<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>1996</year><year>2016</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>supervisor</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
</header>
<module>supervisor</module>
<modulesummary>Generic Supervisor Behaviour</modulesummary>
<description>
<p>A behaviour module for implementing a supervisor, a process which
supervises other processes called child processes. A child
process can either be another supervisor or a worker process.
Worker processes are normally implemented using one of
the <c>gen_event</c>, <c>gen_fsm</c>, or <c>gen_server</c>
behaviours. A supervisor implemented using this module will have
a standard set of interface functions and include functionality
for tracing and error reporting. Supervisors are used to build a
hierarchical process structure called a supervision tree, a
nice way to structure a fault tolerant application. Refer to
<em>OTP Design Principles</em> for more information.</p>
<p>A supervisor expects the definition of which child processes to
supervise to be specified in a callback module exporting a
pre-defined set of functions.</p>
<p>Unless otherwise stated, all functions in this module will fail
if the specified supervisor does not exist or if bad arguments
are given.</p>
</description>
<section>
<title>Supervision Principles</title>
<p>The supervisor is responsible for starting, stopping and
monitoring its child processes. The basic idea of a supervisor is
that it shall keep its child processes alive by restarting them
when necessary.</p>
<p>The children of a supervisor are defined as a list of
<em>child specifications</em>. When the supervisor is started, the child
processes are started in order from left to right according to
this list. When the supervisor terminates, it first terminates
its child processes in reversed start order, from right to left.</p>
<marker id="sup_flags"/>
<p>The properties of a supervisor are defined by the supervisor
flags. This is the type definition for the supervisor flags:
</p>
<pre>sup_flags() = #{strategy => strategy(), % optional
intensity => non_neg_integer(), % optional
period => pos_integer()} % optional
</pre>
<p>A supervisor can have one of the following <em>restart
strategies</em>, specified with the <c>strategy</c> key in the
above map:
</p>
<list type="bulleted">
<item>
<p><c>one_for_one</c> - if one child process terminates and
should be restarted, only that child process is
affected. This is the default restart strategy.</p>
</item>
<item>
<p><c>one_for_all</c> - if one child process terminates and
should be restarted, all other child processes are terminated
and then all child processes are restarted.</p>
</item>
<item>
<p><c>rest_for_one</c> - if one child process terminates and
should be restarted, the 'rest' of the child processes --
i.e. the child processes after the terminated child process
in the start order -- are terminated. Then the terminated
child process and all child processes after it are restarted.</p>
</item>
<item>
<p><c>simple_one_for_one</c> - a simplified <c>one_for_one</c>
supervisor, where all child processes are dynamically added
instances of the same process type, i.e. running the same
code.</p>
<p>The functions <c>delete_child/2</c>
and <c>restart_child/2</c> are invalid for
<c>simple_one_for_one</c> supervisors and will return
<c>{error,simple_one_for_one}</c> if the specified supervisor
uses this restart strategy.</p>
<p>The function <c>terminate_child/2</c> can be used for
children under <c>simple_one_for_one</c> supervisors by
giving the child's <c>pid()</c> as the second argument. If
instead the child specification identifier is used,
<c>terminate_child/2</c> will return
<c>{error,simple_one_for_one}</c>.</p>
<p>Because a <c>simple_one_for_one</c> supervisor could have
many children, it shuts them all down asynchronously. This
means that the children will do their cleanup in parallel,
and therefore the order in which they are stopped is not
defined.</p>
</item>
</list>
<p>To prevent a supervisor from getting into an infinite loop of
child process terminations and restarts, a <em>maximum restart
intensity</em> is defined using two integer values specified
with the <c>intensity</c> and <c>period</c> keys in the above
map. Assuming the values <c>MaxR</c> for <c>intensity</c>
and <c>MaxT</c> for <c>period</c>, then if more than <c>MaxR</c>
restarts occur within <c>MaxT</c> seconds, the supervisor will
terminate all child processes and then itself. The default value
for <c>intensity</c> is <c>1</c>, and the default value
for <c>period</c> is <c>5</c>.
