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

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  <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.
      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
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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 since="">global</module>
  <modulesummary>A global name registration facility.</modulesummary>
  <description>
    <p>This module consists of the following services:</p>
    <list type="bulleted">
      <item>Registration of global names</item>
      <item>Global locks</item>
      <item>Maintenance of the fully connected network</item>
    </list>
    <marker id="prevent_overlapping_partitions"/>
    <warning>
      <p>
        By default <c>global</c> does <i>not</i> take any actions to restore
        a fully connected network when connections are lost due to network
        issues. This is problematic for all applications expecting a fully
        connected network to be provided, such as for example <c>mnesia</c>,
        but also for <c>global</c> itself. A network of overlapping partitions
        might cause the internal state of <c>global</c> to become inconsistent.
        Such an inconsistency can remain even after such partitions have been
        brought together to form a fully connected network again. The effect
        on other applications that expects that a fully connected network is
        maintained may vary, but they might misbehave in very subtle hard to
        detect ways during such a partitioning.
      </p>
      <p>
        In order to prevent such issues, we have introduced a <i>prevent
        overlapping partitions</i> fix which can be enabled using the
        <seeapp marker="kernel_app#prevent_overlapping_partitions">
          <c>prevent_overlapping_partitions</c></seeapp> <c>kernel(6)</c>
        parameter. When this fix has been enabled, <c>global</c> will actively
        disconnect from nodes that reports that they have lost connections to
        other nodes. This will cause fully connected partitions to form
        instead of leaving the network in a state with overlapping partitions.
        Note that this fix <i>has</i> to be enabled on <i>all</i> nodes in the
        network in order to work properly. Since this quite substantially
        changes the behavior, this fix is currently disabled by default. Since
        you might get hard to detect issues without this fix you are, however,
        <i>strongly</i> advised to enable this fix in order to avoid issues
        such as the ones described above. As of OTP 25 this fix  will become
        enabled by default.
      </p>
    </warning>
    <p>These services are controlled through the process
      <c>global_name_server</c> that exists on every node. The global
      name server starts automatically when a node is started.
      With the term <em>global</em> is meant over a system consisting
      of many 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 provided, 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 is
      also 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. For any action resulting 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 example, the specified resource can be a pid.
      When a global lock is set, access to the locked resource is
      denied for all resources other than the lock requester.</p>
    <p>Both the registration and lock services are atomic.
      All nodes involved in these actions 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
      that runs a globally registered process goes down, the name
      is globally unregistered. To this end, the global name
      server subscribes to <c>nodeup</c> and <c>nodedown</c> messages
      sent from module <c>net_kernel</c>. Relevant Kernel
      application variables in this context are <c>net_setuptime</c>,
      <c>net_ticktime</c>, and <c>dist_auto_connect</c>. See also
      <seeapp marker="kernel_app#net_setuptime"><c>kernel(6)</c></seeapp>.</p>
    <p>The name server also maintains 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> ensure that also <c>N1</c>
      and <c>N3</c> are connected. If this is not desired,
      command-line flag <c>-connect_all false</c> can be used (see also
      <seecom marker="erts:erl#connect_all"><c>erl(1)</c></seecom>).
      In this case, the name registration service cannot be used, but the
      lock mechanism still works.</p>
    <p>If the global name server fails to connect nodes (<c>N1</c> and
      <c>N3</c> in the example), a warning event is sent to the
      error logger. The presence of such an event does not exclude the
      nodes to connect later (you can, for
      example, try command <c>rpc:call(N1, net_adm, ping, [N2])</c> in
      the Erlang shell), but it indicates a network problem.</p>
    <note>
      <p>If the fully connected network is not set up properly, try
        first 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" since=""/>
      <name name="del_lock" arity="2" since=""/>
      <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" since=""/>
      <fsummary>Name resolving function that notifies both pids.</fsummary>
      <desc>
        <p>Can be used as a name resolving function for
          <seemfa marker="#register_name/3"><c>register_name/3</c></seemfa>
          and
          <seemfa marker="#re_register_name/3"><c>re_register_name/3</c></seemfa>.</p>
        <p>The function 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" since=""/>
      <fsummary>Name resolving function that kills one pid.</fsummary>
      <desc>
        <p>Can be used as a name resolving function for
          <seemfa marker="#register_name/3"><c>register_name/3</c></seemfa>
          and
          <seemfa marker="#re_register_name/3"><c>re_register_name/3</c></seemfa>.</p>
        <p>The function randomly selects one of the pids for registration and
          kills the other one.</p>
      </desc>
    </func>

