path: root/lib/stdlib/doc/src/supervisor.xml

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  <header>
    <copyright>
      <year>1996</year><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.
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          http://www.apache.org/licenses/LICENSE-2.0

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      distributed under the License is distributed on an "AS IS" BASIS,
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    <title>supervisor</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>supervisor</module>
  <modulesummary>Generic supervisor behavior.</modulesummary>
  <description>
    <p>This behavior module provides a supervisor, a process that
      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
      <seealso marker="gen_event"><c>gen_event</c></seealso>,
      <seealso marker="gen_server"><c>gen_server</c></seealso>, or
      <seealso marker="gen_statem"><c>gen_statem</c></seealso>
      behaviors. A supervisor implemented using this module has
      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. For more
      information, see <seealso marker="doc/design_principles:sup_princ">
      Supervisor Behaviour</seealso> in OTP Design Principles.</p>

    <p>A supervisor expects the definition of which child processes to
      supervise to be specified in a callback module exporting a
      predefined set of functions.</p>

    <p>Unless otherwise stated, all functions in this module fail
      if the specified supervisor does not exist or if bad arguments
      are specified.</p>
  </description>

  <section>
    <marker id="supervision_princ"/>
    <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 must 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 supervisor properties are defined by the supervisor flags.
      The type definition for the supervisor flags is as follows:</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
          is to 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
          is to 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
          is to be restarted, the 'rest' of the child processes (that
          is, 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, that is, running the same
          code.</p>
        <p>Functions
          <seealso marker="#delete_child/2"><c>delete_child/2</c></seealso> and
          <seealso marker="#restart_child/2"><c>restart_child/2</c></seealso>
          are invalid for <c>simple_one_for_one</c> supervisors and return
          <c>{error,simple_one_for_one}</c> if the specified supervisor
          uses this restart strategy.</p>
        <p>Function <seealso marker="#terminate_child/2">
          <c>terminate_child/2</c></seealso> can be used for
          children under <c>simple_one_for_one</c> supervisors by
          specifying the child's <c>pid()</c> as the second argument. If
          instead the child specification identifier is used,
          <c>terminate_child/2</c> return
          <c>{error,simple_one_for_one}</c>.</p>
        <p>As a <c>simple_one_for_one</c> supervisor can have
          many children, it shuts them all down asynchronously. This
          means that the children 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 keys <c>intensity</c> and <c>period</c> 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
      terminates all child processes and then itself. The termination
      reason for the supervisor itself in that case will be <c>shutdown</c>.
      <c>intensity</c> defaults to <c>1</c> and <c>period</c> defaults to
      <c>5</c>.</p>

