View Source shq - Shared inter-process queues
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, 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).
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.
Draining a queue
%% Draining a quee from the front
shq:drain(ServerRef).
%% Draining a queue from the rear
shq:drain_r(ServerRef).
For 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).
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).
For 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).
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.