    <func>
      <name name="random_notify_name" arity="3" since=""/>
      <fsummary>Name resolving function that notifies one pid.</fsummary>
      <desc>
        <p>Can be used as a name resolving function for
          <seemfa marker="#register_name/3"><c>register_name/3</c></seemfa>
          and
          <seemfa marker="#re_register_name/3"><c>re_register_name/3</c></seemfa>.</p>
        <p>The function randomly selects 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="re_register_name" arity="2" since=""/>
      <name name="re_register_name" arity="3" since=""/>
      <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>Function <c><anno>Resolve</anno></c> has the same behavior as in
          <seemfa marker="#register_name/2"><c>register_name/2,3</c></seemfa>.
        </p>
      </desc>
    </func>

    <func>
      <name name="register_name" arity="2" since=""/>
      <name name="register_name" arity="3" since=""/>
      <fsummary>Globally register a name for a pid.</fsummary>
      <type name="method"/>
      <type_desc name="method">{<c>Module</c>, <c>Function</c>} is also
        allowed for backward compatibility, but its use is deprecated.
      </type_desc>
      <desc>
        <p>Globally associates 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,
          function <c><anno>Resolve</anno></c> 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 function <c><anno>Resolve</anno></c>. If you use a
	  local fun, you can never replace the code for the module that
	  the fun belongs to.</p>
	</warning>
        <p>Three predefined resolve functions exist:
          <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 is selected as correct
          while the other is killed.</p>
        <p>This function is completely synchronous, that is,
          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 Erlang/OTP R10 did not check if the
            process was already registered. The global name table could
            therefore 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" since=""/>
      <fsummary>All globally registered names.</fsummary>
      <desc>
        <p>Returns a list of all globally registered names.</p>
      </desc>
    </func>

    <func>
      <name name="send" arity="2" since=""/>
      <fsummary>Send a message to a globally registered pid.</fsummary>
      <desc>
        <p>Sends message <c><anno>Msg</anno></c> to the pid globally registered
          as <c><anno>Name</anno></c>.</p>
        <p>If <c><anno>Name</anno></c> is not a globally registered
          name, the calling function exits with reason
          <c>{badarg, {<anno>Name</anno>, <anno>Msg</anno>}}</c>.</p>
      </desc>
    </func>

    <func>
      <name name="set_lock" arity="1" since=""/>
      <name name="set_lock" arity="2" since=""/>
      <name name="set_lock" arity="3" since=""/>
      <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 <c>0</c>, the process sleeps for a while and tries
          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 specified,
          <c>infinity</c> is used.</p>
        <p>This function is completely synchronous.</p>
        <p>If a process that 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. A deadlock can occur if some processes
          try to lock two or more resources. It is up to the
          application to detect and rectify a deadlock.</p>
        <note>
          <p>Avoid the following values of <c><anno>ResourceId</anno></c>,
            otherwise Erlang/OTP does not work properly:</p>
          <list type="bulleted">
            <item><c>dist_ac</c></item>
            <item><c>global</c></item>
            <item><c>mnesia_adjust_log_writes</c></item>
            <item><c>mnesia_table_lock</c></item>
          </list>
        </note>
      </desc>
    </func>

    <func>
      <name name="sync" arity="0" since=""/>
      <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 that are returned from
          <c>erlang:nodes()</c>. When this function returns,
          the global name server receives 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" since=""/>
      <name name="trans" arity="3" since=""/>
      <name name="trans" arity="4" since=""/>
      <fsummary>Micro transaction facility.</fsummary>
      <type name="retries"/>
      <type name="trans_fun"/>
      <desc>
        <p>Sets a lock on <c><anno>Id</anno></c> (using
          <seemfa marker="#set_lock/3"><c>set_lock/3</c></seemfa>).
          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 fails.
          If <c><anno>Retries</anno></c> is set to <c>infinity</c>,
          the transaction does not abort.</p>
        <p><c>infinity</c> is the default setting and is used if
          no value is specified for <c><anno>Retries</anno></c>.</p>
      </desc>
    </func>

    <func>
      <name name="unregister_name" arity="1" since=""/>
      <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" since=""/>
      <fsummary>Get the pid with a specified 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><seeerl marker="global_group"><c>global_group(3)</c></seeerl>,
      <seeerl marker="net_kernel"><c>net_kernel(3)</c></seeerl></p>
  </section>
</erlref>