path: root/lib/kernel/doc/src/global.xml

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  <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.
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      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
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    </legalnotice>

    <title>global</title>
    <prepared>Martin Bj&ouml;rklund</prepared>
    <docno></docno>
    <date>1997-11-17</date>
    <rev></rev>
  </header>
  <module>global</module>
  <modulesummary>A Global Name Registration Facility</modulesummary>
  <description>
    <p>This documentation describes the Global module which consists
      of the following functionalities:</p>

    <list type="bulleted">
      <item>registration of global names;</item>
      <item>global locks;</item>
      <item>maintenance of the fully connected network.</item>
    </list>

    <p>These services are controlled via the process
      <c>global_name_server</c> which exists on every node. The global
      name server is started automatically when a node is started.
      With the term <em>global</em> is meant over a system consisting
      of several Erlang nodes.</p>
 
    <p>The ability to globally register names is a central concept in
      the programming of distributed Erlang systems. In this module,
      the equivalent of the <c>register/2</c> and <c>whereis/1</c>
      BIFs (for local name registration) are implemented, but for a
      network of Erlang nodes. A registered name is an alias for a
      process identifier (pid). The global name server monitors
      globally registered pids. If a process terminates, the name will
      also be globally unregistered.</p>

    <p>The registered names are stored in replica global name tables on
      every node. There is no central storage point. Thus,
      the translation of a name to a pid is fast, as it is always done
      locally. When any action in taken which results in a change to
      the global name table, all tables on other nodes are automatically
      updated.</p>

    <p>Global locks have lock identities and are set on a specific
      resource. For instance, the specified resource could be a pid.
      When a global lock is set, access to the locked resource is
      denied for all other resources other than the lock requester.</p>

    <p>Both the registration and lock functionalities are atomic. All
      nodes involved in these actions will have the same view of
      the information.</p>

    <p>The global name server also performs the critical task of
      continuously monitoring changes in node configuration: if a node
      which runs a globally registered process goes down, the name
      will be globally unregistered. To this end the global name
      server subscribes to <c>nodeup</c> and <c>nodedown</c> messages
      sent from the <c>net_kernel</c> module. Relevant Kernel
      application variables in this context are <c>net_setuptime</c>,
      <c>net_ticktime</c>, and <c>dist_auto_connect</c>. See also
      <seealso marker="kernel_app#net_setuptime">kernel(6)</seealso>.</p>

    <p>The name server will also maintain a fully connected network. For
      example, if node <c>N1</c> connects to node <c>N2</c> (which is
      already connected to <c>N3</c>), the global name servers on the
      nodes <c>N1</c> and <c>N3</c> will make sure that also <c>N1</c>
      and <c>N3</c> are connected. If this is not desired, the command
      line flag <c>-connect_all false</c> can be used (see also
      <seealso marker="erts:erl#connect_all">erl(1)</seealso>). In
      this case the name registration facility cannot be used, but the
      lock mechanism will still work.</p>

    <p>If the global name server fails to connect nodes (<c>N1</c> and
      <c>N3</c> in the example above) a warning event is sent to the
      error logger. The presence of such an event does not exclude the
      possibility that the nodes will later connect--one can for
      example try the command <c>rpc:call(N1, net_adm, ping, [N2])</c> in
      the Erlang shell--but it indicates some kind of problem with
      the network.</p>

    <note>
      <p>If the fully connected network is not set up properly, the
        first thing to try is to increase the value of
        <c>net_setuptime</c>.</p>
    </note>

  </description>

  <datatypes>
    <datatype>
      <name name="id"/>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="del_lock" arity="1"/>
      <name name="del_lock" arity="2"/>
      <fsummary>Delete a lock</fsummary>
      <desc>
        <p>Deletes the lock <c><anno>Id</anno></c> synchronously.</p>
      </desc>
    </func>

    <func>
      <name name="notify_all_name" arity="3"/>
      <fsummary>Name resolving function that notifies both pids</fsummary>
      <desc>
        <p>This function can be used as a name resolving function for
          <c>register_name/3</c> and <c>re_register_name/3</c>. It
          unregisters both pids, and sends the message
          <c>{global_name_conflict, <anno>Name</anno>, OtherPid}</c> to both
          processes.</p>
      </desc>
    </func>

