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

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <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.
      You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>logger</title>
    <prepared></prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date></date>
    <rev>A</rev>
    <file>logger.xml</file>
  </header>
  <module>logger</module>
  <modulesummary>API module for the logger.</modulesummary>

  <description>
    <p>
      This module is the main logger API. It contains functions that allow
      application to use a single log API and the system to manage those log
      events independently. To log events the logger
      <seealso marker="#macros">macros</seealso> should be used. For instance,
      to log a new error log event:</p>
      <code>
?LOG_ERROR("error happened because: ~p",[Reason]). %% With macro
logger:error("error happened because: ~p",[Reason]). %% Without macro
      </code>
      <p>This log event will then be sent to the configured log handlers which
      by default means that it will be printed to the console. If you want
      your systems logs to be printed to a file instead of the console you
      have to configure the default handler to do so. The simplest way is
      to include the following in your <seealso marker="config"><c>sys.config</c></seealso>.</p>
      <code>
[{kernel,
  [{logger,
    [{handler,default,logger_std_h,
      #{logger_std_h=>#{type=>{file,"path/to/file.log"}}}}]}]}].
      </code>
    <p>
      For more information about:
    </p>
    <list type="bulleted">
      <item>how to use the API,
        see <seealso marker="logger_chapter">the User's Guide</seealso>.</item>
      <item>how to configure logger,
        see the <seealso marker="logger_chapter#configuration">Configuration</seealso>
        section in the User's Guide.</item>
      <item>the convinience macros in logger.hrl,
        see <seealso marker="#macros">the macro section</seealso>.</item>
      <item>what the builtin formatter can do,
        see <seealso marker="logger_formatter">logger_formatter</seealso>.</item>
      <item>what the builtin handlers can do,
        see <seealso marker="logger_std_h">logger_std_h</seealso> and
        <seealso marker="logger_disk_log_h">logger_disk_log_h</seealso>.</item>
      <item>what builtin filters are available,
      see <seealso marker="logger_filters">logger_filters</seealso>.</item>
    </list>
  </description>

  <datatypes>
    <datatype>
      <name name="level"/>
      <desc>
	<p>The severity level for the message to be logged.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="log"/>
      <desc>
	<p></p>
      </desc>
    </datatype>
    <datatype>
      <name name="report"/>
      <desc>
	<p></p>
      </desc>
    </datatype>
    <datatype>
      <name name="msg_fun"/>
      <desc>
	<p></p>
      </desc>
    </datatype>
    <datatype>
      <name name="metadata"/>
      <desc>
	<p>Metadata for the log event.</p>
	<p>Logger adds the following metadata to each log event:</p>
	<list>
	  <item><c>pid  => self()</c></item>
	  <item><c>gl   => group_leader()</c></item>
	  <item><c>time => erlang:monotonic_time(microsecond)</c></item>
	</list>
	<p>When a log macro is used, Logger also inserts location
	  information:</p>
	<list>
	  <item><c>mfa  => {?MODULE,?FUNCTION_NAME,?FUNCTION_ARITY}</c></item>
	  <item><c>file => ?FILE</c></item>
	  <item><c>line => ?LINE</c></item>
	</list>
	<p>You can add custom metadata, either by specifying a map as
	  the last parameter to any of the log macros or the API
	  functions, or by setting process metadata
	  with <seealso marker="#set_process_metadata-1">
	    <c>set_process_metadata/1</c></seealso>
	  or <seealso marker="#update_process_metadata-1">
	    <c>update_process_metadata/1</c></seealso>.</p>
	<p>Logger merges all the metadata maps before forwarding the
	  log event to the handlers. If the same keys occur, values
	  from the log call overwrites process metadata, which in turn
	  overwrites values set by Logger.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="config"/>
      <desc>
	<p>Configuration data for the logger part of Logger, or for a handler.</p>
	<p>The following default values apply:</p>
	<list>
	  <item><c>level => info</c></item>
	  <item><c>filter_default => log</c></item>
	  <item><c>filters => []</c></item>
	  <item><c>formatter => {logger_formatter,DefaultFormatterConfig</c>}</item>
	</list>
	<p>See the <seealso marker="logger_formatter#configuration">
	    <c>logger_formatter(3)</c></seealso> manual page for
	  information about the default configuration for this
	  formatter.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="handler_id"/>
      <desc>
	<p>A unique identifier for a handler instance.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="filter_id"/>
      <desc>
	<p>A unique identifier for a filter.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="filter"/>
      <desc>
	<p>A filter which can be installed for logger or for a handler.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="filter_arg"/>
      <desc>
	<p>The second argument to the filter fun.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="filter_return"/>
      <desc>
	<p>The return value from the filter fun.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="timestamp"/>
      <desc>
	<p>A timestamp produced
	  with <seealso marker="erts:erlang#monotonic_time-1">
	    <c>erlang:monotonic_time(microsecond)</c></seealso>.</p>
      </desc>
    </datatype>
  </datatypes>

