Suppose we have a queue and some producer app is producing the messages into this queue at the rate of 100 messages/second. And the consumer is consuming messages at the rate of 50 messages/sec. So, there is a lag of 50 messages/sec which will cause the queue to full at some point in future. So, how to resolve such issues?
There are different approaches. One is to block the producer until the queue has space. Another is to drop messages that can't be added to the queue. Yet another is to remove low priority messages from the queue to make space for high priority messages. The correct approach depends on your application.
There's another approach - start more consumers. If your consumer can process 50 messages per sec, then two consumers can most likely process more than 50 messages per sec (just don't expect 100 messages per sec, there's bound to be some overhead).