    <func>
      <name name="random_exit_name" arity="3"/>
      <fsummary>Name resolving function that kills one pid</fsummary>
      <desc>
        <p>This function can be used as a name resolving function for
          <c>register_name/3</c> and <c>re_register_name/3</c>. It
          randomly chooses one of the pids for registration and kills
          the other one.</p>
      </desc>
    </func>

    <func>
      <name name="random_notify_name" arity="3"/>
      <fsummary>Name resolving function that notifies one pid</fsummary>
      <desc>
        <p>This function can be used as a name resolving function for
          <c>register_name/3</c> and <c>re_register_name/3</c>. It
          randomly chooses one of the pids for registration, and sends
          the message <c>{global_name_conflict, <anno>Name</anno>}</c> to the other
          pid.</p>
      </desc>
    </func>

    <func>
      <name name="register_name" arity="2"/>
      <name name="register_name" arity="3"/>
      <fsummary>Globally register a name for a pid</fsummary>
      <type name="method"/>
      <type_desc name="method">{<c>Module</c>, <c>Function</c>}
        is currently also allowed for backward compatibility, but its use is
	deprecated
      </type_desc>
      <desc>
        <p>Globally associates the name <c><anno>Name</anno></c> with a pid, that is,
          Globally notifies all nodes of a new global name in a network
          of Erlang nodes.</p>

        <p>When new nodes are added to the network, they are informed
          of the globally registered names that already exist.
          The network is also informed of any global names in newly
          connected nodes. If any name clashes are discovered,
          the <c><anno>Resolve</anno></c> function is called. Its purpose is to
          decide which pid is correct. If the function crashes, or
          returns anything other than one of the pids, the name is
          unregistered. This function is called once for each name
          clash.</p>

	<warning>
	  <p>If you plan to change code without restarting your system,
	  you must use an external fun (<c>fun Module:Function/Arity</c>)
	  as the <c><anno>Resolve</anno></c> function; if you use a
	  local fun you can never replace the code for the module that
	  the fun belongs to.
	  </p>
	</warning>

        <p>There are three pre-defined resolve functions:
          <c>random_exit_name/3</c>, <c>random_notify_name/3</c>, and
          <c>notify_all_name/3</c>. If no <c><anno>Resolve</anno></c> function is
          defined, <c>random_exit_name</c> is used. This means that one
          of the two registered processes will be selected as correct
          while the other is killed.</p>

        <p>This function is completely synchronous. This means that
          when this function returns, the name is either registered on
          all nodes or none.</p>

        <p>The function returns <c>yes</c> if successful, <c>no</c> if
          it fails. For example, <c>no</c> is returned if an attempt
          is made to register an already registered process or to
          register a process with a name that is already in use.</p>

        <note>
          <p>Releases up to and including OTP R10 did not check if the
            process was already registered. As a consequence the
            global name table could become inconsistent. The old
            (buggy) behavior can be chosen by giving the Kernel
            application variable <c>global_multi_name_action</c> the
            value <c>allow</c>.</p>
        </note>

        <p>If a process with a registered name dies, or the node goes
          down, the name is unregistered on all nodes.</p>
      </desc>
    </func>

    <func>
      <name name="registered_names" arity="0"/>
      <fsummary>All globally registered names</fsummary>
      <desc>
        <p>Returns a lists of all globally registered names.</p>
      </desc>
    </func>

    <func>
      <name name="re_register_name" arity="2"/>
      <name name="re_register_name" arity="3"/>
      <fsummary>Atomically re-register a name</fsummary>
      <type name="method"/>
      <type_desc name="method">{<c>Module</c>, <c>Function</c>}
        is also allowed
      </type_desc>
      <desc>
        <p>Atomically changes the registered name <c><anno>Name</anno></c> on all
          nodes to refer to <c><anno>Pid</anno></c>.</p>

        <p>The <c><anno>Resolve</anno></c> function has the same behavior as in
          <c>register_name/2,3</c>.</p>
      </desc>
    </func>

    <func>
      <name name="send" arity="2"/>
      <fsummary>Send a message to a globally registered pid</fsummary>
      <desc>
        <p>Sends the message <c><anno>Msg</anno></c> to the pid globally registered
          as <c><anno>Name</anno></c>.</p>