</p>
<marker id="child_spec"/>
<p>This is the type definition of a child specification:</p>
<pre>child_spec() = #{id => child_id(), % mandatory
start => mfargs(), % mandatory
restart => restart(), % optional
shutdown => shutdown(), % optional
type => worker(), % optional
modules => modules()} % optional</pre>
<p>The old tuple format is kept for backwards compatibility,
see <seealso marker="#type-child_spec">child_spec()</seealso>,
but the map is preferred.
</p>
<list type="bulleted">
<item>
<p><c>id</c> is used to identify the child
specification internally by the supervisor.</p>
<p>The <c>id</c> key is mandatory.</p>
<p>Note that this identifier on occations has been called
"name". As far as possible, the terms "identifier" or "id"
are now used but in order to keep backwards compatibility,
some occurences of "name" can still be found, for example
in error messages.</p>
</item>
<item>
<p><c>start</c> defines the function call used to start the
child process. It must be a module-function-arguments
tuple <c>{M,F,A}</c> used as <c>apply(M,F,A)</c>.</p>
<p>The start function <em>must create and link to</em> the child
process, and must return <c>{ok,Child}</c> or
<c>{ok,Child,Info}</c> where <c>Child</c> is the pid of
the child process and <c>Info</c> an arbitrary term which is
ignored by the supervisor.</p>
<p>The start function can also return <c>ignore</c> if the child
process for some reason cannot be started, in which case
the child specification will be kept by the supervisor
(unless it is a temporary child) but the non-existing child
process will be ignored.</p>
<p>If something goes wrong, the function may also return an
error tuple <c>{error,Error}</c>.</p>
<p>Note that the <c>start_link</c> functions of the different
behaviour modules fulfill the above requirements.</p>
<p>The <c>start</c> key is mandatory.</p>
</item>
<item>
<p><c>restart</c> defines when a terminated child process
shall be restarted. A <c>permanent</c> child process will
always be restarted, a <c>temporary</c> child process will
never be restarted (even when the supervisor's restart strategy
is <c>rest_for_one</c> or <c>one_for_all</c> and a sibling's
death causes the temporary process to be terminated) and a
<c>transient</c> child process will be restarted only if
it terminates abnormally, i.e. with another exit reason
than <c>normal</c>, <c>shutdown</c> or <c>{shutdown,Term}</c>.</p>
<p>The <c>restart</c> key is optional. If it is not given, the
default value <c>permanent</c> will be used.</p>
</item>
<item>
<p><c>shutdown</c> defines how a child process shall be
terminated. <c>brutal_kill</c> means the child process will
be unconditionally terminated using <c>exit(Child,kill)</c>.
An integer timeout value means that the supervisor will tell
the child process to terminate by calling
<c>exit(Child,shutdown)</c> and then wait for an exit signal
with reason <c>shutdown</c> back from the child process. If
no exit signal is received within the specified number of milliseconds,
the child process is unconditionally terminated using
<c>exit(Child,kill)</c>.</p>
<p>If the child process is another supervisor, the shutdown time
should be set to <c>infinity</c> to give the subtree ample
time to shut down. It is also allowed to set it to <c>infinity</c>,
if the child process is a worker.</p>
<warning>
<p>Be careful when setting the shutdown time to
<c>infinity</c> when the child process is a worker. Because, in this
situation, the termination of the supervision tree depends on the
child process, it must be implemented in a safe way and its cleanup
procedure must always return.</p>
</warning>
<p>Note that all child processes implemented using the standard
OTP behaviour modules automatically adhere to the shutdown
protocol.</p>
<p>The <c>shutdown</c> key is optional. If it is not given,
the default value <c>5000</c> will be used if the child is
of type <c>worker</c>; and <c>infinity</c> will be used if
the child is of type <c>supervisor</c>.</p>
</item>
<item>
<p><c>type</c> specifies if the child process is a supervisor or
a worker.</p>
<p>The <c>type</c> key is optional. If it is not given, the
default value <c>worker</c> will be used.</p>
</item>
<item>
<p><c>modules</c> is used by the release handler during code
replacement to determine which processes are using a certain
