path: root/lib/kernel/doc/src/logger_formatter.xml

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  <header>
    <copyright>
      <year>2017</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
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          http://www.apache.org/licenses/LICENSE-2.0

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      distributed under the License is distributed on an "AS IS" BASIS,
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      See the License for the specific language governing permissions and
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    </legalnotice>

    <title>logger_formatter</title>
    <prepared></prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date></date>
    <rev>A</rev>
    <file>logger_formatter.xml</file>
  </header>
  <module>logger_formatter</module>
  <modulesummary>Default formatter for Logger.</modulesummary>

  <description>
    <p>Each log handler has a configured formatter specified as a
      module and a configuration term. The purpose of the formatter is
      to translate the log events to a final printable string
      (<seealso marker="stdlib:unicode#type-chardata"><c>unicode:chardata()</c>
      </seealso>) which can be written to the output
      device of the handler.</p>
    <p><c>logger_formatter</c> is the default formatter used by
      Logger.</p>
  </description>


  <datatypes>
    <datatype>
      <name name="config"/>
      <desc>
	<p>The configuration term for <c>logger_formatter</c> is a
	  <seealso marker="stdlib:maps">map</seealso>, and the
	  following keys can be set as configuration parameters:</p>
	<taglist>
	  <tag><c>chars_limit = pos_integer() | unlimited</c></tag>
	  <item>
	    <p>A positive integer representing the value of the option
	      with the same name to be used when calling
	      <seealso marker="stdlib:io_lib#format-3">
		<c>io_lib:format/3</c></seealso>.
	      This value limits the total number of characters printed
	      for each log event. Notice that this is a soft limit. For a
	      hard truncation limit, see option <c>max_size</c>.</p>
	    <p>Defaults to <c>unlimited</c>.</p>
	    <note>
	      <p><c>chars_limit</c> has no effect on log messages on
		string form. These are expected to be short, but can
		still be truncated by the <c>max_size</c>
		parameter.</p>
	    </note>
	  </item>
	  <tag><marker id="depth"/><c>depth = pos_integer() | unlimited</c></tag>
	  <item>
	    <p>A positive integer representing the maximum depth to
	      which terms shall be printed by this formatter. Format
	      strings passed to this formatter are rewritten. The
	      format controls ~p and ~w are replaced with ~P and ~W,
	      respectively, and the value is used as the depth
	      parameter. For details, see
	      <seealso marker="stdlib:io#format-2">io:format/2,3</seealso>
	      in STDLIB.</p>
	    <p>Defaults to <c>unlimited</c>.</p>
	    <note>
	      <p><c>depth</c> has no effect on log messages on string
		form. These are expected to be short, but can still be
		truncated by the <c>max_size</c> parameter.</p>
	    </note>
	  </item>
	  <tag><c>legacy_header = boolean()</c></tag>
	  <item>
	    <p>If set to <c>true</c> a header field is added to
	      logger_formatter's part of <c>Metadata</c>. The value of
	      this field is a string similar to the header created by
	      the old <c>error_logger</c> event handlers. It can be
	      included in the log event by adding the
	      tuple <c>{logger_formatter,header}</c> to the
	      template. See the description of
	      the <seealso marker="#type-template"><c>template()</c></seealso>
	      type for more information.</p>
	    <p>Defaults to <c>false</c>.</p>
	  </item>
	  <tag><c>max_size = pos_integer() | unlimited</c></tag>
	  <item>
	    <p>A positive integer representing the absolute maximum size a
	      string returned from this formatter can have. If the
	      formatted string is longer, after possibly being limited
	      by <c>chars_limit</c> or <c>depth</c>, it is truncated.</p>
	    <p>Defaults to <c>unlimited</c>.</p>
	  </item>