  <section>
    <title>Macros</title>
    <p>The following macros are defined:</p>

    <list>
      <item><c>?LOG_EMERGENCY(StringOrReport[,Metadata])</c></item>
      <item><c>?LOG_EMERGENCY(FunOrFormat,Args[,Metadata])</c></item>
      <item><c>?LOG_ALERT(StringOrReport[,Metadata])</c></item>
      <item><c>?LOG_ALERT(FunOrFormat,Args[,Metadata])</c></item>
      <item><c>?LOG_CRITICAL(StringOrReport[,Metadata])</c></item>
      <item><c>?LOG_CRITICAL(FunOrFormat,Args[,Metadata])</c></item>
      <item><c>?LOG_ERROR(StringOrReport[,Metadata])</c></item>
      <item><c>?LOG_ERROR(FunOrFormat,Args[,Metadata])</c></item>
      <item><c>?LOG_WARNING(StringOrReport[,Metadata])</c></item>
      <item><c>?LOG_WARNING(FunOrFormat,Args[,Metadata])</c></item>
      <item><c>?LOG_NOTICE(StringOrReport[,Metadata])</c></item>
      <item><c>?LOG_NOTICE(FunOrFormat,Args[,Metadata])</c></item>
      <item><c>?LOG_INFO(StringOrReport[,Metadata])</c></item>
      <item><c>?LOG_INFO(FunOrFormat,Args[,Metadata])</c></item>
      <item><c>?LOG_DEBUG(StringOrReport[,Metadata])</c></item>
      <item><c>?LOG_DEBUG(FunOrFormat,Args[,Metadata])</c></item>
    </list>

    <p>All macros expand to a call to logger, where <c>Level</c> is
      taken from the macro name, and location data is added. See the
      description of
      the <seealso marker="#type-metadata"><c>metadata()</c></seealso>
      type for more information about the location data.</p>

    <p>The call is wrapped in a case statement and will be evaluated
      only if <c>Level</c> is equal to or below the configured log
      level.</p>
  </section>

