How do I get the number of messages in a Spring Integration queue with Micrometer?

Question

Spring's Metrics and Management: MessageChannel Metric Features describes:

If it is a QueueChannel, you also see statistics for the receive operation as well as the count of messages that are currently buffered by this QueueChannel

However:

These legacy metrics will be removed in a future release. See Micrometer Integration.

Where it describes:

The Counter Meters for receive operations on pollable message channels have the following names or tags: name: spring.integration.receive [...]

which sounds like it only counts how many messages have been received. The number of messages in the queue doesn't seem to be available, not even by calculating receive - send (because there is no send).

So, with Spring Integration and Micrometer, is it even possible to read the queue size? How?

Answer 1

I think we need to register a Gauge for this queue size value:

/**
 * A gauge tracks a value that may go up or down. The value that is published for gauges is
 * an instantaneous sample of the gauge at publishing time.
 *
 * @author Jon Schneider
 */
public interface Gauge extends Meter {

And here is a respective builder:

/**
 * A convenience method for building a gauge from a supplying function, holding a strong
 * reference to this function.
 *
 * @param name The gauge's name.
 * @param f    A function that yields a double value for the gauge.
 * @return A new gauge builder.
 * @since 1.1.0
 */
@Incubating(since = "1.1.0")
static Builder<Supplier<Number>> builder(String name, Supplier<Number> f) {

Feel free to raise a GH issue to improve things!

At the moment it is probably possible via an external Gauge instance with QueueChannel.getQueueSize() delegation.