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<erlref>
  <header>
    <copyright>
      <year>1996</year>
      <year>2020</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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  See the License for the specific language governing permissions and
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  The Initial Developer of the Original Code is Ericsson AB.
    </legalnotice>

    <title>supervisor_bridge</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module since="">supervisor_bridge</module>
  <modulesummary>Generic supervisor bridge behavior.</modulesummary>
  <description>
    <p>This behavior module provides a supervisor bridge, a process
      that connects a subsystem not designed according to the OTP design
      principles to a supervision tree. The supervisor bridge sits between
      a supervisor and the subsystem. It behaves like a real supervisor to
      its own supervisor, but has a different interface than a real
      supervisor to the subsystem. For more information, see
      <seeguide marker="system/design_principles:sup_princ">
	Supervisor Behaviour</seeguide> in OTP Design Principles.
    </p>

    <p>A supervisor bridge assumes the functions for starting and stopping
      the subsystem to be located in a callback module exporting a
      predefined set of functions.</p>

    <p>The <seeerl marker="sys"><c>sys(3)</c></seeerl> module can be used
      for debugging a supervisor bridge.</p>

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

  <funcs>
    <func>
      <name name="start_link" arity="2" since=""/>
      <name name="start_link" arity="3" since=""/>
      <fsummary>Create a supervisor bridge process.</fsummary>
      <desc>
        <p>Creates a supervisor bridge process, linked to the calling process,
          which calls <c><anno>Module</anno>:init/1</c> to start the subsystem.
          To ensure a synchronized startup procedure, this function does
          not return until <c><anno>Module</anno>:init/1</c> has returned.</p>
        <list type="bulleted">
          <item>
            <p>If <c><anno>SupBridgeName</anno>={local,<anno>Name</anno>}</c>,
              the supervisor bridge is registered locally as
              <c><anno>Name</anno></c> using <c>register/2</c>.</p>
          </item>
          <item>
            <p>If <c><anno>SupBridgeName</anno>={global,<anno>Name</anno>}</c>,
              the supervisor bridge is registered globally as
              <c><anno>Name</anno></c> using
              <seemfa marker="kernel:global#register_name/2">
              <c>global:register_name/2</c></seemfa>.</p>
          </item>
          <item>
            <p>If
            <c><anno>SupBridgeName</anno>={via,<anno>Module</anno>,<anno>Name</anno>}</c>,
            the supervisor bridge is registered as <c><anno>Name</anno></c>
            using a registry represented by <anno>Module</anno>. The
            <c>Module</c> callback is to export functions
            <c>register_name/2</c>, <c>unregister_name/1</c>, and <c>send/2</c>,
            which are to behave like the corresponding functions in
            <seeerl marker="kernel:global"><c>global</c></seeerl>.
            Thus, <c>{via,global,GlobalName}</c> is a valid reference.</p>
          </item>
        </list>
        <p>If no name is provided, the supervisor bridge 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 that is passed as the
          argument to <c><anno>Module</anno>:init/1</c>.</p>
        <list type="bulleted">
          <item>
            <p>If the supervisor bridge and the subsystem are successfully
              started, the function returns <c>{ok,<anno>Pid</anno>}</c>, where
              <c><anno>Pid</anno></c> is is the pid of the supervisor
              bridge.</p>
          </item>
          <item>
            <p>If there already exists a process with the specified
              <c><anno>SupBridgeName</anno></c>, the function returns
              <c>{error,{already_started,<anno>Pid</anno>}}</c>, where
              <c><anno>Pid</anno></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 bridge
              terminates with reason <c>normal</c>.</p>
          </item>
          <item>
            <p>If <c><anno>Module</anno>:init/1</c> fails or returns an error
              tuple or an incorrect value, this function returns
              <c>{error,<anno>Error</anno>r}</c>, where
              <c><anno>Error</anno></c> is a term with information about the
              error, and the supervisor bridge
              terminates with reason <c><anno>Error</anno></c>.</p>
          </item>
        </list>
      </desc>
    </func>
  </funcs>

 

  <funcs>
    <fsdescription>
      <title>Callback Functions</title>
      <p>The following functions must be exported from a
        <c>supervisor_bridge</c> callback module.</p>
    </fsdescription>
    <func>
      <name since="">Module:init(Args) -> Result</name>
      <fsummary>Initialize process and start subsystem.</fsummary>
      <type>
        <v>Args = term()</v>
        <v>Result = {ok,Pid,State} | ignore | {error,Error}</v>
        <v>&nbsp;Pid = pid()</v>
        <v>&nbsp;State = term()</v>
        <v>&nbsp;Error = term()</v>
      </type>
      <desc>
        <p>Whenever a supervisor bridge is started using
          <seemfa marker="#start_link/2"><c>start_link/2,3</c></seemfa>,
          this function is called
          by the new process to start the subsystem and initialize.</p>
        <p><c>Args</c> is the <c>Args</c> argument provided to the start
          function.</p>
        <p>The function is to return <c>{ok,Pid,State}</c>, where <c>Pid</c>
          is the pid of the main process in the subsystem and <c>State</c>
          is any term.</p>
        <p>If later <c>Pid</c> terminates with a reason <c>Reason</c>,
          the supervisor bridge terminates with reason <c>Reason</c> as well.
          If later the supervisor bridge is stopped by its supervisor with
          reason <c>Reason</c>, it calls
          <c>Module:terminate(Reason,State)</c> to terminate.</p>
        <p>If the initialization fails, the function is to return
          <c>{error,Error}</c>, where <c>Error</c> is any term,
          or <c>ignore</c>.</p>
      </desc>
    </func>

    <func>
      <name since="">Module:terminate(Reason, State)</name>
      <fsummary>Clean up and stop subsystem.</fsummary>
      <type>
        <v>Reason = shutdown | term()</v>
        <v>State = term()</v>
      </type>
      <desc>
        <p>This function is called by the supervisor bridge when it is about
          to terminate. It is to be the opposite of <c>Module:init/1</c>
          and stop the subsystem and do any necessary cleaning up.
          The return value is ignored.</p>
        <p><c>Reason</c> is <c>shutdown</c> if the supervisor bridge is
          terminated by its supervisor. If the supervisor bridge terminates
          because a a linked process (apart from the main process of
          the subsystem) has terminated with reason <c>Term</c>,
          then <c>Reason</c> becomes <c>Term</c>.</p>
        <p><c>State</c> is taken from the return value of
          <c>Module:init/1</c>.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>See Also</title>
    <p><seeerl marker="supervisor"><c>supervisor(3)</c></seeerl>, 
      <seeerl marker="sys"><c>sys(3)</c></seeerl></p>
  </section>
</erlref>

