global(3)

NAME
global - A global name registration facility.

DESCRIPTION

This module consists of the following services:

    • Registration of global names

    • Global locks

    • Maintenance of the fully connected network

As of OTP 25, global will by default prevent overlapping partitions due to network issues by actively disconnecting 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.

WARNING: Prevention of overlapping partitions can be disabled using the prevent_overlapping_partitions Kernel parameter, making global behave like it used to do. This is, however, problematic for all applications expecting a fully connected network to be provided, such as for example mnesia, but also for global itself. A network of overlapping partitions might cause the internal state of global 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. Since you might get hard to detect issues without this fix, you are strongly advised not to disable this fix. Also note that this fix has to be enabled on all nodes in the network in order to work properly.

NOTE: None of the above services will be reliably delivered unless both of the kernel parameters connect_all and prevent_overlapping_partitions are enabled. Calls to the global API will, however, not fail even though one or both of them are disabled. You will just get unreliable results.

These services are controlled through the process global_name_server that exists on every node. The global name server starts automatically when a node is started. With the term global is meant over a system consisting of many Erlang nodes.

The ability to globally register names is a central concept in the programming of distributed Erlang systems. In this module, the equivalent of the register/2 and whereis/1 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.

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.

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.

Both the registration and lock services are atomic. All nodes involved in these actions have the same view of the information.

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 nodeup and nodedown messages sent from module net_kernel. Relevant Kernel application variables in this context are net_setuptime, net_ticktime, and dist_auto_connect.

The name server also maintains a fully connected network. For example, if node N1 connects to node N2 (which is already connected to N3), the global name servers on the nodes N1 and N3 ensure that also N1 and N3 are connected. In this case, the name registration service cannot be used, but the lock mechanism still works.

If the global name server fails to connect nodes (N1 and N3 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 rpc:call(N1, net_adm, ping, [N2]) in the Erlang shell), but it indicates a network problem.

NOTE: If the fully connected network is not set up properly, try first to increase the value of net_setuptime.

See Also

global_group, net_kernel

DATA TYPES

id() :: {ResourceId :: term(), LockRequesterId :: term()}.

    A lock id used to set or delete lock ResourceId on behalf of LockRequesterId.
method() :: fun((Name :: term(), Pid :: pid(), Pid2 :: pid()) -> pid() | none).
retries() :: non_neg_integer() | infinity.
trans_fun() :: function() | {module(), atom()}.

FUNCTIONS

del_lock(Id, Nodes) -> true when Id :: id(), Nodes :: [node()].

    Deletes the lock Id synchronously.
disconnect() -> [node()].

    Disconnect from all other nodes known to global.

    A list of node names (in an unspecified order) is returned which corresponds to the nodes that were disconnected. All disconnect operations performed have completed when global:disconnect/0 returns.

    The disconnects will be made in such a way that only the current node will be removed from the cluster of global nodes. If [prevent_overlapping_partitions] is enabled and you disconnect, from other nodes in the cluster of global nodes, by other means, global on the other nodes may partition the remaining nodes in order to ensure that no overlapping partitions appear. Even if [prevent_overlapping_partitions] is disabled, you should preferably use global:disconnect/0 in order to remove current node from a cluster of global nodes, since you otherwise likely will create overlapping partitions which might cause problems.

    Note that if the node is going to be halted, there is no need to remove it from a cluster of global nodes explicitly by calling global:disconnect/0 before halting it. The removal from the cluster is taken care of automatically when the node halts regardless of whether [prevent_overlapping_partitions] is enabled or not.

    If current node has been configured to be part of a global group, only connected and/or synchronized nodes in that group are known to global, so global:disconnect/0 will only disconnect from those nodes. If current node is not part of a global group, all connected visible nodes will be known to global, so global:disconnect/0 will disconnect from all those nodes.

    Note that information about connected nodes does not instantaneously reach global, so the caller might see a node part of the result returned by nodes() while it still is not known to global. The disconnect operation will, however, still not cause any overlapping partitions when [prevent_overlapping_partitions] is enabled. If [prevent_overlapping_partitions] is disabled, overlapping partitions might form in this case.

