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NAME
global - A Global Name Registration Facility
DESCRIPTION
This documentation describes the Global module which consists of the following
functionalities:
* registration of global names;
* global locks;
* maintenance of the fully connected network.
These services are controlled via the process global_name_server which exists on every
node. The global name server is started automatically when a node is started. With the
term global is meant over a system consisting of several 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 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.
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.
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.
Both the registration and lock functionalities are atomic. All nodes involved in these
actions will 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 which runs a globally registered process goes down, the
name will be globally unregistered. To this end the global name server subscribes to
nodeup and nodedown messages sent from the net_kernel module. Relevant Kernel application
variables in this context are net_setuptime, net_ticktime, and dist_auto_connect. See also
kernel(7).
The name server will also maintain 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 will make sure that also N1 and N3 are connected. If this is not desired,
the command line flag -connect_all false can be used (see also erl(1)). In this case the
name registration facility cannot be used, but the lock mechanism will still work.
If the global name server fails to connect nodes (N1 and N3 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
rpc:call(N1, net_adm, ping, [N2]) in the Erlang shell--but it indicates some kind of
problem with the network.
Note:
If the fully connected network is not set up properly, the first thing to try is 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()
This function can be used as a name resolving function for register_name/3 and
re_register_name/3. It 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()
This function can be used as a name resolving function for register_name/3 and
re_register_name/3. It randomly chooses one of the pids for registration and kills
the other one.
random_notify_name(Name, Pid1, Pid2) -> pid()
Types:
Name = term()
Pid1 = Pid2 = pid()
This function can be used as a name resolving function for register_name/3 and
re_register_name/3. It randomly chooses one of the pids for registration, and sends
the message {global_name_conflict, Name} to the other pid.
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 currently also allowed for backward compatibility, but
its use is deprecated
Globally associates the 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, the Resolve
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.
Warning:
If you plan to change code without restarting your system, you must use an external
fun (fun Module:Function/Arity) as the Resolve function; if you use a local fun you
can never replace the code for the module that the fun belongs to.
There are three pre-defined resolve functions: 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 will
be selected as correct while the other is killed.
This function is completely synchronous. This means that 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 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
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 lists of all globally registered names.
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.
The Resolve function has the same behavior as in register_name/2,3.
send(Name, Msg) -> Pid
Types:
Name = Msg = term()
Pid = pid()
Sends the message Msg to the pid globally registered as Name.
Failure: If Name is not a globally registered name, the calling function will exit
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 will try 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 given, infinity is used.
This function is completely synchronous.
If a process which 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. 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.
Note:
Some values of ResourceId should be avoided or Erlang/OTP will not work properly. A
list of resources to avoid: global, dist_ac, mnesia_table_lock,
mnesia_adjust_log_writes, pg2.
sync() -> ok | {error, Reason :: term()}
Synchronizes the global name server with all nodes known to this node. These are
the nodes which are returned from erlang:nodes(). 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.
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 failed. If Retries is
set to infinity, the transaction will not abort.
infinity is the default setting and will be used if no value is given 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)