module. As a rule of thumb, if the child process is a
<c>supervisor</c>, <c>gen_server</c>, or <c>gen_fsm</c>,
this should be a list with one element <c>[Module]</c>,
where <c>Module</c> is the callback module. If the child
process is an event manager (<c>gen_event</c>) with a
dynamic set of callback modules, the value <c>dynamic</c>
shall be used. See <em>OTP Design Principles</em> for more
information about release handling.</p>
<p>The <c>modules</c> key is optional. If it is not given, it
defaults to <c>[M]</c>, where <c>M</c> comes from the
child's start <c>{M,F,A}</c></p>
</item>
<item>
<p>Internally, the supervisor also keeps track of the pid
<c>Child</c> of the child process, or <c>undefined</c> if no
pid exists.</p>
</item>
</list>
</section>
<datatypes>
<datatype>
<name name="child"/>
</datatype>
<datatype>
<name name="child_id"/>
<desc><p>Not a <c>pid()</c>.</p></desc>
</datatype>
<datatype>
<name name="child_spec"/>
<desc><p>The tuple format is kept for backwards compatibility
only. A map is preferred; see more details
<seealso marker="#child_spec">above</seealso>.</p></desc>
</datatype>
<datatype>
<name name="mfargs"/>
<desc>
<p>The value <c>undefined</c> for <c><anno>A</anno></c> (the
argument list) is only to be used internally
in <c>supervisor</c>. If the restart type of the child
is <c>temporary</c>, then the process is never to be
restarted and therefore there is no need to store the real
argument list. The value <c>undefined</c> will then be
stored instead.</p>
</desc>
</datatype>
<datatype>
<name name="modules"/>
</datatype>
<datatype>
<name name="restart"/>
</datatype>
<datatype>
<name name="shutdown"/>
</datatype>
<datatype>
<name name="strategy"/>
</datatype>
<datatype>
<name name="sup_flags"/>
<desc><p>The tuple format is kept for backwards compatibility
only. A map is preferred; see more details
<seealso marker="#sup_flags">above</seealso>.</p></desc>
</datatype>
<datatype>
<name name="sup_ref"/>
</datatype>
<datatype>
<name name="worker"/>
</datatype>
</datatypes>
<funcs>
<func>
<name name="start_link" arity="2"/>
<name name="start_link" arity="3"/>
<fsummary>Create a supervisor process.</fsummary>
<type name="startlink_ret"/>
<type name="startlink_err"/>
<type name="sup_name"/>
<desc>
<p>Creates a supervisor process as part of a supervision tree.
The function will, among other things, ensure that
the supervisor is linked to the calling process (its
supervisor).</p>
<p>The created supervisor process calls <c><anno>Module</anno>:init/1</c> to
find out about restart strategy, maximum restart intensity
and child processes. To ensure a synchronized start-up
procedure, <c>start_link/2,3</c> does not return until
<c><anno>Module</anno>:init/1</c> has returned and all child processes
have been started.</p>
<p>If <c><anno>SupName</anno>={local,Name}</c>, the supervisor is registered
locally as <c>Name</c> using <c>register/2</c>. If
<c><anno>SupName</anno>={global,Name}</c> the supervisor is registered
globally as <c>Name</c> using <c>global:register_name/2</c>. If
<c><anno>SupName</anno>={via,<anno>Module</anno>,<anno>Name</anno>}</c> the supervisor
is registered as <c>Name</c> using the registry represented by
<c>Module</c>. The <c>Module</c> callback must export the functions
<c>register_name/2</c>, <c>unregister_name/1</c> and <c>send/2</c>,
which shall behave like the corresponding functions in <c>global</c>.
Thus, <c>{via,global,<anno>Name</anno>}</c> is a valid reference.</p>
<p>If no name is provided, the supervisor is not registered.</p>
<p><c><anno>Module</anno></c> is the name of the callback module.</p>
<p><c><anno>Args</anno></c> is an arbitrary term which is passed as
the argument to <c><anno>Module</anno>:init/1</c>.</p>
<p>If the supervisor and its child processes are successfully
created (i.e. if all child process start functions return
<c>{ok,Child}</c>, <c>{ok,Child,Info}</c>, or <c>ignore</c>),
the function returns <c>{ok,Pid}</c>, where <c>Pid</c> is
the pid of the supervisor. If there already exists a process
with the specified <c><anno>SupName</anno></c>, the function returns
<c>{error,{already_started,Pid}}</c>, where <c>Pid</c> is
the pid of that process.</p>
<p>If <c><anno>Module</anno>:init/1</c> returns <c>ignore</c>, this function
returns <c>ignore</c> as well, and the supervisor terminates
with reason <c>normal</c>.
