          (messages are printed on stderr), <command>syslog</command> (messages
          are logged to syslog using default name, <command>syslog:name</command>
          (messages are logged to syslog using specified name). Any other
          value is interpreted as a filename that the logs should be written to.
        </para>

        <para>

          The other options for <option>output_options</option> are:

        </para>

        <!-- configuration of flush is not supported yet. 
        <section>
          <title>flush (true of false)</title>

          <para>
            Flush buffers after each log message. Doing this will
            reduce performance but will ensure that if the program
            terminates abnormally, all messages up to the point of
            termination are output.
          </para>

        </section> -->

        <section>
          <title>maxsize (integer)</title>

          <para>
            Only relevant when destination is file, this is maximum
            file size of output files in bytes. When the maximum
            size is reached, the file is renamed and a new file opened.
            (For example, a ".1" is appended to the name &mdash;
            if a ".1" file exists, it is renamed ".2",
            etc.)
          </para>

          <para>
            If this is 0, no maximum file size is used.
          </para>

          <note>
            <simpara>
	      Due to a limitation of the underlying logging library
	      (log4cplus), rolling over the log files (from ".1" to
	      ".2", etc) may show odd results: There can be
	      multiple small files at the timing of roll over.  This
	      can happen when multiple processes try to roll
	      over the files simultaneously.
	      Version 1.1.0 of log4cplus solved this problem, so if
	      this or higher version of log4cplus is used to build
	      Kea, it shouldn't happen.  Even for older versions
	      it is normally expected to happen rarely unless the log
	      messages are produced very frequently by multiple
	      different processes.
	    </simpara>
	  </note>

        </section>

        <section>
          <title>maxver (integer)</title>

          <para>
            Maximum number of old log files to keep around when
            rolling the output file. Only relevant when
            <option>output</option> is <quote>file</quote>.
          </para>

        </section>

      </section>

      </section>

      <section>
        <title>Example Logger configurations</title>

        <para>
          In this example we want to set the global logging to
          write to the console using standard output.
        </para>

<screen><userinput>
"Logging": {
    "loggers": [
        {
            "name": "kea",
            "output_options": [
                {
                    "output": "stdout"
                }
            ], 
            "severity": "WARN"
        }
    ]
}
</userinput>
</screen>

<para>In this second example, we want to store debug log messages
in a file that is at most 2MB and keep up to 8 copies of old logfiles.
Once the logfile grows to 2MB, it will be renamed and a new file
file be created.</para>

<screen><userinput>
"Logging": {
    "loggers": [
        {
            "name": "kea",
            "output_options": [
                {
                    "output": "/var/log/kea-debug.log",
                    "maxver": 8,
                    "maxsize": 204800,
                    "destination": "file"
                }
            ], 
            "severity": "DEBUG",
            "debuglevel": 99
        }
   ]
}</userinput></screen>
      </section>

    </section>

    <section>
      <title>Logging Message Format</title>

      <para>
          Each message written  to the configured logging
          destinations comprises a number of components that identify
          the origin of the message and, if the message indicates
          a problem, information about the problem that may be
          useful in fixing it.
      </para>

      <para>
          Consider the message below logged to a file:
          <screen>2014-04-11 12:58:01.005 INFO  [kea-dhcp4.dhcpsrv/27456]
    DHCPSRV_MEMFILE_DB opening memory file lease database: type=memfile universe=4</screen>
      </para>

      <para>
        Note: the layout of messages written to the system logging
        file (syslog) may be slightly different.  This message has
        been split across two lines here for display reasons; in the
        logging file, it will appear on one line.)
      </para>

      <para>
        The log message comprises a number of components:

          <variablelist>
          <varlistentry>
          <term>2014-04-11 12:58:01.005</term>
<!-- TODO: timestamp repeated even if using syslog? -->
          <listitem><para>
              The date and time at which the message was generated.
          </para></listitem>
          </varlistentry>

          <varlistentry>
          <term>INFO</term>
          <listitem><para>
              The severity of the message.
          </para></listitem>
          </varlistentry>

          <varlistentry>
          <term>[kea-dhcp4.dhcpsrv/27456]</term>
          <listitem><para>
            The source of the message.  This comprises two components:
            the BIND 10 process generating the message (in this
            case, <command>kea-dhcp4</command>) and the module
            within the program from which the message originated
            (which is the name of the common library used by DHCP server
            implementations).
          </para></listitem>
          </varlistentry>

          <varlistentry>
          <term>DHCPSRV_MEMFILE_DB</term>
          <listitem><para>
            The message identification.  Every message in Kea
            has a unique identification, which can be used as an
            index into the <ulink
            url="kea-messages.html"><citetitle>Kea Messages
            Manual</citetitle></ulink> (<ulink
            url="http://kea.isc.org/docs/kea-messages.html"
            />) from which more information can be obtained.
          </para></listitem>
          </varlistentry>

          <varlistentry>
          <term>opening memory file lease database: type=memfile universe=4</term>
          <listitem><para>
              A brief description.
              Within this text, information relating to the condition
              that caused the message to be logged will be included.
              In this example, the information is logged that the in-memory
              lease database backend will be used to store DHCP leases.
          </para></listitem>
          </varlistentry>
          </variablelist>
      </para>

    </section>

  </chapter>