[rabbitmq-discuss] PubSub - all bindings have to be in memory?

[Ben Browitt]

Matthew,

Let's say RAM can fit all my bindings and that I'm using fixed sized values
with direct exchanges.
Assuming that the incoming and outgoing number of messages is constant,
should I expect the performance to degrade as the number of  bindings
increases or will it stay the same?
In other words, does a large number of queues, exchanges and bindings affect
the load even when
most of them aren't being used?

Ben



On Mon, Feb 1, 2010 at 3:43 PM, Matthew Sackman <matthew at lshift.net> wrote:

> Hi Ben,
>
> On Mon, Feb 01, 2010 at 07:58:52AM +0200, Ben Browitt wrote:
> > Do I have to keep all bindings in memory or is there a mode where
> rabbitmq
> > can read binding keys from disk when needed?
>
> I'm afraid bindings are all kept in RAM. The binding record is quite
> small (dependent on the size of the exchange name, binding key, queue
> name and any args - minimising these will help reduce memory foot print)
> so you should be able to fit quite a lot of them in RAM:
>
> > erts_debug:flat_size(#binding{exchange_name = <<"my_exchange">>,
> queue_name = <<"my_queue">>, key = <<"my_key">>, args = []}).
> 24
>
> So just 24 bytes for that record, though obviously with padding and
> longer field values, this will grow. Fixed size values here would help -
> if for example you're able to use md5sums to get the exchange name and
> queue name, and only use fanout or direct exchanges, and thus have an
> fixed sized hash for the binding key too, then that'll make estimations
> easier.
>
> If you can estimate given, say, twice your expected number of visitors
> and twice the expected number of articles they subscribe to how many
> bindings that would result in, then you may be able to satisfy yourself
> whether or not this is going to be a problem.
>
> Whilst the new persister is able to page messages out to disk, we have
> no plans just at the moment to be able to do the same for bindings.
>
> Please let us know how you get on.
>
> Matthew
>
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