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This documentation describes the Global module which consists
of the following functionalities:
1. Registration of global names
2. Global locks
3. Monitoring nodes
4. Maintenance of the fully connected network
These services are controlled via the process global which exists
on every node. global is started automatically when a node is started.
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 are implemented, but for a network of
Erlang nodes. A registered name is an alias for a process identity
Pid. The system 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 extremely quick because it is never a network operation. 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 of a process. 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 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.
The 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 server on N1 then N3 will make sure
that also N1 and N3 are connected. If this is not
desired, the command line flag -connect_all false must be
passed to init at boot time. In this case, the name
registration facility cannot be used (but the lock mechanism
will still work.)
del_lock(Id)
del_lock(Id, Nodes) -> void()
Types:
Id = {ResourceId, LockRequesterId}
ResourceId = term()
LockRequesterId = term()
Nodes = [node()]
Deletes the lock Id synchronously.
notify_all_name(Name, Pid1, Pid2) -> none
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) -> Pid1 | Pid2
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) -> Pid1 | Pid2
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)
register_name(Name, Pid, Resolve) -> yes | no
Types:
Name = term()
Pid = Pid()
Resolve = {M, F} where M:F(Name, Pid, Pid2) -> Pid | Pid2
| none
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. This function blocks the
global name server during its execution. 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.
There are three pre-defined resolve functions,
random_exit_name, random_notify_name and
notify_all_name. 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 a process with a name that is already
in use.
If a process with a registered name dies, or the node goes down, the name is unregistered on all nodes.
Types:
Name = term()
Returns a lists of all globally registered names.
re_register_name(Name, Pid)
re_register_name(Name, Pid, Resolve) -> void()
Types:
Name = term()
Pid = Pid()
Resolve = {M, F} where M:F(Name, Pid, Pid2) -> Pid | Pid2
| none
Atomically changes the registered name Name on all
nodes to refer to Pid.
The Resolve function has the same behavior as in
register_name.
Types:
Name = term()
Msg = term()
Pid = Pid()
Sends the message Msg to the globally registered
process Name. If Name is not a globally
registered name, the calling function will exit with reason
{badarg, {Name, Msg}}.
set_lock(Id)
set_lock(Id, Nodes)
set_lock(Id, Nodes, Retries) -> boolean()
Types:
Id = {ResourceId, LockRequesterId}
ResourceId = term()
LockRequesterId = term()
Nodes = [node()]
Retries = int() > 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.
global keeps track of all processes sharing the same lock,
i.e. 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.
start()
start_link() -> {ok, Pid} | {error, Reason}
This function starts the global name server. Normally, the server is started automatically.
Stops the global name server.
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.
trans(Id, Fun)
trans(Id, Fun, Nodes)
trans(Id, Fun, Nodes, Retries) -> Res | aborted
Types:
Id = {ResourceId, LockRequesterId}
ResourceId = term()
LockRequesterId = term()
Fun = fun() | {M, F}
Nodes = [node()]
Retries = int() > 0 | infinity
Res = term()
Sets a lock on Id (using set_lock/3).
Evaluates Res = Fun() if successfully locked and returns
Res. 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) -> void()
Types:
Name = term()
Globally removes Name from the network of Erlang nodes.
whereis_name(Name) -> Pid() | undefined
Types:
Name = term()
Returns either an atom undefined, or a Pid which
is globally associated with Name.