If <c><anno>Module</anno>:init/1</c> fails or returns an incorrect value,
this function returns <c>{error,Term}</c> where <c>Term</c>
is a term with information about the error, and the supervisor
terminates with reason <c>Term</c>.</p>
<p>If any child process start function fails or returns an error
tuple or an erroneous value, the supervisor will first terminate
all already started child processes with reason <c>shutdown</c>
and then terminate itself and return
<c>{error, {shutdown, Reason}}</c>.</p>
</desc>
</func>
<func>
<name name="start_child" arity="2"/>
<fsummary>Dynamically add a child process to a supervisor.</fsummary>
<type name="startchild_ret"/>
<type name="startchild_err"/>
<desc>
<p>Dynamically adds a child specification to the supervisor
<c><anno>SupRef</anno></c> which starts the corresponding child process.</p>
<p><marker id="SupRef"/><c><anno>SupRef</anno></c> can be:</p>
<list type="bulleted">
<item>the pid,</item>
<item><c>Name</c>, if the supervisor is locally registered,</item>
<item><c>{Name,Node}</c>, if the supervisor is locally
registered at another node, or</item>
<item><c>{global,Name}</c>, if the supervisor is globally
registered.</item>
<item><c>{via,Module,Name}</c>, if the supervisor is registered
through an alternative process registry.</item>
</list>
<p><c><anno>ChildSpec</anno></c> must be a valid child specification
(unless the supervisor is a <c>simple_one_for_one</c>
supervisor; see below). The child process will be started by
using the start function as defined in the child
specification.</p>
<p>In the case of a <c>simple_one_for_one</c> supervisor,
the child specification defined in <c>Module:init/1</c> will
be used, and <c><anno>ChildSpec</anno></c> shall instead be an arbitrary
list of terms <c><anno>List</anno></c>. The child process will then be
started by appending <c><anno>List</anno></c> to the existing start
function arguments, i.e. by calling
<c>apply(M, F, A++<anno>List</anno>)</c> where <c>{M,F,A}</c> is the start
function defined in the child specification.</p>
<p>If there already exists a child specification with
the specified identifier, <c><anno>ChildSpec</anno></c> is discarded, and
the function returns <c>{error,already_present}</c> or
<c>{error,{already_started,<anno>Child</anno>}}</c>, depending on if
the corresponding child process is running or not.</p>
<p>If the child process start function returns <c>{ok,<anno>Child</anno>}</c>
or <c>{ok,<anno>Child</anno>,<anno>Info</anno>}</c>, the child specification and pid are
added to the supervisor and the function returns the same
value.</p>
<p>If the child process start function returns <c>ignore</c>,
the child specification is added to the supervisor (unless the
supervisor is a <c>simple_one_for_one</c> supervisor, see below),
the pid is set to <c>undefined</c> and the function returns
<c>{ok,undefined}</c>.
</p>
<p>In the case of a <c>simple_one_for_one</c> supervisor, when a child
process start function returns <c>ignore</c> the functions returns
<c>{ok,undefined}</c> and no child is added to the supervisor.
