[erlang-questions] Need some advice on implementing a Queue Server

[Torben Hoffmann]

Slightly off topic, but I have used pg2 to create a simple pub/sub library for Elixir
at https://github.com/lehoff/ogma

Might be a source for inspiration.

Keep in mind that pg2 merely provides a way to send a message to multiple processes
in one go. All the semantics of your queue has to be dealt with in your code, but I'm
sure you are taking this into account.

Cheers,
Torben

avinash D'silva writes:

> Thank you both.
>
> Gilberio, your example taught me something new today :)
>
> BTW, if I want a named queue, should I use   pg2:create(<<"Q_1">>). or is
> there a better way?
>
> On Fri, Jul 31, 2015 at 12:55 AM, Gilberio Carmenates Garcia <
> co7eb@REDACTED> wrote:
>
>> Regards, James, I think using queue module from otp and gen_server
>> behavior, maintaining the Queue in the state, would help you to implement
>> something like that. queue module have many interesting APIs that will do
>> the job for you very faster. Just remember not to use queue:len/1 or any
>> other O(n) order function  if you don´t  need it, for len you can just keep
>> it in track in the gen_server state along with  the queue. NOTE: some of
>> the queue APIs, throws exceptions, use the simple API if you don want to
>> handle exceptions.
>>
>>
>>
>> Ie:
>>
>> Init([])->
>>
>>       {ok, {queue:new(), 0}}.
>>
>>
>>
>> Handle_call({push, Item}, _From, {MyQueue, Len})
>>
>>     {reply, ok, {queue:in(Item, MyQueue), Len + 1}}
>>
>>
>>
>> Handle_call(pop, _From, {MyQueue, Len})
>>
>>     case queue:out(MyQueue) of
>>
>>         {{value, Item}, Q} ->
>>
>>            {reply, Item, {Q, Len - 1}}
>>
>>         {empty, Q} ->
>>
>>            {reply, no_item, {Q, Len}}
>>
>>
>>
>> If you want to retrieve some specific item you can always use
>> queue:filter/2 function instead.
>>
>> Something like this:
>>
>> pop_item(Item, Queue)->
>>
>>     queue:filter(fun(I)-> I =/= Item end, Queue).
>>
>>
>>
>> but that is function of order O(n) so…
>>
>>
>>
>> NOTE: I just wrote  the example without testing it, so I apologize for any
>> mistake.
>>
>>
>>
>> Best regards,
>>
>> Ivan.
>>
>>
>>
>>
>>
>> *De:* erlang-questions-bounces@REDACTED [mailto:
>> erlang-questions-bounces@REDACTED] *En nombre de *Wes James
>> *Enviado el:* jueves, 30 de julio de 2015 15:02
>> *Para:* avinash D'silva
>> *CC:* erlang-questions@REDACTED
>> *Asunto:* Re: [erlang-questions] Need some advice on implementing a Queue
>> Server
>>
>>
>>
>>
>>
>> On Thu, Jul 30, 2015 at 12:22 PM, avinash D'silva <evnix.com@REDACTED>
>> wrote:
>> >
>> > Hi,
>> >
>> > I am trying to learn Erlang by implementing a small Queueing Server.
>> >
>> > What have I done so far?
>> > I have created a basic websocket server using cowboy.
>> >
>> > I am trying to implement a Queue which all processes can access.
>> >
>> > I have used lists which can act as queues.
>> >
>> > The basic insert function looks like this:
>> > Queue(Q) ->
>> >         receive
>> >                Msg ->
>> >                     Queue(lists:append(Q,[Msg]))
>> >         end.
>> >
>> > To create a Queue called "Q_1"
>> >
>> > >  pg2:create(<<"Q_1">>).
>> > >  P = spawn(test,Queue,[[1]]). % spawn queue and insert 1.
>> > >  pg2:join(<<"Q_1">>,P).
>> > >  P ! 2.
>> > >  P ! 3. % add some elements to queue.
>> >
>> > to retrieve the elements from the named Queue "Q_1":
>> > 1. I use [Px|_]=pg2:get_members()
>> > 2. then I would do something like: Px ! POP_ELEMENT
>> >
>> >
>> > Is there a better or more scalable way of doing this?
>> >
>> > Will this work when a lot of clients are connected( around 60), having a
>> insert rate of 100k elements per hour?
>> >
>> > Any help is appreciated :)
>> >
>>
>> I did a search on yahoo for "erlang queue example" and this was the first
>> hit:
>>
>> http://www.erlang.org/doc/man/queue.html
>>
>> There were several other examples that might help.
>>
>> -wes
>>
>>
>>
>>

-- 
Torben Hoffmann
CTO, erlang-solutions.com
T: +45 25 14 05 38