A global name registration facility.
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(6)
.
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
.
Functions
del_lock(Id) -> true
Id = id()
del_lock(Id, Nodes) -> true
Id = id()
Nodes = [node()]
Deletes the lock
synchronously.
notify_all_name(Name, Pid1, Pid2) -> none
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,
to both
processes.
random_exit_name(Name, Pid1, Pid2) -> pid()
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()
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,
to
the other pid.
re_register_name(Name, Pid) -> yes
Name = term()
Pid = pid()
re_register_name(Name, Pid, Resolve) -> yes
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
on
all nodes to refer to
.
Function
has the same behavior as in
register_name/2,3
.
register_name(Name, Pid) -> yes | no
Name = term()
Pid = pid()
register_name(Name, Pid, Resolve) -> yes | no
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
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
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
. 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
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]
Name = term()
Returns a list of all globally registered names.
send(Name, Msg) -> Pid
Name = Msg = term()
Pid = pid()
Sends message
to the pid globally registered
as
.
If
is not a globally registered
name, the calling function exits with reason
{badarg, {
.
set_lock(Id) -> boolean()
Id = id()
set_lock(Id, Nodes) -> boolean()
Id = id()
Nodes = [node()]
set_lock(Id, Nodes, Retries) -> boolean()
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
for
. If a lock already exists on
for another requester than
, and
is not equal to 0
, the process sleeps for a while and tries
to execute the action later. When
attempts have been made, false
is returned, otherwise
true
. If
is infinity
,
true
is eventually returned (unless the lock is never
released).
If no value for
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
,
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
Id = id()
Fun = trans_fun()
Res = term()
trans(Id, Fun, Nodes) -> Res | aborted
Id = id()
Fun = trans_fun()
Nodes = [node()]
Res = term()
trans(Id, Fun, Nodes, Retries) -> Res | aborted
Id = id()
Fun = trans_fun()
Nodes = [node()]
Retries = retries()
Res = term()
retries() = integer() >= 0 | infinity
trans_fun() = function() | {module(), atom()}
Sets a lock on
(using
set_lock/3
).
If this succeeds,
is evaluated and the
result
is returned. Returns aborted
if the lock attempt fails.
If
is set to infinity
,
the transaction does not abort.
infinity
is the default setting and is used if
no value is specified for
.
unregister_name(Name) -> term()
Name = term()
Removes the globally registered name
from
the network of Erlang nodes.
whereis_name(Name) -> pid() | undefined
Name = term()
Returns the pid with the globally registered name
. Returns undefined
if the name is not
globally registered.