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<?xml version="1.0" encoding="latin1" ?>
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<!DOCTYPE erlref SYSTEM "erlref.dtd">
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<holder>Ericsson AB, All Rights Reserved</holder>
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The contents of this file are subject to the Erlang Public License,
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Version 1.1, (the "License"); you may not use this file except in
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compliance with the License. You should have received a copy of the
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Erlang Public License along with this software. If not, it can be
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retrieved online at http://www.erlang.org/.
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Software distributed under the License is distributed on an "AS IS"
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basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
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the License for the specific language governing rights and limitations
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The Initial Developer of the Original Code is Ericsson AB.
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<prepared>Martin Björklund</prepared>
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<date>1997-11-17</date>
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<module>global</module>
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<modulesummary>A Global Name Registration Facility</modulesummary>
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<p>This documentation describes the Global module which consists
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of the following functionalities:
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<list type="bulleted">
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<item>registration of global names;</item>
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<item>global locks;</item>
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<item>maintenance of the fully connected network.</item>
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<p>These services are controlled via the process
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<c>global_name_server</c> which exists on every node. The global
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name server is started automatically when a node is started.
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With the term <em>global</em> is meant over a system consisting
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of several Erlang nodes.</p>
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<p>The ability to globally register names is a central concept in
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the programming of distributed Erlang systems. In this module,
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the equivalent of the <c>register/2</c> and <c>whereis/1</c>
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BIFs (for local name registration) are implemented, but for a
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network of Erlang nodes. A registered name is an alias for a
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process identifier (pid). The global name server monitors
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globally registered pids. If a process terminates, the name will
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also be globally unregistered.</p>
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<p>The registered names are stored in replica global name tables on
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every node. There is no central storage point. Thus,
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the translation of a name to a pid is fast, as it is always done
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locally. When any action in taken which results in a change to
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the global name table, all tables on other nodes are automatically
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<p>Global locks have lock identities and are set on a specific
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resource. For instance, the specified resource could be a pid.
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When a global lock is set, access to the locked resource is
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denied for all other resources other than the lock requester.</p>
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<p>Both the registration and lock functionalities are atomic. All
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nodes involved in these actions will have the same view of
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<p>The global name server also performs the critical task of
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continuously monitoring changes in node configuration: if a node
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which runs a globally registered process goes down, the name
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will be globally unregistered. To this end the global name
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server subscribes to <c>nodeup</c> and <c>nodedown</c> messages
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sent from the <c>net_kernel</c> module. Relevant Kernel
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application variables in this context are <c>net_setuptime</c>,
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<c>net_ticktime</c>, and <c>dist_auto_connect</c>. See also
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<seealso marker="kernel_app#net_setuptime">kernel(6)</seealso>.
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<p>The server will also maintain a fully connected network. For
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example, if node <c>N1</c> connects to node <c>N2</c> (which is
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already connected to <c>N3</c>), the global name servers on the
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nodes <c>N1</c> and <c>N3</c> will make sure that also <c>N1</c>
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and <c>N3</c> are connected. If this is not desired, the command
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line flag <c>-connect_all false</c> can be used (see also
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<seealso marker="erts:erl#connect_all">erl(1)</seealso>). In
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this case the name registration facility cannot be used, but the
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lock mechanism will still work.
