Detecting Slow Consumers

In this section we will discuss how Apache ActiveMQ Artemis can be configured to deal with slow consumers. A slow consumer with a server-side queue (e.g. JMS topic subscriber) can pose a significant problem for broker performance. If messages build up in the consumer's server-side queue then memory will begin filling up and the broker may enter paging mode which would impact performance negatively. However, criteria can be set so that consumers which don't acknowledge messages quickly enough can potentially be disconnected from the broker which in the case of a non-durable JMS subscriber would allow the broker to remove the subscription and all of its messages freeing up valuable server resources.

Configuration required for detecting slow consumers

By default the server will not detect slow consumers. If slow consumer detection is desired then see queue attributes chapter for more details.

The calculation to determine whether or not a consumer is slow only inspects the number of messages a particular consumer has acknowledged. It doesn't take into account whether or not flow control has been enabled on the consumer, whether or not the consumer is streaming a large message, etc. Keep this in mind when configuring slow consumer detection.

Please note that slow consumer checks are performed using the scheduled thread pool and that each queue on the broker with slow consumer detection enabled will cause a new entry in the internal java.util.concurrent.ScheduledThreadPoolExecutor instance. If there are a high number of queues and the slow-consumer-check-period is relatively low then there may be delays in executing some of the checks. However, this will not impact the accuracy of the calculations used by the detection algorithm. See thread pooling for more details about this pool.