    <marker id="child_spec"/>
    <p>The type definition of a child specification is as follows:</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>Notice that this identifier on occations has been called
          "name". As far as possible, the terms "identifier" or "id"
          are now used but to keep backward 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> any term that 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 is kept by the supervisor
          (unless it is a temporary child) but the non-existing child
          process is ignored.</p>
        <p>If something goes wrong, the function can also return an
          error tuple <c>{error,Error}</c>.</p>
        <p>Notice that the <c>start_link</c> functions of the different
          behavior 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
          must be restarted. A <c>permanent</c> child process is
          always restarted. A <c>temporary</c> child process is
          never 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).
          A <c>transient</c> child process is restarted only if
          it terminates abnormally, that is, 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 specified,
          it defaults to <c>permanent</c>.</p>
      </item>
      <item>
        <p><c>shutdown</c> defines how a child process must be
          terminated. <c>brutal_kill</c> means that the child process
          is unconditionally terminated using <c>exit(Child,kill)</c>.
          An integer time-out value means that the supervisor tells
          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
          is to 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>Notice that all child processes implemented using the standard
          OTP behavior modules automatically adhere to the shutdown
          protocol.</p>
        <p>The <c>shutdown</c> key is optional. If it is not specified,
          it defaults to <c>5000</c> if the child is
          of type <c>worker</c> and it defaults to <c>infinity</c> 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 specified,
          it defaults to <c>worker</c>.</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_statem</c>,
          this is to 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, value <c>dynamic</c>
          must be used. For more information about release handling, see
          <seealso marker="doc/design_principles:release_handling">
          Release Handling</seealso>
	  in OTP Design Principles.</p>
        <p>The <c>modules</c> key is optional. If it is not specified, 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 backward compatibility
        only. A map is preferred; see more details
        <seealso marker="#child_spec">above</seealso>.</p></desc>
    </datatype>
    <datatype>
      <name name="mfargs"/>
      <desc><p>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>, the process is never to be
        restarted and therefore there is no need to store the real
        argument list. Value <c>undefined</c> is then 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 backward 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="check_childspecs" arity="1"/>
      <fsummary>Check if children specifications are syntactically correct.
      </fsummary>
      <desc>
        <p>Takes a list of child specification as argument
          and returns <c>ok</c> if all of them are syntactically
          correct, otherwise <c>{error,<anno>Error</anno>}</c>.</p>
      </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 <seealso marker="proplists">
          <c>proplists</c></seealso>) 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. For a
              <c>simple_one_for_one</c> supervisors, no check is done to ensure
              that each child process is still alive, although 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
              <c>child_type = supervisor</c> in the specification list,
              regardless if the child process is still alive.</p>
          </item>
          <item>
            <p><c>workers</c> - The count of all children marked as
              <c>child_type = worker</c> in the specification list,
              regardless if the child process is still alive.</p>
          </item>
        </list>
        <p>For a description of <c><anno>SupRef</anno></c>, see
          <seealso marker="#SupRef"><c>start_child/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="delete_child" arity="2"/>
      <fsummary>Delete a child specification from a supervisor.</fsummary>
      <desc>
        <p>Tells 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
          <seealso marker="#terminate_child/2">
          <c>terminate_child/2</c></seealso> to terminate it.</p>
        <p>For a description of <c><anno>SupRef</anno></c>, see
          <seealso marker="#SupRef"><c>start_child/2</c></seealso>.</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 is 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="get_childspec" arity="2"/>
      <fsummary>Return the child specification map for the specified
        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>For a description of <c><anno>SupRef</anno></c>, see
          <seealso marker="#SupRef"><c>start_child/2</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name name="restart_child" arity="2"/>
      <fsummary>Restart a terminated child process belonging to a supervisor.
      </fsummary>
      <desc>
        <p>Tells 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>Notice 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>For a description of <c><anno>SupRef</anno></c>, see
          <seealso marker="#SupRef"><c>start_child/2</c></seealso>.</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="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 supervisor
          <c><anno>SupRef</anno></c>, which starts the corresponding child
          process.</p>
        <p><marker id="SupRef"/><c><anno>SupRef</anno></c> can be any of the
          following:</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</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 is started by
          using the start function as defined in the child specification.</p>
        <p>For a <c>simple_one_for_one</c> supervisor,
          the child specification defined in <c>Module:init/1</c> is used,
          and <c><anno>ChildSpec</anno></c> must instead be an arbitrary
          list of terms <c><anno>List</anno></c>. The child process is then
          started by appending <c><anno>List</anno></c> to the existing start
          function arguments, that is, 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>
        <list type="bulleted">
          <item>
            <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>
          </item>
          <item>
            <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>
          </item>
          <item>
            <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>
          </item>
        </list>
        <p>For 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="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.
          For example, the function ensures 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 startup
          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>
        <list type="bulleted">
          <item>
            <p>If <c><anno>SupName</anno>={local,Name}</c>, the supervisor is
              registered locally as <c>Name</c> using <c>register/2</c>.</p>
          </item>
          <item>
            <p>If <c><anno>SupName</anno>={global,Name}</c>, the supervisor is
              registered globally as <c>Name</c> using
              <seealso marker="kernel:global#register_name/2">
              <c>global:register_name/2</c></seealso>.</p>
          </item>
          <item>
            <p>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 must behave
              like the corresponding functions in
              <seealso marker="kernel:global"><c>global</c></seealso>. Thus,
              <c>{via,global,<anno>Name</anno>}</c> is a valid reference.</p>
          </item>
        </list>
        <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 any term that is passed as
          the argument to <c><anno>Module</anno>:init/1</c>.</p>
        <list type="bulleted">
          <item>
            <p>If the supervisor and its child processes are successfully
              created (that is, 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.</p>
          </item>
          <item>
            <p>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>
          </item>
          <item>
            <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>.</p>
          </item>
          <item>
            <p>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>
          </item>
          <item>
            <p>If any child process start function fails or returns an error
              tuple or an erroneous value, the supervisor first terminates
              all already started child processes with reason <c>shutdown</c>
              and then terminate itself and returns
              <c>{error, {shutdown, Reason}}</c>.</p>
          </item>
        </list>
      </desc>
    </func>