    Note that when [prevent_overlapping_partitions] is enabled, you may see warning reports on other nodes when they detect that current node has disconnected. These are in this case completely harmless and can be ignored.

    kernel_app.md#prevent_overlapping_partitions
notify_all_name(Name, Pid1, Pid2) -> none when Name :: term(), Pid1 :: pid(), Pid2 :: pid().

    The function unregisters both pids and sends the message {global_name_conflict, Name, OtherPid} to both processes.

    Can be used as a name resolving function for register_name/3 and re_register_name/3.
random_exit_name(Name, Pid1, Pid2) -> pid() when Name :: term(), Pid1 :: pid(), Pid2 :: pid().

    The function randomly selects one of the pids for registration and kills the other one.

    Can be used as a name resolving function for register_name/3 and re_register_name/3.
random_notify_name(Name, Pid1, Pid2) -> pid() when Name :: term(), Pid1 :: pid(), Pid2 :: pid().

    The function randomly selects one of the pids for registration, and sends the message {global_name_conflict, Name} to the other pid.

    Can be used as a name resolving function for register_name/3 and re_register_name/3.
re_register_name(Name, Pid, Resolve) -> yes when Name :: term(), Pid :: pid(), Resolve :: method().

    Atomically changes the registered name Name on all nodes to refer to Pid.

    Function Resolve has the same behavior as in register_name/2,3.
register_name(Name, Pid, Resolve) -> yes | no when Name :: term(), Pid :: pid(), Resolve :: method().

    Globally associates name Name with a pid, that is, globally notifies all nodes of a new global name in a network of Erlang nodes.

    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 Resolve 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.

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

    Three predefined resolve functions exist: random_exit_name/3, random_notify_name/3, and notify_all_name/3.

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

    The function returns yes if successful, no if it fails. For example, no 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.

    NOTE: 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 global_multi_name_action the value allow.

    If a process with a registered name dies, or the node goes down, the name is unregistered on all nodes.
registered_names() -> [Name] when Name :: term().

    Returns a list of all globally registered names.
send(Name, Msg) -> Pid when Name :: term(), Msg :: term(), Pid :: pid().

    Sends message Msg to the pid globally registered as Name.

    If Name is not a globally registered name, the calling function exits with reason {badarg, {Name, Msg}}.
set_lock(Id, Nodes, Retries) -> boolean() when Id :: id(), Nodes :: [node()], Retries :: retries().

    Sets a lock on the specified nodes on using id/0.

    If a lock already exists on ResourceId for another requester than LockRequesterId, and Retries is not equal to 0, the process sleeps for a while and tries to execute the action later. When Retries attempts have been made, false is returned, otherwise true. If Retries is infinity, true is eventually returned (unless the lock is never released).

    This function is completely synchronous.

    If a process that holds a lock dies, or the node goes down, the locks held by the process are deleted.

    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.

    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.

    NOTE: Avoid the following values of ResourceId, otherwise Erlang/OTP does not work properly:

        • dist_ac

        • global

        • mnesia_adjust_log_writes

        • mnesia_table_lock
sync() -> ok | {error, Reason :: term()}.

    Synchronizes the global name server with all nodes known to this node.

    These are the nodes that are returned from nodes(). 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.

    The only possible error reason Reason is {"global_groups definition error", Error}.
trans(Id, Fun, Nodes, Retries) -> Res | aborted when Id :: id(), Fun :: trans_fun(), Nodes :: [node()], Retries :: retries(), Res :: term().

    Sets a lock on Id (using set_lock/3).

    If this succeeds, Fun() is evaluated and the result Res is returned. Returns aborted if the lock attempt fails. If Retries is set to infinity, the transaction does not abort.

    infinity is the default setting and is used if no value is specified for Retries.
unregister_name(Name) -> _ when Name :: term().

    Removes the globally registered name Name from the network of Erlang nodes.
whereis_name(Name) -> pid() | undefined when Name :: term().

    Returns the pid with the globally registered name Name. Returns undefined if the name is not globally registered.