JMS topic time dependency

Question

I don't quite understand what this means:

Each message may have multiple subscribers. There are time dependencies that exist between publishers and subscribers; subscribers to a topic may only consume messages published after a subscription was created.

What does it mean that there exists "time dependencies"? The above says that "subscribers may only consume messages after a subscription was created", I think I'm missing something vital here because I understood that as a subscriber cannot consume a message that is not yet created (but isn't this common sense).

I couldn't find any explanation on Google, so thank you for any help!

Edit: I found an excellent channel on youtube explaining spring boot and other technologies. In particular, this video helped me understand the difference between Queue and Topic (also what durable subscriber is).

Answer 1

What does it mean that there exists "time dependencies"?

To understand this better, compare it with a JMS Queue in which the message broker preserves all the messages (till TimeToLive expiration) published by the producer even if there are NO consumers CREATED.

Now, coming to a JMS topic, the message broker DOES NOT preserve the messages published by the producer if there are NO subscribers CREATED.

During the creation of topic subscription, you can tell the Message Broker that I don't want to loose the messages for this topic published FROM NOW ON by creating the Topic subscription as non-durable (using topicSession.createDurableSubscriber() method). Again, even in the case of non-durable subscription, the broker preserves the messages published after (subscription creation time) till TimeToLive expiration.

Answer 2

Simply that if you publish messages in to the topic before the consumer has subscribed, they will not see the messages.

Similarly, this is related to persistent subscribers. Normal subscribers only see messages while they are connected to the broker. Any messages before or after the subscriber has disconnected, will not be see by the subscriber.

However, with a persistent subscription, a subscriber can disconnect, yet the broker will continue to keep messages destined for that subscriber. When the subscriber reconnects, it will fetch all of those stored messages as well as any new ones.