[rabbitmq-discuss] When are messages persisted to disk?

[Matthew Sackman]

On Thu, Sep 01, 2011 at 12:14:07AM +0200, Alvaro Videla wrote:
> I would assume that if the message is published with: delivery-mode = 2  then it will be persisted ALWAYS. Is that the case?

No. delivery-mode=2 _AND_ msg arrives in durable queue. This is the only
case in which msgs _MUST_ be written to disk.

When memory is plentiful, even msgs with delivery-mode=2 and in durable
queues will be held in RAM _in addition_ to being on disk.

When memory is tight, these RAM copies will be forgotten. When memory is
tight, other messages that are currently _only_ in RAM may be written to
disk, and yes, they'll go to the transient msg store.

> Is it correct to say that transient queues get their messages saved to
> the TRANSIENT_MSG_STORE which is wiped out if RabbitMQ restarts?

Yes, but equally delivery-mode=1 msgs in durable queues go to the
transient msg store.

> Are there situations where the message is directly delivered to the consumer without being persisted to disk?

Yes. Or at least once a certain bug makes it through QA, then it will
be. If a msg that should be written to disk arrives in a queue and that
queue is able to send the msg out to a consumer and that consumer acks
the msg (or the consumer is consuming/getting with noAck=true) then
there's a possibility that the msg will never get written to disk. It's
a race. If we find out soon enough that the msg has been sent to a
consumer that's taken responsibility for the msg then we'll avoid
writing it. That doesn't quite happen in 2.6.0, but it hopefully will by
the next release.

Matthew