  <funcs>
    <func>
      <name>emergency(StringOrReport[,Metadata])</name>
      <name>emergency(Format,Args[,Metadata])</name>
      <name>emergency(Fun,FunArgs[,Metadata])</name>
      <fsummary>Logs the given message as level <c>emergency</c>.</fsummary>
      <desc>
        <p>Equivalent to
	<seealso marker="#log-2"><c>log(emergency,...)</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>alert(StringOrReport[,Metadata])</name>
      <name>alert(Format,Args[,Metadata])</name>
      <name>alert(Fun,FunArgs[,Metadata])</name>
      <fsummary>Logs the given message as level <c>alert</c>.</fsummary>
      <desc>
        <p>Equivalent to
	<seealso marker="#log-2"><c>log(alert,...)</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>critical(StringOrReport[,Metadata])</name>
      <name>critical(Format,Args[,Metadata])</name>
      <name>critical(Fun,FunArgs[,Metadata])</name>
      <fsummary>Logs the given message as level <c>critical</c>.</fsummary>
      <desc>
        <p>Equivalent to
	<seealso marker="#log-2"><c>log(critical,...)</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>error(StringOrReport[,Metadata])</name>
      <name>error(Format,Args[,Metadata])</name>
      <name>error(Fun,FunArgs[,Metadata])</name>
      <fsummary>Logs the given message as level <c>error</c>.</fsummary>
      <desc>
        <p>Equivalent to
	<seealso marker="#log-2"><c>log(error,...)</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>warning(StringOrReport[,Metadata])</name>
      <name>warning(Format,Args[,Metadata])</name>
      <name>warning(Fun,FunArgs[,Metadata])</name>
      <fsummary>Logs the given message as level <c>warning</c>.</fsummary>
      <desc>
        <p>Equivalent to
	<seealso marker="#log-2"><c>log(warning,...)</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>notice(StringOrReport[,Metadata])</name>
      <name>notice(Format,Args[,Metadata])</name>
      <name>notice(Fun,FunArgs[,Metadata])</name>
      <fsummary>Logs the given message as level <c>notice</c>.</fsummary>
      <desc>
        <p>Equivalent to
	<seealso marker="#log-2"><c>log(notice,...)</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>info(StringOrReport[,Metadata])</name>
      <name>info(Format,Args[,Metadata])</name>
      <name>info(Fun,FunArgs[,Metadata])</name>
      <fsummary>Logs the given message as level <c>info</c>.</fsummary>
      <desc>
        <p>Equivalent to
	<seealso marker="#log-2"><c>log(info,...)</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>debug(StringOrReport[,Metadata])</name>
      <name>debug(Format,Args[,Metadata])</name>
      <name>debug(Fun,FunArgs[,Metadata])</name>
      <fsummary>Logs the given message as level <c>debug</c>.</fsummary>
      <desc>
        <p>Equivalent to
	<seealso marker="#log-2"><c>log(debug,...)</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="log" arity="2"/>
      <name name="log" arity="3" clause_i="1"/>
      <name name="log" arity="3" clause_i="2"/>
      <name name="log" arity="3" clause_i="3"/>
      <name name="log" arity="4" clause_i="1"/>
      <name name="log" arity="4" clause_i="2"/>
      <fsummary>Logs the given message.</fsummary>
      <type variable="Level"/>
      <type variable="StringOrReport" name_i="1"/>
      <type variable="Format" name_i="3"/>
      <type variable="Args" name_i="3"/>
      <type variable="Fun" name_i="4"/>
      <type variable="FunArgs" name_i="4"/>
      <type variable="Metadata"/>
      <desc>
        <p>Log the given message.</p>
      </desc>
    </func>

    <func>
      <name name="get_logger_config" arity="0"/>
      <fsummary>Lookup the current configuration for logger.</fsummary>
      <desc>
        <p>Lookup the current configuration for logger.</p>
      </desc>
    </func>

    <func>
      <name name="get_handler_config" arity="1"/>
      <fsummary>Lookup the current configuration for the given handler.</fsummary>
      <desc>
        <p>Lookup the current configuration for the given handler.</p>
      </desc>
    </func>

    <func>
      <name name="i" arity="0"/>
      <fsummary>Get all logger configurations</fsummary>
      <desc>
        <p>Same as <seealso marker="#i/1"><c>logger:i(term)</c></seealso></p>
      </desc>
    </func>

    <func>
      <name name="i" arity="1" clause_i="1"/>
      <name name="i" arity="1" clause_i="2"/>
      <name name="i" arity="1" clause_i="3"/>
      <fsummary>Get all logger configurations</fsummary>
      <desc>
        <p>Display or return all current logger configuration.</p>
        <taglist>
          <tag><c><anno>Action</anno> = string</c></tag>
          <item>
	    <p>Return the pretty printed current logger configuration
              as iodata.</p>
	  </item>
          <tag><c><anno>Action</anno> = term</c></tag>
          <item>
	    <p>Return the current logger configuration as a term. The
              format of this term may change inbetween releases. For a
              stable format use <seealso marker="#get_handler_config/1">
	      <c>logger:get_handler_config/1</c></seealso>
              and <seealso marker="#get_logger_config/0">
	      <c>logger:get_logger_config/0</c></seealso>.
              The same as calling <c>logger:i()</c>.</p>
	  </item>
	  <tag><c><anno>Action</anno> = print</c></tag>
          <item>
	    <p>Pretty print all the current logger configuration to
              standard out. Example:</p>
            <code><![CDATA[1> logger:i(print).
Current logger configuration:
  Level: info
  Filter Default: log
  Filters:
  Handlers:
    Id: default
      Module:    logger_std_h
      Level:     info
      Formatter:
        Module: logger_formatter
        Config: #{legacy_header => true,single_line => false,
                  template => [{logger_formatter,header},"\n",msg,"\n"]}
      Filter Default: stop
      Filters:
        Id: stop_progress
          Fun:    fun logger_filters:progress/2
          Config: stop
        Id: remote_gl
          Fun:    fun logger_filters:remote_gl/2
          Config: stop
        Id: domain
          Fun:    fun logger_filters:domain/2
          Config: {log,prefix_of,[beam,erlang,otp,sasl]}
        Id: no_domain
          Fun:    fun logger_filters:domain/2
          Config: {log,no_domain,[]}
      Handler Config:
        logger_std_h: #{type => standard_io}
  Level set per module:
    Module: my_module
      Level:    debug]]></code>
          </item>
        </taglist>
      </desc>
    </func>

