Selectors are a way for a consumer to tell the broker, "Don't send me messages like this when you do normal dispatch for this destination."
For a queue, all messages are essentially communal between the set of consumers, so the skipped messages stay on the queue for some other consumer (one with a matching selector) to consume them. It's very important that you ensure that the set of selectors from the different consumers covers all messages that will be sent, because any messages that don't match any consumer will sit on the top of the queue and if there are enough of them they can prevent messages from being dispatched to any consumer and work will stop on the queue. For a topic, each consumer has their own copy of the message stream, so messages that don't match their consumer's selector are simply discarded rather than left behind. This means it's not critical to ensure that the set of selectors fully covers the set of messages. So yes, they're specified by the consumers, but they will have their desired effect even though they haven't been specified when the first message is sent. As long as your routing rules can be computed at consumer startup (i.e. there's nothing dependent on application state), selectors could give you what you need in the more-limited scenarios you described. One caution, though: Art Naseef always warned that selectors were an easy way to kill broker performance, because they must be evaluated on the broker for each message for each consumer. So if you are going to have large numbers of consumers and/or large numbers of messages (and unfortunately I don't have good heuristics for what constitutes "large numbers" here), there might be a performance impact to using the selector approach. With that said, lots of people use selectors in ActiveMQ without performance problems, so if you think, "we don't have all THAT many messages and consumers," you'll probably be fine. Tim On May 2, 2017 3:54 AM, "Vince Cole" <the.vince.c...@gmail.com> wrote: > Hi Tim > > OK - just one more question: > If having a single consumer per queue wasn't a problem, would Selectors > still be a workable solution? > Or can they only be used if you have consumers (i.e. is it the consumers > who > have to specify them)? > > OK OK - another: > If it was OK to lose messages when consumers weren't attached, then topics > could be used. > Would Selectors work in the same way with topics? > > thanks again in advance > > > > -- > View this message in context: http://activemq.2283324.n4. > nabble.com/destination-unspecified-can-ActiveMQ-set- > it-tp4725338p4725420.html > Sent from the ActiveMQ - User mailing list archive at Nabble.com. >
