Gauges

Gauges are used to measure the instantaneous value of something.

They’re often useful for things that aren’t events, like “users currently in the database”. If you can perform a query of some kind at any time to get a value, it’s probably something suitable for a gauge.

Examples of metrics you might want to track with a gauge:

• Number of users in your database.

TODO: More examples.

Creating

Create your gauge using gauge-fn:: but you have to pass it a function, not just a body:

(require '[metrics.core :refer [new-registry]])
(require '[metrics.gauges :refer [gauge-fn gauge]])

(def reg (new-registry))
(def files-open
  (gauge-fn reg "files-open"
         #(return-number-of-files-open ...)))

Once a gauge has been registered, a call to (gauge reg "files-open") will return the existing gauge.

You can also use the defgauge macro to create a gauge and bind it to a var in one concise, easy step:

(require '[metrics.gauges :refer [defgauge]])

(defgauge reg files-open
  (fn []
    (return-number-of-files-open ...)))

defgauge takes a function like gauge-fn.

All the def[metric] macros do some magic to the metric title to make it easier to define.

Writing

With gauges there is no writing. Gauges execute the form(s) (or function) you passed when creating them every time they’re read. You don’t need to do anything else.

Reading

There’s only one way to get data from a gauge.

value

You can read the value of a gauge at any time with value:

(require '[metrics.gauges :refer [value]])

(value files-open)

Or if you haven’t held a reference to files-open, you can do the following:

(value (gauge reg "files-open"))