[erlang-questions] Re: A discussion in application design and event propagation

Garrett Smith <>
Mon Apr 19 22:20:48 CEST 2010


On Mon, Apr 19, 2010 at 12:24 PM, Fred Hebert <> wrote:
>
>
> On Mon, Apr 19, 2010 at 12:14 PM, Garrett Smith <> wrote:
>>
>>
>> Thinking in architectural terms, this strikes me as a pub/sub problem
>> that a message broker would handle well. RabbitMQ (AMQP), e.g., would
>> let you publish events to an exchange, which can then be routed to
>> various queues via bindings. RabbitMQ in particular is a nice option
>> for Erlang developers.
>>
>> Garrett
>>
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>
> I'm not too familiar with RabbitMQ and queue usage past using them to send
> the load from an application's frontend to a backend (and also surviving
> huge peaks in users).
>
> How well does it lend itself to real-time applications? How would you make
> it work with N tables and 4N users? From my little experience with queuing
> systems like this, it sounds like the number of queues or topics required
> would be huge, otherwise the amount of filtering even higher. What's your
> take on that?

The overhead of relaying messages is very low. In a distributed system
though you'll have network latencies as well as costs in decoding and
encoding messages. I'm pretty sure this doesn't qualify as "real
time", but for your use case it's probably fine. If you have specific
requirements, I'd post to the rabbit list.

I haven't paid close attention to your card game design, but at first
stab, I'd use what AMQP calls a "topic" exchange. This delivers
messages to queues based on routing key patterns. Some of the possible
message keys might be:

  blackjack.table.setup
  blackjack.table.deal
  blackjack.table.teardown
  blackjack.player.join
  blackjack.player.leave

Message producers, e.g. a "table" process, would send messages to the
exchange with a particular routing key designating an event. So if you
spawned a new table process, you might send a message keyed with
"blackjack.table.setup". The message would contain information about
the table (number, capacity, etc.)

Message consumers would each create a queue for receiving messages and
"bind" that queue using routing key patterns -- think of this as
subscribing to a message stream based on their keys. E.g. a player
process is interested when a table deal event occurs -- it would bind
using "blackjack.table.deal". You could "sniff" the messages for those
applying to your table(s) or push the routing keys further by
including table IDs (in which case you'd receive table events only for
the tables you bind to -- e.g. "blackjack.table.4.deal").

As for the proliferation of queues, you'd need at least one queue per
participating process. I think it's fair to say that rabbitmq will
handily support tens of thousands of queues on a single machine
(throughput aside), though this is also something you'd want to pass
by the rabbit list. As always there are lots of variable when scale is
an issue.

But in general, message brokers specialize in routing messages
involving lots and lots of producers and consumers. RabbitMQ happens
to be one excellent option (there are certainly others) and has the
benefit in your case of being very Erlang friendly.

Garrett


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