Hi!
Welcome to the ActiveMQ community! :-)
Here's some interesting reading:
http://www.activemq.org/site/how-does-a-queue-compare-to-a-topic.html
http://www.activemq.org/site/how-do-durable-queues-and-topics-work.html
In a nutshell I guess, when you use queues, each message is stored in
the queue until a consumer consumes them (each message will be consumed
by only one consumer), while for topics, each message will be send to
all consumers currently subscribed to the topic. If there is no
consumers, the message is discarded. If you want to receive messages
using topics while your consumer is offline, you could use durable
subscriptions, but the condition is that you must first subscribe to the
topic before going offline. (i.e. tell the broker that you are
interested in the topic's messages.)
Another workaround is to use the recovery policies of ActiveMQ. This
allows you to somewhat go back in time and receive some of the messages
you've missed. You must specify a retroactive consumer to use this:
http://www.activemq.org/site/retroactive-consumer.html
http://www.activemq.org/site/subscription-recovery-policy.html
Hope these helps!
Regards,
Adrian Co
wizard wrote:
Hi, I'm new to amq, so I have a basic question, I guess.
I've created a queue, with a producer and a consumer, and I was able to send
messages, and keep them in queue.
Now I've changed to using a topic, and I still send and receive messages,
but the behaviour has changed: when I produce a message and the consumer is
running, it instantly gets the message. But when I produce a message and the
consumer isn't running, I seem to lose the message, because when I connect
the consumer, it doesn't read any message at all, and my queue size is
always 0.
Is this how topics are supposed to work? What is wrong?
