50. Metrics

Spring Boot Actuator includes a metrics service with ‘gauge’ and ‘counter’ support. A ‘gauge’ records a single value; and a ‘counter’ records a delta (an increment or decrement). Spring Boot Actuator also provides a PublicMetrics interface that you can implement to expose metrics that you cannot record via one of those two mechanisms. Look at SystemPublicMetrics for an example.

Metrics for all HTTP requests are automatically recorded, so if you hit the metrics endpoint you should see a response similar to this:

{
    "counter.status.200.root": 20,
    "counter.status.200.metrics": 3,
    "counter.status.200.star-star": 5,
    "counter.status.401.root": 4,
    "gauge.response.star-star": 6,
    "gauge.response.root": 2,
    "gauge.response.metrics": 3,
    "classes": 5808,
    "classes.loaded": 5808,
    "classes.unloaded": 0,
    "heap": 3728384,
    "heap.committed": 986624,
    "heap.init": 262144,
    "heap.used": 52765,
    "nonheap": 0,
    "nonheap.committed": 77568,
    "nonheap.init": 2496,
    "nonheap.used": 75826,
    "mem": 986624,
    "mem.free": 933858,
    "processors": 8,
    "threads": 15,
    "threads.daemon": 11,
    "threads.peak": 15,
    "threads.totalStarted": 42,
    "uptime": 494836,
    "instance.uptime": 489782,
    "datasource.primary.active": 5,
    "datasource.primary.usage": 0.25
}

Here we can see basic memory, heap, class loading, processor and thread pool information along with some HTTP metrics. In this instance the root (‘/’) and /metrics URLs have returned HTTP 200 responses 20 and 3 times respectively. It also appears that the root URL returned HTTP 401 (unauthorized) 4 times. The double asterisks (star-star) comes from a request matched by Spring MVC as /** (normally a static resource).

The gauge shows the last response time for a request. So the last request to root took 2ms to respond and the last to /metrics took 3ms.

	Note
In this example we are actually accessing the endpoint over HTTP using the /metrics URL, this explains why metrics appears in the response.