3.3. Prefetch Limit
Overview
If a consumer is slow to acknowledge messages, it can happen that the broker sends it another message before the previous message is acknowledged. If the consumer continues to be slow, moreover, the number of unacknowledged messages can grow continuously larger. The broker does not continue to send messages indefinitely in these circumstances. When the number of unacknowledged messages reaches a set limit—the prefetch limit—the server ceases sending new messages to the consumer. No more messages will be sent until the consumer starts sending back some acknowledgements.
Prefetch limits
Different prefetch limits can be set for each consumer type:
- Queue consumer
- Default prefetch limit is 1000.If you are using a collection of consumers to distribute the workload (many consumers processing messages from the same queue), you typically want this limit to be small. If one consumer is allowed to accumulate a large number of unacknowledged messages, it could starve the other consumers of messages. Also, if the consumer fails, there would be a large number of messages unavailable for processing until the failed consumer is restored.
- Queue browser
- Default prefetch limit is 500.
- Topic consumer
- Default prefetch limit is 32766.The default limit of 32766 is the largest value of a short and is the maximum possible value of the prefetch limit.
- Durable topic subscriber
- Default prefetch limit is 100.You can typically improve the efficiency of a consumer by increasing this prefetch limit.
Optimizing prefetch limits
Typically, it is a good idea to optimize queue consumers and durable topic subscribers as follows:
- Queue consumers—if you have just a single consumer attached to a queue, you can leave the prefetch limit at a fairly large value. But if you are using a group of consumers to distribute the workload, it is usually better to restrict the prefetch limit to a very small number—for example, 0 or 1.
- Durable topic subscribers—the efficiency of topic subscribers is generally improved by increasing the prefetch limit. Try increasing the limit to 1000.
