# pg

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Module

pg

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Module Summary

Distributed named process groups.

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Since

Module pg was introduced in OTP 23.0.

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Description

This module implements process groups. A message can be sent to one, some, or all group members.

Up until OTP 17 there used to exist an experimental pg module in stdlib. This pg module is not the same module as that experimental pg module, and only share the same module name.

A group of processes can be accessed by a common name. For example, if there is a group named foobar, there can be a set of processes (which can be located on different nodes) that are all members of the group foobar. There are no special functions for sending a message to the group. Instead, client functions are to be written with the functions get_members/1 and get_local_members/1 to determine which processes are members of the group. Then the message can be sent to one or more group members.

If a member terminates, it is automatically removed from the group.

A process may join multiple groups. It may join the same group multiple times. It is only allowed to join processes running on local node.

Process Groups implement strong eventual consistency. Process Groups membership view may temporarily diverge. For example, when processes on node1 and node2 join concurrently, node3 and node4 may receive updates in a different order.

Membership view is not transitive. If node1 is not directly connected to node2, they will not see each other groups. But if both are connected to node3, node3 will have the full view.

Groups are automatically created when any process joins, and are removed when all processes leave the group. Non-existing group is considered empty (containing no processes).

Process groups can be organised into multiple scopes. Scopes are completely independent of each other. A process may join any number of groups in any number of scopes. Scopes are designed to decouple single mesh into a set of overlay networks, reducing amount of traffic required to propagate group membership information. Default scope pg is started automatically when kernel(6) is configured to do so.

Note

Scope name is used to register process locally, and to name an ETS table. If there is another process registered under this name, or another ETS table exists, scope fails to start.

Local membership is not preserved if scope process exits and restarts.

A scope can be kept local-only by using a scope name that is unique cluster-wide, e.g. the node name:

  pg:start_link(node()).

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Data Types

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group() = any()

The identifier of a process group.

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Exports

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start_link() -> {ok, pid()} | {error, any()}
OTP 23.0

Starts the default pg scope within supervision tree. Kernel may be configured to do it automatically, see kernel(6) configuration manual.

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start(Scope :: atom()) -> {ok, pid()} | {error, any()}
OTP 23.0

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start_link(Scope :: atom()) -> {ok, pid()} | {error, any()}
OTP 23.0

Starts additional scope.

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join(Group :: group(), PidOrPids :: pid() | [pid()]) -> ok
OTP 23.0

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join(Scope :: atom(),
Group :: group(),
PidOrPids :: pid() | [pid()]) ->
ok
OTP 23.0

Joins single process or multiple processes to the group Group. A process can join a group many times and must then leave the group the same number of times.

PidOrPids may contain the same process multiple times.

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leave(Group :: group(), PidOrPids :: pid() | [pid()]) -> ok
OTP 23.0

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leave(Scope :: atom(),
Group :: group(),
PidOrPids :: pid() | [pid()]) ->
ok | not_joined
OTP 23.0

Makes the process PidOrPids leave the group Group. If the process is not a member of the group, not_joined is returned.

When list of processes is passed as PidOrPids, function returns not_joined only when all processes of the list are not joined.

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monitor_scope() -> {reference(), #{group() => [pid()]}}
OTP 25.1

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monitor_scope(Scope :: atom()) ->
{reference(), #{group() => [pid()]}}
OTP 25.1

Subscribes the caller to updates from the specified scope. Returns content of the entire scope and a reference to match the upcoming notifications.

Whenever any group membership changes, an update message is sent to the subscriber:

    {Ref, join, Group, [JoinPid1, JoinPid2]}

    {Ref, leave, Group, [LeavePid1]}

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monitor(Group :: group()) -> {reference(), [pid()]}
OTP 25.1

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monitor(Scope :: atom(), Group :: group()) ->
{reference(), [pid()]}
OTP 25.1

Subscribes the caller to updates for the specified group. Returns list of processes currently in the group, and a reference to match the upcoming notifications.

See monitor_scope/0 for the update message structure.

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demonitor(Ref :: reference()) -> ok | false
OTP 25.1

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demonitor(Scope :: atom(), Ref :: reference()) -> ok | false
OTP 25.1

Unsubscribes the caller from updates (scope or group). Flushes all outstanding updates that were already in the message queue of the calling process.

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get_local_members(Group :: group()) -> [pid()]
OTP 23.0

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get_local_members(Scope :: atom(), Group :: group()) -> [pid()]
OTP 23.0

Returns all processes running on the local node in the group Group. Processes are returned in no specific order. This function is optimised for speed.

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get_members(Group :: group()) -> [pid()]
OTP 23.0

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get_members(Scope :: atom(), Group :: group()) -> [pid()]
OTP 23.0

Returns all processes in the group Group. Processes are returned in no specific order. This function is optimised for speed.

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which_groups() -> [Group :: group()]
OTP 23.0

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which_groups(Scope :: atom()) -> [Group :: group()]
OTP 23.0

Returns a list of all known groups.

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See Also

kernel(6)