Long Task Timers

The long task timer is a special type of timer that lets you measure time while an event being measured is still running. A normal Timer only records the duration after the task is complete.

Long task timers publish at least the following statistics:

Active task count
Total duration of active tasks
The maximum duration of active tasks

Unlike a regular Timer, a long task timer does not publish statistics about completed tasks.

Consider a background process to refresh metadata from a data store. For example, Edda caches AWS resources, such as instances, volumes, auto-scaling groups, and others. Normally all data can be refreshed in a few minutes. If the AWS services have problems, it can take much longer. A long task timer can be used to track the active time for refreshing the metadata.

For example, in a Spring application, it is common for such long running processes to be implemented with @Scheduled. Micrometer provides a special @Timed annotation for instrumenting these processes with a long task timer:

@Timed(value = "aws.scrape", longTask = true)
@Scheduled(fixedDelay = 360000)
void scrapeResources() {
    // find instances, volumes, auto-scaling groups, etc...
}

It is up to the application framework to make something happen with @Timed. If your framework of choice does not support it, you can still use the long task timer:

LongTaskTimer scrapeTimer = registry.more().longTaskTimer("scrape");
void scrapeResources() {
    scrapeTimer.record(() => {
        // find instances, volumes, auto-scaling groups, etc...
    });
}

If we wanted to alert when this process exceeds a threshold, with a long task timer, we receive that alert at the first reporting interval after we have exceeded the threshold. With a regular timer, we would not receive the alert until the first reporting interval after the process completed, over an hour later!

The interface contains a fluent builder for long task timers:

LongTaskTimer longTaskTimer = LongTaskTimer
    .builder("long.task.timer")
    .description("a description of what this timer does") // optional
    .tags("region", "test") // optional
    .register(registry);