</p>
<p>If the child process start function returns an error tuple or
an erroneous value, or if it fails, the child specification is
discarded, and the function returns <c>{error,Error}</c> where
<c>Error</c> is a term containing information about the error
and child specification.</p>
</desc>
</func>
<func>
<name name="terminate_child" arity="2"/>
<fsummary>Terminate a child process belonging to a supervisor.</fsummary>
<desc>
<p>Tells the supervisor <c><anno>SupRef</anno></c> to terminate the given
child.</p>
<p>If the supervisor is not <c>simple_one_for_one</c>,
<c><anno>Id</anno></c> must be the child specification
identifier. The process, if there is one, is terminated and,
unless it is a temporary child, the child specification is
kept by the supervisor. The child process may later be
restarted by the supervisor. The child process can also be
restarted explicitly by calling
<c>restart_child/2</c>. Use <c>delete_child/2</c> to remove
the child specification.</p>
<p>If the child is temporary, the child specification is deleted as
soon as the process terminates. This means
that <c>delete_child/2</c> has no meaning,
and <c>restart_child/2</c> can not be used for these
children.</p>
<p>If the supervisor is <c>simple_one_for_one</c>, <c><anno>Id</anno></c>
must be the child process' <c>pid()</c>. If the specified
process is alive, but is not a child of the given
supervisor, the function will return
<c>{error,not_found}</c>. If the child specification
identifier is given instead of a <c>pid()</c>, the
function will return <c>{error,simple_one_for_one}</c>.</p>
<p>If successful, the function returns <c>ok</c>. If there is
no child specification with the specified <c><anno>Id</anno></c>, the
function returns <c>{error,not_found}</c>.</p>
<p>See <seealso marker="#SupRef"><c>start_child/2</c></seealso>
for a description of <c><anno>SupRef</anno></c>.</p>
</desc>
</func>
<func>
<name name="delete_child" arity="2"/>
<fsummary>Delete a child specification from a supervisor.</fsummary>
<desc>
<p>Tells the supervisor <c><anno>SupRef</anno></c> to delete the child
specification identified by <c><anno>Id</anno></c>. The corresponding child
process must not be running. Use <c>terminate_child/2</c> to
terminate it.</p>
<p>See <seealso marker="#SupRef"><c>start_child/2</c></seealso>
for a description of <c><anno>SupRef</anno></c>.</p>
<p>If successful, the function returns <c>ok</c>. If the child
specification identified by <c><anno>Id</anno></c> exists but
the corresponding child process is running or about to be restarted,
the function returns <c>{error,running}</c> or
<c>{error,restarting}</c>, respectively. If the child specification
identified by <c><anno>Id</anno></c> does not exist, the function
returns <c>{error,not_found}</c>.</p>
</desc>
</func>
<func>
<name name="restart_child" arity="2"/>
<fsummary>Restart a terminated child process belonging to a supervisor.</fsummary>
<desc>
<p>Tells the supervisor <c><anno>SupRef</anno></c> to restart
a child process corresponding to the child specification
identified by <c><anno>Id</anno></c>. The child
specification must exist, and the corresponding child process
must not be running.</p>
<p>Note that for temporary children, the child specification
is automatically deleted when the child terminates; thus
it is not possible to restart such children.</p>
<p>See <seealso marker="#SupRef"><c>start_child/2</c></seealso>
for a description of <c>SupRef</c>.</p>
<p>If the child specification identified
by <c><anno>Id</anno></c> does not exist, the function
returns <c>{error,not_found}</c>. If the child specification
exists but the corresponding process is already running, the
function returns
<c>{error,running}</c>.</p>
<p>If the child process start function
returns <c>{ok,<anno>Child</anno>}</c>
or <c>{ok,<anno>Child</anno>,<anno>Info</anno>}</c>, the pid
is added to the supervisor and the function returns the same
value.</p>
<p>If the child process start function returns <c>ignore</c>,
the pid remains set to <c>undefined</c>, and the function
returns <c>{ok,undefined}</c>.</p>
<p>If the child process start function returns an error tuple
or an erroneous value, or if it fails, the function returns
<c>{error,<anno>Error</anno>}</c>
where <c><anno>Error</anno></c> is a term containing
information about the error.</p>
</desc>
</func>
<func>
<name name="which_children" arity="1"/>
<fsummary>Return information about all children specifications and
child processes belonging to a supervisor.</fsummary>
<desc>
<p>Returns a newly created list with information about all child
specifications and child processes belonging to
the supervisor <c><anno>SupRef</anno></c>.</p>
<p>Note that calling this function when supervising a large
number of children under low memory conditions can cause an
out of memory exception.</p>
<p>See <seealso marker="#SupRef"><c>start_child/2</c></seealso> for a description of