    <func>
      <name name="terminate_child" arity="2"/>
      <fsummary>Terminate a child process belonging to a supervisor.</fsummary>
      <desc>
        <p>Tells supervisor <c><anno>SupRef</anno></c> to terminate the
          specified 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 any, is terminated and,
          unless it is a temporary child, the child specification is
          kept by the supervisor. The child process can later be
          restarted by the supervisor. The child process can also be
          restarted explicitly by calling
          <seealso marker="#restart_child/2"><c>restart_child/2</c></seealso>.
          Use
          <seealso marker="#delete_child/2"><c>delete_child/2</c></seealso>
          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> cannot 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 <c>pid()</c> of the child process. If the specified
          process is alive, but is not a child of the specified
          supervisor, the function returns
          <c>{error,not_found}</c>. If the child specification
          identifier is specified instead of a <c>pid()</c>, the
          function returns <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>For a description of <c><anno>SupRef</anno></c>, see
          <seealso marker="#SupRef"><c>start_child/2</c></seealso>.</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
          supervisor <c><anno>SupRef</anno></c>.</p>
        <p>Notice that calling this function when supervising many
          childrens under low memory conditions can cause an
          out of memory exception.</p>
        <p>For a description of <c><anno>SupRef</anno></c>, see
          <seealso marker="#SupRef"><c>start_child/2</c></seealso>.</p>
        <p>The following information is given for each child
          specification/process:</p>
        <list type="bulleted">
          <item>
            <p><c><anno>Id</anno></c> - As defined in the child specification or
              <c>undefined</c> for 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>
  </funcs>

  <section>
    <title>Callback Functions</title>
    <p>The following function 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>&nbsp;SupFlags =
          <seealso marker="#type-sup_flags"><c>sup_flags()</c></seealso></v>
        <v>&nbsp;ChildSpec =
          <seealso marker="#type-child_spec"><c>child_spec()</c></seealso></v>
      </type>
      <desc>
        <p>Whenever a supervisor is started using
          <seealso marker="#start_link/2"><c>start_link/2,3</c></seealso>,
          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 maximum restart intensity for the
          supervisor. <c>[ChildSpec]</c> is a list of valid child
          specifications defining which child processes the supervisor
          must start and monitor. See the discussion in section
          <seealso marker="#supervision_princ">
          <c>Supervision Principles</c></seealso> earlier.</p>
        <p>Notice 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
          <seealso marker="#start_child/2"><c>start_child/2</c></seealso>.</p>
        <p>The function can also return <c>ignore</c>.</p>
        <p>Notice that this function can also be called as a part of a code
          upgrade procedure. Therefore, the function is not to have any side
          effects. For more information about code upgrade of supervisors, see
          section
          <seealso marker="doc/design_principles:appup_cookbook#sup">Changing
          a Supervisor</seealso> in OTP Design Principles.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>See Also</title>
    <p><seealso marker="gen_event"><c>gen_event(3)</c></seealso>, 
      <seealso marker="gen_statem"><c>gen_statem(3)</c></seealso>,
      <seealso marker="gen_server"><c>gen_server(3)</c></seealso>, 
      <seealso marker="sys"><c>sys(3)</c></seealso></p>
  </section>
</erlref>