shq - Shared inter-process queues View Source

shq requires OTP 24 or later.

Usage

Starting a queue

%% Start a non-linked queue
{ok, Pid} = shq:start(Opts).
{ok, Pid} = shq:start(ServerName, Opts).

%% Start a linked queue
{ok, Pid} = shq:start_link(Opts).
{ok, Pid} = shq:start_link(ServerName, Opts).

%% Start a monitored queue
{ok, {Pid, Mon}} = shq:start_monitor(Opts).
{ok, {Pid, Mon}} = shq:start_monitor(ServerName, Opts).

ServerName is a server name as used when starting a named gen_server.

Opts is a map or property list of options. Currently, the only accepted option is max, which is the maximum number of items the queue can hold. Allowed values are either a non-negative integer or the atom infinity (default: infinity).

Inserting an item

%% Insert an item at the rear
shq:in(ServerRef, Item).
shq:in(ServerRef, Item, Timeout).

%% Insert an item at the front
shq:in_r(ServerRef, Item).
shq:in_r(ServerRef, Item, Timeout).

ServerRef is a server reference as used with gen_server.

Item is the item to insert.

Timeout is a timeout to wait for a slot to become available if the queue is full (default: 0).

The return value is either the atom ok if the item was queued, or the atom full if not.

When these functions return full, it is guaranteed that the given item was not inserted, and will not be inserted later without calling this function again.

Retrieving an item

%% Retrieving an item from the front
shq:out(ServerRef).
shq:out(ServerRef, Timeout).

%% Retrieving an item from the rear
shq:out_r(ServerRef).
shq:out_r(ServerRef, Timeout).

For ServerRef, see "Inserting".

Timeout is a timeout to wait for an item to become available if the queue is empty (default: 0).

The return value is either a tuple {ok, Item}, or the atom empty.

When these functions return empty, it is guaranteed that no item was removed from the queue, and none will be removed later without calling this function again.

Peeking

%% Peek at the front
shq:peek(ServerRef).

%% Peek at the rear
shq:peek_r(ServerRef).

Peeking is the same as retrieving, but without removing the item from the queue. The operation will always return instantly, ie there is no way to wait for an item to become available if the queue is empty.

Retrieving queue size

shq:size(ServerRef).

Retrieves the number of items in the queue.

The return value is a non-negative integer.

Stopping a queue

ok=shq:stop(ServerRef).

All items left will be lost when a queue is stopped.

Notes

As shq queues are backed by ets tables of type set, the number of items in a queue does not affect its performance.

Queue performance may be affected by how you use them, however.