global(3erl)
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
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. See also kernel(7).
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. If this is not desired, command-line flag -connect_all
false can be used (see also erl(1)). 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.
DATA TYPES
id() = {ResourceId :: term(), LockRequesterId :: term()}
EXPORTS
del_lock(Id) -> true
del_lock(Id, Nodes) -> true
Types:
Id = id()
Nodes = [node()]
Deletes the lock Id synchronously.
notify_all_name(Name, Pid1, Pid2) -> none
Types:
Name = term()
Pid1 = Pid2 = pid()
Can be used as a name resolving function for register_name/3 and
re_register_name/3.
The function unregisters both pids and sends the message
{global_name_conflict, Name, OtherPid} to both processes.
random_exit_name(Name, Pid1, Pid2) -> pid()
Types:
Name = term()
Pid1 = Pid2 = pid()
Can be used as a name resolving function for register_name/3 and
re_register_name/3.
The function randomly selects one of the pids for registration
and kills the other one.
random_notify_name(Name, Pid1, Pid2) -> pid()
Types:
Name = term()
Pid1 = Pid2 = pid()
Can be used as a name resolving function for register_name/3 and
re_register_name/3.
The function randomly selects one of the pids for registration,
and sends the message {global_name_conflict, Name} to the other
pid.
re_register_name(Name, Pid) -> yes
re_register_name(Name, Pid, Resolve) -> yes
Types:
Name = term()
Pid = pid()
Resolve = method()
method() =
fun((Name :: term(), Pid :: pid(), Pid2 :: pid()) ->
pid() | none)
{Module, Function} is also allowed.
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) -> yes | no
register_name(Name, Pid, Resolve) -> yes | no
Types:
Name = term()
Pid = pid()
Resolve = method()
method() =
fun((Name :: term(), Pid :: pid(), Pid2 :: pid()) ->
pid() | none)
{Module, Function} is also allowed for backward
compatibility, but its use is deprecated.
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. If no Resolve
function is defined, random_exit_name is used. This means that
one of the two registered processes is selected as correct while
the other is killed.
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]
Types:
Name = term()
Returns a list of all globally registered names.
send(Name, Msg) -> Pid
Types:
Name = 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) -> boolean()
set_lock(Id, Nodes) -> boolean()
set_lock(Id, Nodes, Retries) -> boolean()
Types:
Id = id()
Nodes = [node()]
Retries = retries()
id() = {ResourceId :: term(), LockRequesterId :: term()}
retries() = integer() >= 0 | infinity
Sets a lock on the specified nodes (or on all nodes if none are
specified) on ResourceId for LockRequesterId. 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).
If no value for Retries is specified, infinity is used.
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
* pg2
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 erlang: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) -> Res | aborted
trans(Id, Fun, Nodes) -> Res | aborted
trans(Id, Fun, Nodes, Retries) -> Res | aborted
Types:
Id = id()
Fun = trans_fun()
Nodes = [node()]
Retries = retries()
Res = term()
retries() = integer() >= 0 | infinity
trans_fun() = function() | {module(), atom()}
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) -> term()
Types:
Name = term()
Removes the globally registered name Name from the network of
Erlang nodes.
whereis_name(Name) -> pid() | undefined
Types:
Name = term()
Returns the pid with the globally registered name Name. Returns
undefined if the name is not globally registered.
SEE ALSO
global_group(3erl), net_kernel(3erl)
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