        <p>Failure: If <c><anno>Name</anno></c> is not a globally registered
          name, the calling function will exit with reason
          <c>{badarg, {<anno>Name</anno>, <anno>Msg</anno>}}</c>.</p>
      </desc>
    </func>

    <func>
      <name name="set_lock" arity="1"/>
      <name name="set_lock" arity="2"/>
      <name name="set_lock" arity="3"/>
      <fsummary>Set a lock on the specified nodes</fsummary>
      <type name="id"/>
      <type name="retries"/>
      <desc>
        <p>Sets a lock on the specified nodes (or on all nodes if none
          are specified) on <c><anno>ResourceId</anno></c> for
          <c><anno>LockRequesterId</anno></c>. If a lock already exists on
          <c><anno>ResourceId</anno></c> for another requester than
          <c><anno>LockRequesterId</anno></c>, and <c><anno>Retries</anno></c> is not equal to 0,
          the process sleeps for a while and will try to execute
          the action later. When <c><anno>Retries</anno></c> attempts have been made,
          <c>false</c> is returned, otherwise <c>true</c>. If
          <c><anno>Retries</anno></c> is <c>infinity</c>, <c>true</c> is eventually
          returned (unless the lock is never released).</p>

        <p>If no value for <c><anno>Retries</anno></c> is given, <c>infinity</c> is
          used.</p>

        <p>This function is completely synchronous.</p>

        <p>If a process which holds a lock dies, or the node goes
          down, the locks held by the process are deleted.</p>

        <p>The global name server keeps track of all processes sharing
          the same lock, that is, if two processes set the same lock,
          both processes must delete the lock.</p>

        <p>This function does not address the problem of a deadlock. A
          deadlock can never occur as long as processes only lock one
          resource at a time. But if some processes try to lock two or
          more resources, a deadlock may occur. It is up to the
          application to detect and rectify a deadlock.</p>

        <note>
          <p>Some values of <c><anno>ResourceId</anno></c> should be avoided or
            Erlang/OTP will not work properly. A list of resources to
            avoid: <c>global</c>, <c>dist_ac</c>,
            <c>mnesia_table_lock</c>, <c>mnesia_adjust_log_writes</c>,
            <c>pg2</c>.</p>
        </note>

      </desc>
    </func>

    <func>
      <name name="sync" arity="0"/>
      <fsummary>Synchronize the global name server</fsummary>
      <desc>
        <p>Synchronizes the global name server with all nodes known to
          this node. These are the nodes which are returned from
          <c>erlang:nodes()</c>. When this function returns,
          the global name server will receive global information from
          all nodes. This function can be called when new nodes are
          added to the network.</p>
        <p>The only possible error reason <c>Reason</c> is
          <c>{"global_groups definition error", Error}</c>.</p>
      </desc>
    </func>

    <func>
      <name name="trans" arity="2"/>
      <name name="trans" arity="3"/>
      <name name="trans" arity="4"/>
      <fsummary>Micro transaction facility</fsummary>
      <type name="retries"/>
      <type name="trans_fun"/>
      <desc>
        <p>Sets a lock on <c><anno>Id</anno></c> (using <c>set_lock/3</c>). If this
          succeeds, <c><anno>Fun</anno>()</c> is evaluated and the result <c><anno>Res</anno></c>
          is returned.  Returns <c>aborted</c> if the lock attempt
          failed. If <c><anno>Retries</anno></c> is set to <c>infinity</c>,
          the transaction will not abort.</p>

        <p><c>infinity</c> is the default setting and will be used if 
          no value is given for <c><anno>Retries</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="unregister_name" arity="1"/>
      <fsummary>Remove a globally registered name for a pid</fsummary>
      <desc>
        <p>Removes the globally registered name <c><anno>Name</anno></c> from
          the network of Erlang nodes.</p>
      </desc>
    </func>

    <func>
      <name name="whereis_name" arity="1"/>
      <fsummary>Get the pid with a given globally registered name</fsummary>
      <desc>
        <p>Returns the pid with the globally registered name
          <c><anno>Name</anno></c>. Returns <c>undefined</c> if the name is not
          globally registered.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>See Also</title>
    <p><seealso marker="global_group">global_group(3)</seealso>,
       <seealso marker="net_kernel">net_kernel(3)</seealso></p>
  </section>
</erlref>