    <func>
      <name name="add_logger_filter" arity="2"/>
      <fsummary>Add a filter to the logger.</fsummary>
      <desc>
        <p>Add a filter to the logger.</p>
	<p>The filter fun is called with the log event as the first
	  parameter, and the specified <c>filter_args()</c> as the
	  second parameter.</p>
	<p>The return value of the fun specifies if a log event is to
	  be discarded or forwarded to the handlers:</p>
	<taglist>
	  <tag><c>log()</c></tag>
	  <item>
	    <p>The filter <em>passed</em>. The next logger filter, if
	      any, is applied. If no more logger filters exist, the
	      log event is forwarded to the handler part of the
	      logger, where handler filters are applied.</p>
	  </item>
	  <tag><c>stop</c></tag>
	  <item>
	    <p>The filter <em>did not pass</em>, and the log event is
	      immediately discarded.</p>
	  </item>
	  <tag><c>ignore</c></tag>
	  <item>
	    <p>The filter has no knowledge of the log event. The next
	      logger filter, if any, is applied. If no more logger
	      filters exist, the value of the <c>filter_default</c>
	      configuration parameter for the logger specifies if the
	      log event shall be discarded or forwarded to the handler
	      part.</p>
	  </item>
	</taglist>
	<p>See section <seealso marker="logger_chapter#Filter">
	    Filter</seealso> in the User's Guide for more information
	    about filters.</p>
	<p>Some built-in filters exist. These are defined
	  in <seealso marker="logger_filters"><c>logger_filters</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="add_handler_filter" arity="3"/>
      <fsummary>Add a filter to the specified handler.</fsummary>
      <desc>
        <p>Add a filter to the specified handler.</p>
	<p>The filter fun is called with the log event as the first
	  parameter, and the specified <c>filter_args()</c> as the
	  second parameter.</p>
	<p>The return value of the fun specifies if a log event is to
	  be discarded or forwarded to the handler callback:</p>
	<taglist>
	  <tag><c>log()</c></tag>
	  <item>
	    <p>The filter <em>passed</em>. The next handler filter, if
	      any, is applied. If no more filters exist for this
	      handler, the log event is forwarded to the handler
	      callback.</p>
	  </item>
	  <tag><c>stop</c></tag>
	  <item>
	    <p>The filter <em>did not pass</em>, and the log event is
	      immediately discarded.</p>
	  </item>
	  <tag><c>ignore</c></tag>
	  <item>
	    <p>The filter has no knowledge of the log event. The next
	      handler filter, if any, is applied. If no more filters
	      exist for this handler, the value of
	      the <c>filter_default</c> configuration parameter for
	      the handler specifies if the log event shall be
	      discarded or forwarded to the handler callback.</p>
	  </item>
	</taglist>
	<p>See
	  section <seealso marker="logger_chapter#Filter">Filter</seealso>
	  in the User's Guide for more information about filters.</p>
	<p>Some built-in filters exist. These are defined in
	  <seealso marker="logger_filters"><c>logger_filters</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="remove_logger_filter" arity="1"/>
      <fsummary>Remove a filter from the logger.</fsummary>
      <desc>
        <p>Remove the filter identified
          by <c><anno>FilterId</anno></c> from the logger.</p>
      </desc>
    </func>

    <func>
      <name name="remove_handler_filter" arity="2"/>
      <fsummary>Remove a filter from the specified handler.</fsummary>
      <desc>
        <p>Remove the filter identified
          by <c><anno>FilterId</anno></c> from the handler identified
          by <c><anno>HandlerId</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="add_handler" arity="3"/>
      <fsummary>Add a handler with the given configuration.</fsummary>
      <desc>
        <p>Add a handler with the given configuration.</p>
	<p><c><anno>HandlerId</anno></c> is a unique identifier which
	  must be used in all subsequent calls referring to this
	  handler.</p>
      </desc>
    </func>

