activemq-artemis/docs/user-manual/en/slow-consumers.md

# 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.

## Required Configuration

By default the server will not detect slow consumers. If slow consumer
detection is desired then see [address model chapter](address-model.md)
for more details on the required address settings.

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](thread-pooling.md) for more details about this pool.

## Example

See the [slow consumer example](examples.md#slow-consumer) which shows how to detect a slow consumer
with Apache ActiveMQ Artemis.