	  <tag><c>report_cb = fun((</c><seealso marker="logger#type-report"><c>logger:report()</c></seealso><c>) -> {</c><seealso marker="stdlib:io#type-format"><c>io:format()</c></seealso><c>,[term()]})</c></tag>
	  <item>
	    <p>A report callback is used by the formatter to transform
	      log messages on report form to a format string and
	      arguments. The report callback can be specified in the
	      metadata for the log event. If no report callback exist
	      in metadata, <c>logger_formatter</c> will
	      use <seealso marker="logger#format_report-1">
		<c>logger:format_report/1</c></seealso> as default
	      callback.</p>
	    <p>If this configuration parameter is set, it replaces
	      both the default report callback, and any report
	      callback found in metadata. That is, all reports are
	      converted by this configured function.</p>
	    <p>The value must be a function with arity 1,
	      returning <c>{Format,Args}</c>, and it will be called
	      with a report as only argument.</p>
	  </item>
	  <tag><c>single_line = boolean()</c></tag>
	  <item>
	    <p>If set to <c>true</c>, all newlines in the message are
	      replaced with <c>", "</c>, and whitespaces following
	      directly after newlines are removed. Note that newlines
	      added by the <c>template</c> parameter are not replaced.</p>
	    <p>Defaults to <c>true</c>.</p>
	  </item>
	  <tag><c>template = </c><seealso marker="#type-template"><c>template()</c></seealso></tag>
	  <item>
	    <p>The template describes how the formatted string is
	      composed by combining different data values from the log
	      event. See the description of
	      the <seealso marker="#type-template"><c>template()</c></seealso>
	      type for more information about this.</p>
	  </item>
	  <tag><c>time_designator = byte()</c></tag>
	  <item>
	    <p>Timestamps are formatted according to RFC3339, and the
	      time designator is the character used as date and time
	      separator.</p>
	    <p>Defaults to <c>$T</c>.</p>
	    <p>The value of this parameter is used as
	      the <c>time_designator</c> option
	      to <seealso marker="stdlib:calendar#system_time_to_rfc3339-2">
		<c>calendar:system_time_to_rcf3339/2</c></seealso>.</p>
	  </item>
	  <tag><c>time_offset = integer() | [byte()]</c></tag>
	  <item>
	    <p>The time offset, either a string or an integer, to be
	      used when formatting the timestamp.</p>
	    <p>An empty string is interpreted as local time. The
	      values <c>"Z"</c>, <c>"z"</c> or <c>0</c> are
	      interpreted as Universal Coordinated Time (UTC).</p>
	    <p>Strings, other than <c>"Z"</c>, <c>"z"</c>,
	      or <c>""</c>, must be on the form <c>±[hh]:[mm]</c>, for
	      example <c>"-02:00"</c> or <c>"+00:00"</c>.</p>
	    <p>Integers must be in microseconds, meaning that the
	      offset <c>7200000000</c> is equivalent
	      to <c>"+02:00"</c>.</p>
	    <p>Defaults to an empty string, meaning that timestamps
	      are displayed in local time. However, for backwards
	      compatibility, if the SASL configuration
	      parameter <seealso marker="sasl:sasl_app#utc_log">
		<c>utc_log</c></seealso><c>=true</c>, the default is
	      changed to <c>"Z"</c>, meaning that timestamps are displayed
	      in UTC.</p>
	    <p>The value of this parameter is used as
	      the <c>offset</c> option
	      to <seealso marker="stdlib:calendar#system_time_to_rfc3339-2">
		<c>calendar:system_time_to_rcf3339/2</c></seealso>.</p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="template"/>
      <desc>
	<p>The template is a list of atoms, tuples and strings. The
	  atoms <c>level</c> or <c>msg</c>, are treated as
	  placeholders for the severity level and the log message,
	  respectively. Other atoms or tuples are interpreted as
	  placeholders for metadata, where atoms are expected to match
	  top level keys, and tuples represent paths to sub keys when
	  the metadata is a nested map. For example the
	  tuple <c>{key1,key2}</c> is replaced by the value of
	  the <c>key2</c> field in the nested map below. The
	  atom <c>key1</c> on its own is replaced by the complete
	  value of the <c>key1</c> field. The values are converted to
	  strings.</p>

