shq - Shared inter-process queues

🔗 Hex package

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

A server name as used when starting a named gen_server.

Opts

A map or property list of options

max => non_neg_integer() | infinity

The maximum number of items the queue can hold (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

A server reference as used with gen_server.

Item

The item to insert.

Timeout

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, the atom full if the queue was full, or the atom closed if the queue was not accepting new items at the time of the call.

When these functions return full or closed, 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).

ServerRef

See "Inserting".

Timeout

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).

ServerRef

See "Inserting".

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.

Draining a queue

%% Draining a quee from the front
shq:drain(ServerRef).

%% Draining a queue from the rear
shq:drain_r(ServerRef).

ServerRef

See "Inserting".

The return value is a list of all items that were in the queue at the time when the call was made, including the ones that were waiting for insertion.

Retrieving queue size

shq:size(ServerRef).

ServerRef

See "Inserting".

Retrieves the number of items in the queue.

The return value is a non-negative integer.

Closing and Reopening a queue

%% Closing a queue
ok=shq:close(ServerRef).

%% Reopening a queue
ok=shq:open(ServerRef).

ServerRef

See "Inserting".

When a queue is closed, all subsequent insert operations will be rejected and return closed until it is reopened.

The queue status can be queried via shq:status(ServerRef).

Stopping a queue

ok=shq:stop(ServerRef).

ServerRef

See "Inserting".

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. However, as ets tables live in memory, shq queues are not suitable for storing vast numbers of large items. It is also noteworthy that items waiting to be inserted into a queue that is currently full will still be stored in the backing table, albeit temporarily, and thus still against available memory.

Authors

• Maria Scott (Maria-12648430)
• Jan Uhlig (juhlig)