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The global group function makes it possible to group the nodes
in a system into partitions, each partition having its own global
name space, refer to global(3). These partitions are
called global groups.
The main advantage of dividing systems to 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(6),
config(4):
{global_groups, [GroupTuple]}
Types:
GroupTuple = {GroupName, [Node]}
| {GroupName, PublishType, [Node]}
GroupName = atom() (naming a global group)
PublishType = normal | hidden
Node = atom() (naming a node)
A GroupTuple without PublishType is the same as a
GroupTuple with PublishType == normal.
A node started with the command line flag -hidden, see
erl(1), is said to be a
hidden node. A hidden node will establish 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 == hidden, is
said to be a hidden global group. All nodes in a hidden global
group are hidden nodes, regardless if they are started with
the -hidden command line flag or not.
For the processes and nodes to run smoothly using the global group functiontionality, the following criteria must be met:
global_group,
must be running on each node. The processes are automatically
started and synchronized when a node is started.In the following description, a group node is a node belonging to the same global group as the local node.
global_groups() -> {GroupName, GroupNames} | undefined
Types:
GroupName = atom()
GroupNames = [GroupName]
Returns a tuple containing the name of the global group
the local node belongs to, and the list of all other known
group names. Returns undefined if no global groups are
defined.
Types:
Item, Info -- see below
Returns a list containing information about the global groups. Each element of the list is a tuple. The order of the tuples is not defined.
{state, State}
State == synced. If no global groups are defined,
State == no_conf.{own_group_name, GroupName}
{own_group_nodes, Nodes}
{synced_nodes, Nodes}
{sync_error, Nodes}
{no_contact, Nodes}
{other_groups, Groups}
Groups is a list of tuples
{GroupName, Nodes}, specifying the name and nodes
of the other global groups.{monitoring, Pids}
nodeup and nodedown messages.
Types:
Flag = bool()
Depending on Flag, the calling process starts
subscribing (Flag == true) or stops subscribing
(Flag == false) to node status change messages.
A process which has subscribed will receive the messages
{nodeup, Node} and {nodedown, Node} when a
group node connects or disconnects, respectively.
Types:
Nodes = [Node]
Node = node()
Returns the names of all group nodes, regardless of their current status.
registered_names(Where) -> Names
Types:
Where = {node, Node} | {group, GroupName}
Node = node()
GroupName = atom()
Names = [Name]
Name = atom()
Returns a list of all names which 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 = {node, Node} | {group, GroupName}
Node = node()
GroupName = atom()
Name = atom()
Msg = term()
Searches for Name, globally registered on
the specified node or in the specified global group, or --
if the Where argument is not provided -- in any global
group. The global groups are searched in the order in which
they appear in the value of the global_groups
configuration parameter.
If Name is found, the 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}}.
Synchronizes the group nodes, that is, the global name servers on the group nodes. Also check 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(3)).
Failure:
{error, {'invalid global_groups definition', Bad}} if
the global_groups configuration parameter has an
invalid value Bad.
whereis_name(Name) -> pid() | undefined
whereis_name(Where, Name) -> pid() | undefined
Types:
Where = {node, Node} | {group, GroupName}
Node = node()
GroupName = atom()
Name = atom()
Searches for Name, globally registered on
the specified node or in the specified global group, or -- if
the Where argument is not provided -- in any global
group. The global groups are searched in the order in which
they appear in the value of the global_groups
configuration parameter.
If Name is found, the corresponding pid is returned.
If the name is not found, the function returns
undefined.
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 may produce an incorrect or misleading result. For example, the isolated node may not have accurate information about registered names in its global group.
Note also that the send/2,3 function is not secure.
Distribution of applications is highly dependent of the global group definitions. It is not recommended that an application is distributed over several global groups of the obvious reason that the registered names may be moved to another global group at failover/takeover. There is nothing preventing doing this, but the application code must in such case handle the situation.