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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
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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
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</legalnotice>
<title>logger_disk_log_h</title>
<prepared></prepared>
<responsible></responsible>
<docno></docno>
<approved></approved>
<checked></checked>
<date></date>
<rev>A</rev>
<file>logger_disk_log_h.xml</file>
</header>
<module>logger_disk_log_h</module>
<modulesummary>A disk_log based handler for the Logger
application.</modulesummary>
<description>
<p>This is a handler for the Logger application that offers circular
(wrapped) logs by using the disk_log application. Multiple instances
of this handler can be added to logger, and each instance will print to
its own disk_log file, created with the name and settings specified in
the handler configuration.</p>
<p>The default standard handler,
<seealso marker="logger_std_h"><c>logger_std_h</c></seealso>, can be
replaced by a disk_log handler at startup of the kernel application.
See an example of this below.</p>
<p>The handler has an overload protection mechanism that will keep the handler
process and the kernel application alive during a high load of log
requests. How this feature works, and how to modify the configuration,
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 disk_log handler, use
<seealso marker="logger#add_handler-3"><c>logger:add_handler/3</c>
</seealso>. The handler configuration argument is a map which may contain
general configuration parameters, as documented in the
<seealso marker="logger_chapter#handler_configuration"><c>User's Guide</c>
</seealso>, as well as handler specific parameters.</p>
<p>The settings for the disk_log log file should be specified with the
key <c>disk_log_opts</c>. These settings are a subset of the disk_log
datatype
<seealso marker="disk_log#open-1"><c>dlog_option()</c></seealso>.</p>
<p>Parameters in the <c>disk_log_opts</c> map:</p>
<taglist>
<tag><c>file</c></tag>
<item>This is the full name of the disk_log log file.</item>
<tag><c>type</c></tag>
<item>This is the disk_log type, <c>wrap</c> or <c>halt</c>. The
default value is <c>wrap</c>.</item>
<tag><c>max_no_files</c></tag>
<item>This is the maximum number of files that disk_log will use
for its circular logging. The default value is <c>10</c>. (The setting
has no effect on a halt log).</item>
<tag><c>max_no_bytes</c></tag>
<item>This is the maximum number of bytes that will be written to
a log file before disk_log proceeds with the next file in order (or
generates an error in case of a full halt log). The default value for
a wrap log is <c>1048576</c> bytes, and <c>infinity</c> for a halt
log.</item>
</taglist>
<p>Specific configuration for the handler (represented as a sub map)
is specified with the key <c>logger_disk_log_h</c>. It may contain the
following parameter:</p>
<taglist>
<tag><c>filesync_repeat_interval</c></tag>
<item>
<p>This value (in milliseconds) specifies how often the handler will
do a disk_log sync operation in order to make sure that buffered data
gets written to disk. The handler will repeatedly attempt this
operation, but only perform it if something has actually been logged
since the last sync. The default value is <c>5000</c> milliseconds.
If <c>no_repeat</c> is set as value, the repeated sync operation is
disabled. The user can also call the
<seealso marker="logger_disk_log_h#disk_log_sync-1"><c>disk_log_sync/1</c>
</seealso> function to perform a disk_log sync.</p></item>
</taglist>
<p>There are a number of other configuration parameters available, that are
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>Note that when changing the configuration of the handler in runtime, by
calling
<seealso marker="logger#set_handler_config-2"><c>logger:set_handler_config/2
or logger:set_handler_config/3</c></seealso>, the <c>disk_log_opts</c>
settings may not be modified.</p>
<p>Example of adding a disk_log handler:</p>
<code type="none">
logger:add_handler(my_disk_log_h, logger_disk_log_h,
#{level => error,
filter_default => log,
disk_log_opts =>
#{file => "./my_disk_log",
type => wrap,
max_no_files => 4,
max_no_bytes => 10000},
logger_disk_log_h =>
#{filesync_repeat_interval => 1000}}).
</code>
<p>In order to use the disk_log handler instead of the default standard
handler when starting en Erlang node, use the kernel configuration parameter
<seealso marker="kernel_app#configuration"><c>logger_dest</c></seealso> with
value <c>{disk_log,FileName}</c>. Example:</p>
<code type="none">
erl -kernel logger_dest '{disk_log,"./system_disk_log"}'
</code>
</description>
<funcs>
<func>
<name name="disk_log_sync" arity="1" clause_i="1"/>
<fsummary>Writes buffered data to disk.</fsummary>
<desc>
<p>Write buffered data to disk.</p>
</desc>
</func>
</funcs>
</erlref>
