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# pg (kernel v10.4)

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Distributed named process groups.

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 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()).`

## See Also

Kernel

# Summary

## Types

group()

The identifier of a process group.

## Functions

demonitor(Ref)

Equivalent to `demonitor(pg, Ref)`.

demonitor(Scope, Ref)

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

get_local_members(Group)

Equivalent to `get_local_members(pg, Group)`.

get_local_members(Scope, Group)

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.

get_members(Group)

Equivalent to `get_members(pg, Group)`.

get_members(Scope, Group)

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

join(Group, PidOrPids)

Equivalent to `join(pg, Group, PidOrPids)`.

join(Scope, Group, PidOrPids)

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.

leave(Group, PidOrPids)

Equivalent to `leave(pg, Group, PidOrPids)`.

leave(Scope, Group, PidOrPids)

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

monitor(Group)

Equivalent to `monitor(pg, Group)`.

monitor(Scope, Group)

Subscribes the caller to updates for the specified group.

monitor_scope()

Equivalent to `monitor_scope(pg)`.

monitor_scope(Scope)

Subscribes the caller to updates from the specified scope.

start(Scope)

Starts additional scope.

start_link()

Starts the default `pg` scope within supervision tree.

start_link(Scope)

Equivalent to `start(Scope)`, except that it also creates a `link/1` with the calling process.

which_groups()

Equivalent to `which_groups(pg)`.

which_groups(Scope)

Returns a list of all known groups.

# Types

# group()

(not exported) (since OTP 23.0)

    -type group() :: any().

The identifier of a process group.

# Functions

# demonitor(Ref)

(since OTP 25.1)

    -spec demonitor(Ref :: reference()) -> ok | false.

Equivalent to `demonitor(pg, Ref)`.

# demonitor(Scope, Ref)

(since OTP 25.1)

    -spec demonitor(Scope :: atom(), Ref :: reference()) -> ok | false.

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

# get_local_members(Group)

(since OTP 23.0)

    -spec get_local_members(Group :: group()) -> [pid()].

Equivalent to `get_local_members(pg, Group)`.

# get_local_members(Scope, Group)

(since OTP 23.0)

    -spec get_local_members(Scope :: atom(), Group :: group()) -> [pid()].

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.

# get_members(Group)

(since OTP 23.0)

    -spec get_members(Group :: group()) -> [pid()].

Equivalent to `get_members(pg, Group)`.

# get_members(Scope, Group)

(since OTP 23.0)

    -spec get_members(Scope :: atom(), Group :: group()) -> [pid()].

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

# join(Group, PidOrPids)

(since OTP 23.0)

    -spec join(Group :: group(), PidOrPids :: pid() | [pid()]) -> ok.

Equivalent to `join(pg, Group, PidOrPids)`.

# join(Scope, Group, PidOrPids)

(since OTP 23.0)

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

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.

# leave(Group, PidOrPids)

(since OTP 23.0)

    -spec leave(Group :: group(), PidOrPids :: pid() | [pid()]) -> ok.

Equivalent to `leave(pg, Group, PidOrPids)`.

# leave(Scope, Group, PidOrPids)

(since OTP 23.0)

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

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.

# monitor(Group)

(since OTP 25.1)

    -spec monitor(Group :: group()) -> {reference(), [pid()]}.

Equivalent to `monitor(pg, Group)`.

# monitor(Scope, Group)

(since OTP 25.1)

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

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.

# monitor_scope()

(since OTP 25.1)

    -spec monitor_scope() -> {reference(), #{group() => [pid()]}}.

Equivalent to `monitor_scope(pg)`.

# monitor_scope(Scope)

(since OTP 25.1)

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

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]}

# start(Scope)

(since OTP 23.0)

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

Starts additional scope.

# start_link()

(since OTP 23.0)

    -spec start_link() -> {ok, pid()} | {error, any()}.

Starts the default `pg` scope within supervision tree.

Kernel may be configured to do it automatically by setting the Kernel configuration parameter `start_pg`.

# start_link(Scope)

(since OTP 23.0)

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

Equivalent to `start(Scope)`, except that it also creates a `link/1` with the calling process.

# which_groups()

(since OTP 23.0)

    -spec which_groups() -> [Group :: group()].

Equivalent to `which_groups(pg)`.

# which_groups(Scope)

(since OTP 23.0)

    -spec which_groups(Scope :: atom()) -> [Group :: group()].

Returns a list of all known groups.