<?xml version="1.0" encoding="utf-8" ?>
<!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 application.</modulesummary>
<description>
</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
FilterDefault: log
Filters:
Handlers:
Id: logger_std_h
Module: logger_std_h
Level: info
Formatter:
Module: logger_formatter
Config: #{template => [{logger_formatter,header},"\n",msg,"\n"],
legacy_header => true}
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 reffering 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="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="#set_logger_config-2"><c>set_logger_config/2</c>
</seealso>, or read the current configuration
with <seealso marker="#get_logger_config-0"><c>get_logger_config/0</c>
</seealso>, then merge in your added or updated
associations before writing it back.</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
doesn't exist, it will be added.</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="#set_handler_config-3"><c>set_handler_config/3</c>
</seealso>, or read the current configuration
with <seealso marker="#get_handler_config-1"><c>get_handler_config/1</c>
</seealso>, then merge in your added or updated
associations before writing it back.</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 doesn't exist, it will
be added.</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>