<code>
#{key1=>#{key2=>my_value,
          ...}
  ...}</code>

        <p>Strings in the template are printed literally.</p>
	<p>The default template differs depending on the values
	  of <c>legacy_header</c>
	  and <c>single_line</c>:</p>

	<p>The default value for the <c>template</c> configuration
	  parameter depends on the value of the <c>single_line</c>
	  and <c>legacy_header</c> configuration parameters as
	  follows.</p>

	<p>The log event used in the examples is:</p>
	<code>
?LOG_ERROR("name: ~p~nexit_reason: ~p",[my_name,"It crashed"])</code>

	<taglist>
	  <tag><c>legacy_header=true, single_line=false</c></tag>
	  <item>
	    <p>Default
	    template: <c>[{logger_formatter,header},"\n",msg,"\n"]</c></p>

	    <p>Example log entry:</p>
	    <code type="none">
=ERROR REPORT==== 17-May-2018::18:30:19.453447 ===
name: my_name
exit_reason: "It crashed"</code>

	    <p>Notice that all eight levels can occur in the heading,
	      not only <c>ERROR</c>, <c>WARNING</c> or <c>INFO</c> as the
	      old <c>error_logger</c> produced. And microseconds are
	      added at the end of the timestamp.</p>
	  </item>

	  <tag><c>legacy_header=true, single_line=true</c></tag>
	  <item>
	    <p>Default
	    template: <c>[{logger_formatter,header},"\n",msg,"\n"]</c></p>

	    <p>Notice that the template is here the same as
	      for <c>single_line=false</c>, but the resulting log entry
	      differs in that there is only one line after the
	      heading:</p>
	    <code type="none">
=ERROR REPORT==== 17-May-2018::18:31:06.952665 ===
name: my_name, exit_reason: "It crashed"</code>
	  </item>

	  <tag><c>legacy_header=false, single_line=true</c></tag>
	  <item>
	    <p>Default template: <c>[time," ",level,": ",msg,"\n"]</c></p>

	    <p>Example log entry:</p>
	    <code type="none">
2018-05-17T18:31:31.152864+02:00 error: name: my_name, exit_reason: "It crashed"</code>
	  </item>

	  <tag><c>legacy_header=false, single_line=false</c></tag>
	  <item>
	    <p>Default template: <c>[time," ",level,":\n",msg,"\n"]</c></p>

	    <p>Example log entry:</p>
	    <code type="none">
2018-05-17T18:32:20.105422+02:00 error:
name: my_name
exit_reason: "It crashed"</code>
	  </item>
	</taglist>
      </desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="check_config" arity="1"/>
      <fsummary>Validates the given formatter configuration.</fsummary>
      <desc>
	<p>This callback function is called by Logger when the
	  formatter configuration for a handler is set or modified. It
	  returns <c>ok</c> if the configuration is valid,
	  and <c>{error,term()}</c> if it is faulty.</p>
      </desc>
    </func>
    <func>
      <name name="format" arity="2"/>
      <fsummary>Formats the given message.</fsummary>
      <desc>
	<p>This the formatter callback function to be called from
	  handlers. The log event is processed as follows:</p>
	<list>
	  <item>If the message is on report form, it is converted to
	    <c>{Format,Args}</c> by calling the report
	    callback.</item>
	  <item>The size is limited according to the values of
	    configuration parameters <c>chars_limit</c>
	    and <c>depth</c>. Notice that this does not apply to
	    messages on string form.</item>
	  <item>The full log entry is composed according to
	    the <c>template</c>.</item>
	  <item>If the final string is too long, it is truncated
	    according to the value of configuration
	    parameter <c>max_size</c>.</item>
	</list>
      </desc>
    </func>
  </funcs>

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