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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    <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>

  <section>
    <title>Configuration</title>
    <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>Default is <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>Default is <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>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>Default is <c>unlimited</c>.</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>Default is <c>true</c>.</p>
      </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
	  section <seealso marker="#default_templates">Default
	  Templates</seealso> for more information.</p>
	<p>Default is <c>false</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>template = </c><seealso marker="#type-template"><c>template()</c></seealso></tag>
      <item>
	<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,
	  repectively. 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>. See <seealso marker="#default_templates">Default
	  Templates</seealso> for more information</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>Default is <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>The default value is 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>
  </section>

  <section>
    <marker id="default_templates"/>
    <title>Default templates</title>

    <p>The default value for the <c>template</c> configuration
      parameter depends on the value of <c>single_line</c>
      and <c>legacy_header</c> 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>
  </section>

  <datatypes>
    <datatype>
      <name name="template"/>
      <desc>
      </desc>
    </datatype>
  </datatypes>

  <funcs>
    <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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