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NAME
global_group - Grouping nodes to global name registration groups.
DESCRIPTION
This module makes it possible to partition the nodes of a system into global groups. Each global group
has its own global namespace, see global(3erl).
The main advantage of dividing systems into global groups is that the background load decreases while the
number of nodes to be updated is reduced when manipulating globally registered names.
The Kernel configuration parameter global_groups defines the global groups (see also kernel(7) and
config(5)):
{global_groups, [GroupTuple :: group_tuple()]}
For the processes and nodes to run smoothly using the global group functionality, the following criteria
must be met:
* An instance of the global group server, global_group, must be running on each node. The processes are
automatically started and synchronized when a node is started.
* All involved nodes must agree on the global group definition, otherwise the behavior of the system is
undefined.
* All nodes in the system must belong to exactly one global group.
In the following descriptions, a group node is a node belonging to the same global group as the local
node.
DATA TYPES
group_tuple() =
{GroupName :: group_name(), [node()]} |
{GroupName :: group_name(),
PublishType :: publish_type(),
[node()]}
A GroupTuple without PublishType is the same as a GroupTuple with PublishType equal to normal.
group_name() = atom()
publish_type() = hidden | normal
A node started with command-line flag -hidden (see erl(1)) is said to be a hidden node. A hidden
node establishes hidden connections to nodes not part of the same global group, but normal
(visible) connections to nodes part of the same global group.
A global group defined with PublishType equal to hidden is said to be a hidden global group. All
nodes in a hidden global group are hidden nodes, whether they are started with command-line flag
-hidden or not.
name() = atom()
A registered name.
where() = {node, node()} | {group, group_name()}
EXPORTS
global_groups() -> {GroupName, GroupNames} | undefined
Types:
GroupName = group_name()
GroupNames = [GroupName]
Returns a tuple containing the name of the global group that the local node belongs to, and the
list of all other known group names. Returns undefined if no global groups are defined.
info() -> [info_item()]
Types:
info_item() =
{state, State :: sync_state()} |
{own_group_name, GroupName :: group_name()} |
{own_group_nodes, Nodes :: [node()]} |
{synched_nodes, Nodes :: [node()]} |
{sync_error, Nodes :: [node()]} |
{no_contact, Nodes :: [node()]} |
{other_groups, Groups :: [group_tuple()]} |
{monitoring, Pids :: [pid()]}
sync_state() = no_conf | synced
Returns a list containing information about the global groups. Each list element is a tuple. The
order of the tuples is undefined.
{state, State}:
If the local node is part of a global group, State is equal to synced. If no global groups are
defined, State is equal to no_conf.
{own_group_name, GroupName}:
The name (atom) of the group that the local node belongs to.
{own_group_nodes, Nodes}:
A list of node names (atoms), the group nodes.
{synced_nodes, Nodes}:
A list of node names, the group nodes currently synchronized with the local node.
{sync_error, Nodes}:
A list of node names, the group nodes with which the local node has failed to synchronize.
{no_contact, Nodes}:
A list of node names, the group nodes to which there are currently no connections.
{other_groups, Groups}:
Groups is a list of tuples {GroupName, Nodes}, specifying the name and nodes of the other
global groups.
{monitoring, Pids}:
A list of pids, specifying the processes that have subscribed to nodeup and nodedown messages.
monitor_nodes(Flag) -> ok
Types:
Flag = boolean()
Depending on Flag, the calling process starts subscribing (Flag equal to true) or stops
subscribing (Flag equal to false) to node status change messages.
A process that has subscribed receives the messages {nodeup, Node} and {nodedown, Node} when a
group node connects or disconnects, respectively.
own_nodes() -> Nodes
Types:
Nodes = [Node :: node()]
Returns the names of all group nodes, regardless of their current status.
registered_names(Where) -> Names
Types:
Where = where()
Names = [Name :: name()]
Returns a list of all names that are globally registered on the specified node or in the specified
global group.
send(Name, Msg) -> pid() | {badarg, {Name, Msg}}
send(Where, Name, Msg) -> pid() | {badarg, {Name, Msg}}
Types:
Where = where()
Name = name()
Msg = term()
Searches for Name, globally registered on the specified node or in the specified global group, or
(if argument Where is not provided) in any global group. The global groups are searched in the
order that they appear in the value of configuration parameter global_groups.
If Name is found, message Msg is sent to the corresponding pid. The pid is also the return value
of the function. If the name is not found, the function returns {badarg, {Name, Msg}}.
sync() -> ok
Synchronizes the group nodes, that is, the global name servers on the group nodes. Also checks the
names globally registered in the current global group and unregisters them on any known node not
part of the group.
If synchronization is not possible, an error report is sent to the error logger (see also
error_logger(3erl).
Returns {error, {'invalid global_groups definition', Bad}} if configuration parameter
global_groups has an invalid value Bad.
whereis_name(Name) -> pid() | undefined
whereis_name(Where, Name) -> pid() | undefined
Types:
Where = where()
Name = name()
Searches for Name, globally registered on the specified node or in the specified global group, or
(if argument Where is not provided) in any global group. The global groups are searched in the
order that they appear in the value of configuration parameter global_groups.
If Name is found, the corresponding pid is returned. If the name is not found, the function
returns undefined.
NOTES
* In the situation where a node has lost its connections to other nodes in its global group, but has
connections to nodes in other global groups, a request from another global group can produce an
incorrect or misleading result. For example, the isolated node can have inaccurate information about
registered names in its global group.
* Function send/2,3 is not secure.
* Distribution of applications is highly dependent of the global group definitions. It is not
recommended that an application is distributed over many global groups, as the registered names can
be moved to another global group at failover/takeover. Nothing prevents this to be done, but the
application code must then handle the situation.
SEE ALSO
global(3erl), erl(1)