    <func>
      <name name="remove_handler" arity="1"/>
      <fsummary>Remove the handler with the specified identity.</fsummary>
      <desc>
        <p>Remove the handler identified by <c><anno>HandlerId</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="set_module_level" arity="2"/>
      <fsummary>Set the log level for the specified module.</fsummary>
      <desc>
        <p>Set the log level for the
          specified <c><anno>Module</anno></c>.</p>
	<p>The log level for a module overrides the global log level
	  of the logger for log event originating from the module in
	  question. Notice, however, that it does not override the
	  level configuration for any handler.</p>
	<p>For example: Assume that the global log level for the
	  logger is <c>info</c>, and there is one handler, <c>h1</c>,
	  with level <c>info</c> and one handler, <c>h2</c>, with
	  level <c>debug</c>.</p>
	<p>With this configuration, no debug messages will be logged,
	  since they are all stopped by the global log level.</p>
	<p>If the level for <c>mymodule</c> is set now set
	  to <c>debug</c>, then debug events from this module will be
	  logged by the handler <c>h2</c>, but not by
	  handler <c>h1</c>.</p>
	<p>Debug events from other modules are still not logged.</p>
        <p>To change the global log level for the logger, use
          <seealso marker="#set_logger_config/2">
	    <c>logger:set_logger_config(level,Level)</c></seealso>.</p>
        <p>To change the log level for a handler, use
          <seealso marker="#set_handler_config/3">
	    <c>logger:set_handler_config(HandlerId,level,Level)</c></seealso>.</p>
	<note>
	  <p>The originating module for a log event is only detected
	    if <c>mfa=>{Module,Function,Arity}</c> exists in the
	    metadata. When log macros are used, this association is
	    automatically added to all log events. If the logger API
	    is called directly, without using a macro, the logging
	    client must explicitly add this information.</p>
	</note>
      </desc>
    </func>

    <func>
      <name name="reset_module_level" arity="1"/>
      <fsummary>Remove a module specific log setting.</fsummary>
      <desc>
        <p>Remove a module specific log setting. After this, the
          global log level is used for the specified module.</p>
      </desc>
    </func>

    <func>
      <name name="add_handlers" arity="1" clause_i="1"/>
      <fsummary>Setup logger handlers from the applications configuration parameters.</fsummary>
      <desc>
        <p>Reads the application configuration parameter <c>logger</c> and
          calls <c>logger:add_handlers/1</c> with it contents.</p>
      </desc>
    </func>

    <func>
      <name name="add_handlers" arity="1" clause_i="2"/>
      <fsummary>Setup logger handlers.</fsummary>
      <type name="config_handler"/>
      <desc>
        <p>This function should be used by custom logger handlers to make
        configuration consistent no matter which handler the system uses.
        Normal usage to to add a call to <c>logger:add_handlers/1</c>
        just after the processes that the handler needs are started
        and pass the applications logger config as an argument. Eg.</p>
        <code>
-behaviour(application).
start(_, []) ->
    case supervisor:start_link({local, my_sup}, my_sup, []) of
        {ok, Pid} ->
            ok = logger:add_handlers(my_app),
            {ok, Pid, []};
        Error -> Error
     end.</code>
       <p>This will read the <c>logger</c> configuration parameter from
         the handler application and start the configured handlers. The contents
         of the configuration use the same rules as the
         <seealso marker="logger_chapter#handler-configuration">logger handler configuration</seealso>.
       </p>
       <p>If the handler is meant to replace the default handler the kernels
         default handlers have to be disabled before the new handler is added.
         A <c>sys.config</c> file that disables the kernel handler and adds
         a custom handler could looks like this:</p>
         <code>
[{kernel,
  [{logger,
    %% Disable the default kernel handler
    [{handler,default,undefined}]}]},
 {my_app,
  [{logger,
    %% Enable this handler as the default
    [{handler,default,my_handler,#{}}]}]}].
         </code>
      </desc>
    </func>

    <func>
      <name name="set_logger_config" arity="1"/>
      <fsummary>Set configuration data for the logger.</fsummary>
      <desc>
        <p>Set configuration data for the logger. This overwrites the
          current logger configuration.</p>
	<p>To modify the existing configuration,
	  use <seealso marker="#update_logger_config-1">
	    <c>update_logger_config/1</c></seealso>, or, if a more
	  complex merge is needed, read the current configuration
	  with <seealso marker="#get_logger_config-0"><c>get_logger_config/0</c>
	  </seealso>, then do the merge before writing the new
	  configuration back with this function.</p>
	<p>If a key is removed compared to the current configuration,
	  the default value is used.</p>
      </desc>
    </func>