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<p>If the global name server fails to connect nodes (<c>N1</c> and
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<c>N3</c> in the example above) a warning event is sent to the
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error logger. The presence of such an event does not exclude the
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possibility that the nodes will later connect--one can for
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example try the command <c>rpc:call(N1, net_adm, ping, [N2]) in the Erlang shell</c>--but it indicates some kind of problem with
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<p>If the fully connected network is not set up properly, the
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first thing to try is to increase the value of
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<c>net_setuptime</c>.</p>
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<name>del_lock(Id)</name>
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<name>del_lock(Id, Nodes) -> void()</name>
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<fsummary>Delete a lock</fsummary>
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<v>Id = {ResourceId, LockRequesterId}</v>
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<v> ResourceId = term()</v>
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<v> LockRequesterId = term()</v>
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<v>Nodes = [node()]</v>
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<p>Deletes the lock <c>Id</c> synchronously.</p>
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<name>notify_all_name(Name, Pid1, Pid2) -> none</name>
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<fsummary>Name resolving function that notifies both pids</fsummary>
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<v>Pid1 = Pid2 = pid()</v>
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<p>This function can be used as a name resolving function for
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<c>register_name/3</c> and <c>re_register_name/3</c>. It
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unregisters both pids, and sends the message
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<c>{global_name_conflict, Name, OtherPid}</c> to both
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<name>random_exit_name(Name, Pid1, Pid2) -> Pid1 | Pid2</name>
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<fsummary>Name resolving function that kills one pid</fsummary>
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<v>Pid1 = Pid2 = pid()</v>
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<p>This function can be used as a name resolving function for
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<c>register_name/3</c> and <c>re_register_name/3</c>. It
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randomly chooses one of the pids for registration and kills
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<name>random_notify_name(Name, Pid1, Pid2) -> Pid1 | Pid2</name>
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<fsummary>Name resolving function that notifies one pid</fsummary>
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<v>Pid1 = Pid2 = pid()</v>
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<p>This function can be used as a name resolving function for
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<c>register_name/3</c> and <c>re_register_name/3</c>. It
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randomly chooses one of the pids for registration, and sends
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the message <c>{global_name_conflict, Name}</c> to the other
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<name>register_name(Name, Pid)</name>
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<name>register_name(Name, Pid, Resolve) -> yes | no</name>
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<fsummary>Globally register a name for a pid</fsummary>
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<v>Resolve = fun() or {Module, Function} where</v>
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<v> Resolve(Name, Pid, Pid2) -> Pid | Pid2 | none</v>
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<p>Globally associates the name <c>Name</c> with a pid, that is,
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Globally notifies all nodes of a new global name in a network
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<p>When new nodes are added to the network, they are informed
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of the globally registered names that already exist.
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The network is also informed of any global names in newly
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connected nodes. If any name clashes are discovered,
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the <c>Resolve</c> function is called. Its purpose is to
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decide which pid is correct. If the function crashes, or
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returns anything other than one of the pids, the name is
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unregistered. This function is called once for each name
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<p>There are three pre-defined resolve functions:
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<c>random_exit_name/3</c>, <c>random_notify_name/3</c>, and
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<c>notify_all_name/3</c>. If no <c>Resolve</c> function is
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defined, <c>random_exit_name</c> is used. This means that one
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of the two registered processes will be selected as correct
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while the other is killed.</p>
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<p>This function is completely synchronous. This means that
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when this function returns, the name is either registered on
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all nodes or none.</p>
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<p>The function returns <c>yes</c> if successful, <c>no</c> if
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it fails. For example, <c>no</c> is returned if an attempt
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is made to register an already registered process or to
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register a process with a name that is already in use.</p>
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<p>Releases up to and including OTP R10 did not check if
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the process was already registered. As a consequence the
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global name table could become inconsistent. The old (buggy)
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behavior can be chosen by giving the Kernel application
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variable <c>global_multi_name_action</c> the value
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<p>If a process with a registered name dies, or the node goes
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down, the name is unregistered on all nodes.</p>
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<name>registered_names() -> [Name]</name>
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<fsummary>All globally registered names</fsummary>
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<p>Returns a lists of all globally registered names.</p>
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<name>re_register_name(Name, Pid)</name>
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<name>re_register_name(Name, Pid, Resolve) -> void()</name>
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<fsummary>Atomically re-register a name</fsummary>
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<v>Resolve = fun() or {Module, Function} where</v>
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<v> Resolve(Name, Pid, Pid2) -> Pid | Pid2 | none</v>
