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

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<erlref>
  <header>
    <copyright>
      <year>2017</year><year>2018</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_std_h</title>
    <prepared></prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date></date>
    <rev>A</rev>
    <file>logger_std_h.xml</file>
  </header>
  <module since="OTP 21.0">logger_std_h</module>
  <modulesummary>Standard handler for Logger.</modulesummary>

  <description>
    <p>This is the standard handler for Logger.
      Multiple instances of this handler can be added to
      Logger, and each instance prints logs to <c>standard_io</c>,
      <c>standard_error</c>, or to file.</p>
    <p>The handler has an overload protection mechanism that keeps the handler
      process and the Kernel application alive during high loads of log 
      events. How overload protection works, and how to configure it, is
      described in the
      <seealso marker="logger_chapter#overload_protection"><c>User's Guide</c>
      </seealso>.</p>
    <p>To add a new instance of the standard handler, use
      <seealso marker="logger#add_handler-3"><c>logger:add_handler/3</c>
      </seealso>. The handler configuration argument is a map which can contain
      general configuration parameters, as documented in the
       <seealso marker="logger_chapter#handler_configuration"><c>User's Guide</c>
       </seealso>, and handler specific parameters. The specific data
       is stored in a sub map with the key <c>config</c>, and can contain the
       following parameters:</p>
    <taglist>
      <tag><marker id="type"/><c>type</c></tag>
      <item>
	<p>This has the value <c>standard_io</c>, <c>standard_error</c>,
	<c>{file,LogFileName}</c>, or <c>{file,LogFileName,LogFileOpts}</c>.</p>
	<p>If <c>LogFileOpts</c> is specified, it replaces the default
	  list of options used when opening the log file. The default
	  list is <c>[raw,append,delayed_write]</c>. One reason to do
	  so can be to change <c>append</c> to, for
	  example, <c>write</c>, ensuring that the old log is
	  truncated when a node is restarted. See the reference manual
	  for <seealso marker="file#open-2"><c>file:open/2</c></seealso>
	  for more information about file options.</p>
	<p>Log files are always UTF-8 encoded. The encoding can not be
	  changed by setting the option <c>{encoding,Encoding}</c>
	  in <c>LogFileOpts</c>.</p>
	<p>Notice that the standard handler does not have support for
	  circular logging. Use the disk_log handler,
	  <seealso marker="logger_disk_log_h"><c>logger_disk_log_h</c></seealso>,
	  for this.</p>
	<p>The value is set when the handler is added, and it can not
	  be changed in runtime.</p>
	<p>Defaults to <c>standard_io</c>.</p>
      </item>
      <tag><c>filesync_repeat_interval</c></tag>
      <item>
	<p>This value, in milliseconds, specifies how often the handler does
	a file sync operation to write buffered data to disk. The handler attempts
	the operation repeatedly, but only performs a new sync if something has
	actually been logged.</p>
	<p>If <c>no_repeat</c> is set as value, the repeated file sync operation
	is disabled, and it is the operating system settings that determine
	how quickly or slowly data is written to disk. The user can also call
	the <seealso marker="logger_std_h#filesync-1"><c>filesync/1</c></seealso>
        function to perform a file sync.</p>
	<p>Defaults to <c>5000</c> milliseconds.</p>
      </item>
    </taglist>
    <p>Other configuration parameters exist, to be used for customizing
    the overload protection behaviour. The same parameters are used both in the
    standard handler and the disk_log handler, and are documented in the
    <seealso marker="logger_chapter#overload_protection"><c>User's Guide</c>
    </seealso>.</p>
    <p>Notice that if changing the configuration of the handler in runtime,
    the <c>type</c> parameter must not be modified.</p>
    <p>Example of adding a standard handler:</p>
    <code type="none">
logger:add_handler(my_standard_h, logger_std_h,
                   #{config => #{type => {file,"./system_info.log"},
                                 filesync_repeat_interval => 1000}}).
    </code>
    <p>To set the default handler, that starts initially with
    the Kernel application, to log to file instead of <c>standard_io</c>,
    change the Kernel default logger configuration. Example:</p>
    <code type="none">
erl -kernel logger '[{handler,default,logger_std_h,
                      #{config => #{type => {file,"./log.log"}}}}]'
    </code>
    <p>An example of how to replace the standard handler with a disk_log handler
    at startup is found in the
    <seealso marker="logger_disk_log_h"><c>logger_disk_log_h</c></seealso>
    manual.</p>
  </description>

  <funcs>

    <func>
      <name name="filesync" arity="1" clause_i="1" since="OTP 21.0"/>
      <fsummary>Writes buffered data to disk.</fsummary>
      <desc>
        <p>Write buffered data to disk.</p>
      </desc>
    </func>

  </funcs>

  <section>
    <title>See Also</title>
    <p><seealso marker="logger"><c>logger(3)</c></seealso>,
      <seealso marker="logger_disk_log_h">
	<c>logger_disk_log_h(3)</c></seealso></p>
  </section>
</erlref>