    <func>
      <name name="set_logger_config" arity="2"/>
      <fsummary>Add or update configuration data for the logger.</fsummary>
      <desc>
        <p>Add or update configuration data for the logger. If the
          given <c><anno>Key</anno></c> already exists, its associated
          value will be changed to <c><anno>Value</anno></c>. If it
          does not exist, it will be added.</p>
      </desc>
    </func>

    <func>
      <name name="update_logger_config" arity="1"/>
      <fsummary>Update configuration data for the logger.</fsummary>
      <desc>
        <p>Update configuration data for the logger. This function
          behaves as if it was implemented as follows:</p>
	<code type="erl">
{ok,Old} = logger:get_logger_config(),
logger:set_logger_config(maps:merge(Old,Config)).
	</code>
	<p>To overwrite the existing configuration without any merge,
	  use <seealso marker="#set_logger_config-1"><c>set_logger_config/1</c>
	  </seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="set_handler_config" arity="2"/>
      <fsummary>Set configuration data for the specified handler.</fsummary>
      <desc>
        <p>Set configuration data for the specified handler. This
          overwrites the current handler configuration.</p>
	<p>To modify the existing configuration,
	  use <seealso marker="#update_handler_config-2">
	    <c>update_handler_config/2</c></seealso>, or, if a more
	  complex merge is needed, read the current configuration
	  with <seealso marker="#get_handler_config-1"><c>get_handler_config/1</c>
	  </seealso>, then do the merge before writing the new
	  configuration back with this function.</p>
	<p>If a key is removed compared to the current configuration,
	  and the key is know by Logger, the default value is used. If
	  it is a custom key, then it is up to the handler
	  implementation if the value is removed or a default value is
	  inserted.</p>
      </desc>
    </func>

    <func>
      <name name="set_handler_config" arity="3"/>
      <fsummary>Add or update configuration data for the specified
        handler.</fsummary>
      <desc>
        <p>Add or update configuration data for the specified
          handler. If the given <c><anno>Key</anno></c> already
          exists, its associated value will be changed
          to <c><anno>Value</anno></c>. If it does not exist, it will
          be added.</p>
      </desc>
    </func>

    <func>
      <name name="update_handler_config" arity="2"/>
      <fsummary>Update configuration data for the specified handler.</fsummary>
      <desc>
        <p>Update configuration data for the specified handler. This function
          behaves as if it was implemented as follows:</p>
	<code type="erl">
{ok,{_,Old}} = logger:get_handler_config(HandlerId),
logger:set_handler_config(HandlerId,maps:merge(Old,Config)).
	</code>
	<p>To overwrite the existing configuration without any merge,
	  use <seealso marker="#set_handler_config-2"><c>set_handler_config/2</c>
	  </seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="compare_levels" arity="2"/>
      <fsummary>Compare the severity of two log levels.</fsummary>
      <desc>
        <p>Compare the severity of two log levels. Returns <c>gt</c>
          if <c>Level1</c> is more severe than
          <c>Level2</c>, <c>lt</c> if <c>Level1</c> is less severe,
          and <c>eq</c> if the levels are equal.</p>
      </desc>
    </func>