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<p>Atomically changes the registered name <c>Name</c> on all
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nodes to refer to <c>Pid</c>.</p>
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<p>The <c>Resolve</c> function has the same behavior as in
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<c>register_name/2,3</c>.</p>
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<name>send(Name, Msg) -> Pid</name>
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<fsummary>Send a message to a globally registered pid</fsummary>
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<p>Sends the message <c>Msg</c> to the pid globally registered
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<p>Failure: If <c>Name</c> is not a globally registered
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name, the calling function will exit with reason
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<c>{badarg, {Name, Msg}}</c>.</p>
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<name>set_lock(Id)</name>
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<name>set_lock(Id, Nodes)</name>
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<name>set_lock(Id, Nodes, Retries) -> boolean()</name>
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<fsummary>Set a lock on the specified nodes</fsummary>
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<v>Id = {ResourceId, LockRequesterId}</v>
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<v> ResourceId = term()</v>
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<v> LockRequesterId = term()</v>
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<v>Nodes = [node()]</v>
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<v>Retries = int() >= 0 | infinity</v>
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<p>Sets a lock on the specified nodes (or on all nodes if none
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are specified) on <c>ResourceId</c> for
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<c>LockRequesterId</c>. If a lock already exists on
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<c>ResourceId</c> for another requester than
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<c>LockRequesterId</c>, and <c>Retries</c> is not equal to 0,
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the process sleeps for a while and will try to execute
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the action later. When <c>Retries</c> attempts have been made,
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<c>false</c> is returned, otherwise <c>true</c>. If
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<c>Retries</c> is <c>infinity</c>, <c>true</c> is eventually
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returned (unless the lock is never released).</p>
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<p>If no value for <c>Retries</c> is given, <c>infinity</c> is
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<p>This function is completely synchronous.</p>
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<p>If a process which holds a lock dies, or the node goes
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down, the locks held by the process are deleted.</p>
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<p>The global name server keeps track of all processes sharing
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the same lock, that is, if two processes set the same lock,
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both processes must delete the lock.</p>
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<p>This function does not address the problem of a deadlock. A
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deadlock can never occur as long as processes only lock one
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resource at a time. But if some processes try to lock two or
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more resources, a deadlock may occur. It is up to the
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application to detect and rectify a deadlock.</p>
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<p>Some values of <c>ResourceId</c> should be avoided or
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Erlang/OTP will not work properly. A list of
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resources to avoid: <c>global</c>, <c>dist_ac</c>,
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<c>mnesia_table_lock</c>, <c>mnesia_adjust_log_writes</c>,
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<name>sync() -> void()</name>
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<fsummary>Synchronize the global name server</fsummary>
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<p>Synchronizes the global name server with all nodes known to
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this node. These are the nodes which are returned from
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<c>erlang:nodes()</c>. When this function returns,
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the global name server will receive global information from
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all nodes. This function can be called when new nodes are
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added to the network.</p>
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<name>trans(Id, Fun)</name>
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<name>trans(Id, Fun, Nodes)</name>
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<name>trans(Id, Fun, Nodes, Retries) -> Res | aborted</name>
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<fsummary>Micro transaction facility</fsummary>
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<v>Id = {ResourceId, LockRequesterId}</v>
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<v> ResourceId = term()</v>
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<v> LockRequesterId = term()</v>
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<v>Fun = fun() | {M, F}</v>
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<v>Nodes = [node()]</v>
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<v>Retries = int() >= 0 | infinity</v>
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<p>Sets a lock on <c>Id</c> (using <c>set_lock/3</c>). If this
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succeeds, <c>Fun()</c> is evaluated and the result <c>Res</c>
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is returned. Returns <c>aborted</c> if the lock attempt
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failed. If <c>Retries</c> is set to <c>infinity</c>,
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the transaction will not abort.</p>
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<p><c>infinity</c> is the default setting and will be used if
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no value is given for <c>Retries</c>.</p>
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<name>unregister_name(Name) -> void()</name>
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<fsummary>Remove a globally registered name for a pid</fsummary>
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<p>Removes the globally registered name <c>Name</c> from
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the network of Erlang nodes.</p>
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<name>whereis_name(Name) -> pid() | undefined</name>
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<fsummary>Get the pid with a given globally registered name</fsummary>
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<p>Returns the pid with the globally registered name
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<c>Name</c>. Returns <c>undefined</c> if the name is not
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globally registered.</p>
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<title>See Also</title>
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<p><seealso marker="global_group">global_group(3)</seealso>,
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<seealso marker="net_kernel">net_kernel(3)</seealso>,</p>
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lib/kernel/doc/src/global.xml