<c>SupRef</c>.</p>
<p>The information given for each child specification/process
is:</p>
<list type="bulleted">
<item>
<p><c><anno>Id</anno></c> - as defined in the child specification or
<c>undefined</c> in the case of a
<c>simple_one_for_one</c> supervisor.</p>
</item>
<item>
<p><c><anno>Child</anno></c> - the pid of the corresponding child
process, the atom <c>restarting</c> if the process is about to be
restarted, or <c>undefined</c> if there is no such process.</p>
</item>
<item>
<p><c><anno>Type</anno></c> - as defined in the child specification.</p>
</item>
<item>
<p><c><anno>Modules</anno></c> - as defined in the child specification.</p>
</item>
</list>
</desc>
</func>
<func>
<name name="count_children" arity="1"/>
<fsummary>Return counts for the number of child specifications,
active children, supervisors, and workers.</fsummary>
<desc>
<p>Returns a property list (see <c>proplists</c>) containing the
counts for each of the following elements of the supervisor's
child specifications and managed processes:</p>
<list type="bulleted">
<item>
<p><c>specs</c> - the total count of children, dead or alive.</p>
</item>
<item>
<p><c>active</c> - the count of all actively running child processes
managed by this supervisor. In the case of <c>simple_one_for_one</c>
supervisors, no check is carried out to ensure that each child process
is still alive, though the result provided here is likely to be very
accurate unless the supervisor is heavily overloaded.</p>
</item>
<item>
<p><c>supervisors</c> - the count of all children marked as
child_type = supervisor in the spec list, whether or not the
child process is still alive.</p>
</item>
<item>
<p><c>workers</c> - the count of all children marked as
child_type = worker in the spec list, whether or not the child
process is still alive.</p>
</item>
</list>
<p>See <seealso marker="#SupRef"><c>start_child/2</c></seealso>
for a description of <c><anno>SupRef</anno></c>.</p>
</desc>
</func>
<func>
<name name="check_childspecs" arity="1"/>
<fsummary>Check if children specifications are syntactically correct.</fsummary>
<desc>
<p>This function takes a list of child specification as argument
and returns <c>ok</c> if all of them are syntactically
correct, or <c>{error,<anno>Error</anno>}</c> otherwise.</p>
</desc>
</func>
<func>
<name name="get_childspec" arity="2"/>
<fsummary>Return the child specification map for the given
child.</fsummary>
<desc>
<p>Returns the child specification map for the child identified
by <c>Id</c> under supervisor <c>SupRef</c>. The returned
map contains all keys, both mandatory and optional.</p>
<p>See <seealso marker="#SupRef"><c>start_child/2</c></seealso>
for a description of <c><anno>SupRef</anno></c>.</p>
</desc>
</func>
</funcs>
<section>
<title>CALLBACK FUNCTIONS</title>
<p>The following functions must be exported from a
<c>supervisor</c> callback module.</p>
</section>
<funcs>
<func>
<name>Module:init(Args) -> Result</name>
<fsummary>Return a supervisor specification.</fsummary>
<type>
<v>Args = term()</v>
<v>Result = {ok,{SupFlags,[ChildSpec]}} | ignore</v>
<v> SupFlags = <seealso marker="#type-sup_flags">sup_flags()</seealso></v>
<v> ChildSpec = <seealso marker="#type-child_spec">child_spec()</seealso></v>
</type>
<desc>
<p>Whenever a supervisor is started using
<c>supervisor:start_link/2,3</c>, this function is called by
the new process to find out about restart strategy, maximum
restart intensity, and child specifications.</p>
<p><c>Args</c> is the <c>Args</c> argument provided to the start
function.</p>
<p><c>SupFlags</c> is the supervisor flags defining the
restart strategy and max restart intensity for the
supervisor. <c>[ChildSpec]</c> is a list of valid child
specifications defining which child processes the supervisor
shall start and monitor. See the discussion about
Supervision Principles above.</p>
<p>Note that when the restart strategy is
<c>simple_one_for_one</c>, the list of child specifications
must be a list with one child specification only.
(The child specification identifier is ignored.) No child process is then started
during the initialization phase, but all children are assumed
to be started dynamically using
<c>supervisor:start_child/2</c>.</p>
<p>The function may also return <c>ignore</c>.</p>
<p>Note that this function might also be called as a part of a
code upgrade procedure. For this reason, the function should
not have any side effects. See
<seealso marker="doc/design_principles:appup_cookbook#sup">Design
Principles</seealso> for more information about code upgrade
of supervisors.</p>
</desc>
</func>
</funcs>
<section>
<title>SEE ALSO</title>
<p><seealso marker="gen_event">gen_event(3)</seealso>,
<seealso marker="gen_fsm">gen_fsm(3)</seealso>,
<seealso marker="gen_server">gen_server(3)</seealso>,
<seealso marker="sys">sys(3)</seealso></p>
</section>
</erlref>