    <func>
      <name name="set_process_metadata" arity="1"/>
      <fsummary>Set metadata to use when logging from current process.</fsummary>
      <desc>
        <p>Set metadata which Logger shall automatically insert in
          all log events produced on the current process.</p>
	<p>Location data produced by the log macros, and/or metadata
	  given as argument to the log call (API function or macro),
	  are merged with the process metadata. If the same keys
	  occur, values from the metadata argument to the log call
	  overwrite values from the process metadata, which in turn
	  overwrite values from the location data.</p>
	<p>Subsequent calls to this function overwrites previous data
          set. To update existing data instead of overwriting it,
          see <seealso marker="#update_process_metadata-1">
	    <c>update_process_metadata/1</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="update_process_metadata" arity="1"/>
      <fsummary>Set or update metadata to use when logging from
	current process.</fsummary>
      <desc>
	<p>Set or update metadata to use when logging from current
	  process</p>
	<p>If process metadata exists for the current process, this
	  function behaves as if it was implemented as follows:</p>
	<code type="erl">
logger:set_process_metadata(maps:merge(logger:get_process_metadata(),Meta)).
	</code>
	<p>If no process metadata exists, the function behaves as
	  <seealso marker="#set_process_metadata-1">
	    <c>set_process_metadata/1</c>
	  </seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="get_process_metadata" arity="0"/>
      <fsummary>Retrieve data set with set_process_metadata/1.</fsummary>
      <desc>
        <p>Retrieve data set
          with <seealso marker="#set_process_metadata-1">
	    <c>set_process_metadata/1</c></seealso> or
	  <seealso marker="#update_process_metadata-1">
	    <c>update_process_metadata/1</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="unset_process_metadata" arity="0"/>
      <fsummary>Delete data set with set_process_metadata/1.</fsummary>
      <desc>
        <p>Delete data set
          with <seealso marker="#set_process_metadata-1">
	    <c>set_process_metadata/1</c></seealso> or
	  <seealso marker="#update_process_metadata-1">
	    <c>update_process_metadata/1</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="format_report" arity="1"/>
      <fsummary>Convert a log message on report form to {Format,Args}.</fsummary>
      <desc>
        <p>Convert a log message on report form to <c>{Format,Args}</c>.</p>
	<p>This is the default report callback used
	  by <seealso marker="logger_formatter">
	    <c>logger_formatter</c></seealso> when no custom report
	    callback is found.</p>
	<p>The function produces lines of <c>Key: Value</c> from
	  key-value lists. Strings are printed with <c>~ts</c> and
	  other terms with <c>~tp</c>.</p>
	<p>If the <c><anno>Report</anno></c> is a map, it is
	  converted to a key-value list before formatting as such.</p>
      </desc>
    </func>

  </funcs>

  <section>
    <title>Callback Functions</title>
    <p>The following functions are to be exported from a handler
      callback module.</p>
  </section>

  <funcs>
    <func>
      <name>Module:adding_handler(HandlerId,Config1) -> {ok,Config2} | {error,Reason}</name>
      <fsummary>An instance of this handler is about to be added.</fsummary>
      <type>
	<v>HandlerId =
	  <seealso marker="#type-handler_id">handler_id()</seealso></v>
	<v>Config1 = Config2 =
	  <seealso marker="#type-config">config()</seealso></v>
	<v>Reason = term()</v>
      </type>
      <desc>
	<p>This callback function is optional.</p>
	<p>The function is called when an new handler is about to be
	  added, and the purpose is to verify the configuration and
	  initiate all resourced needed by the handler.</p>
	<p>If everything succeeds, the callback function can add
	  possible default values or internal state values to the
	  configuration, and return the adjusted map
	  in <c>{ok,Config2}</c>.</p>
	<p>If the configuration is faulty, or if the initiation fails,
	  the callback function must return <c>{error,Reason}</c>.</p>
      </desc>
    </func>

    <func>
      <name>Module:removing_handler(HandlerId,Config) -> ok</name>
      <fsummary>The given handler is about to be removed.</fsummary>
      <type>
	<v>HandlerId =
	  <seealso marker="#type-handler_id">handler_id()</seealso></v>
	<v>Config =
	  <seealso marker="#type-config">config()</seealso></v>
      </type>
      <desc>
	<p>This callback function is optional.</p>
	<p>The function is called when a handler is about to be
	  removed, and the purpose is to release all resources used by
	  the handler. The return value is ignored by Logger.</p>
      </desc>
    </func>

    <func>
      <name>Module:changing_config(HandlerId,Config1,Config2) -> {ok,Config3} | {error,Reason}</name>
      <fsummary>The configuration for this handler is about to change.</fsummary>
      <type>
	<v>HandlerId =
	  <seealso marker="#type-handler_id">handler_id()</seealso></v>
	<v>Config1 = Config2 = Config3 =
	  <seealso marker="#type-config">config()</seealso></v>
	<v>Reason = term()</v>
      </type>
      <desc>
	<p>This callback function is optional.</p>
	<p>The function is called when the configuration for a handler
	  is about to change, and the purpose is to verify and act on
	  the new configuration.</p>
	<p><c>Config1</c> is the existing configuration
	  and <c>Config2</c> is the new configuration.</p>
	<p>If everything succeeds, the callback function must return a
	  possibly adjusted configuration in <c>{ok,Config3}</c>.</p>
	<p>If the configuration is faulty, the callback function must
	  return <c>{error,Reason}</c>.</p>
      </desc>
    </func>
  </funcs>
</erlref>