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metrics-clojure

metrics-clojure is a Clojure façade around Coda Hale's metrics library.

Installation

Add this to your project.clj's dependencies:

[metrics-clojure "0.1.0"]

That's it.

Usage

metrics-clojure is a thin wrapper around metrics, so if you don't know what any of these words mean you should probably read the metrics documentation and/or watch the talk:

Creating Metrics

metrics-clojure wraps up the metrics classes to make them more Clojurey.

Gauges

Create your gauge:

(use '[metrics.gauges :only (gauge)])

(def files-open
  (gauge "files-open"
         (return-number-of-files-open ...)))

That's it. Pretty simple.

gauge is a macro. If you need a function instead you can use gauge-fn, but you have to pass it a function, not just a body:

(use '[metrics.gauges :only (gauge-fn)])

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

Counters

Create your counter:

(use '[metrics.counters :only (counter)])

(def users-connected (counter "users-connected"))

Now increment and decrement it:

(use '[metrics.counters :only (inc! dec!)])

(inc! users-connected)
(inc! users-connected 2)

(dec! users-connected)
(dec! users-connected 2)

Meters

Create your meter:

(use '[metrics.meters :only (meter)])

(def files-served (meter "files-served" "files"))

The second argument to meter is a string describing the "units" for the meter. In this example it's "files", as in "18732 files".

Mark the meter every time the event happens:

(use '[metrics.meters :only (mark!)])

(mark! files-served)

Histograms

Create your histogram:

(use '[metrics.histograms :only (histogram)])

(def search-results-returned
  (meter "search-results-returned"))

You can create a biased histogram by passing an extra boolean argument:

(def search-results-returned-biased
  (meter "search-results-returned-biased" true))

Update the histogram when you have a new value to record:

(use '[metrics.histograms :only (update!)])

(update! search-results-returned 10)

Timers

Create your timer:

(use '[metrics.timers :only (timer)])

(def image-processing-time (timer "image-processing-time"))

Now time something:

(use '[metrics.timers :only (time!)])

(time! image-processing-time
       (process-image-part-1 ...)
       (process-image-part-2 ...))

time! is a macro. If you need a function instead, you can use time-fn!, but you'll need to pass it a function instead of just a body:

(use '[metrics.timers :only (time-fn!)])

(time-fn! image-processing-time
          (fn []
            (process-image-part-1 ...)
            (process-image-part-2 ...)))

Metric Names

In all these examples we've used a string to name the metric. The metrics library actually names metrics with three-part names. In Java and Scala those names are usually the package and class where the timer is being used.

In Clojure you usually won't have a meaningful class name to record, and the package name would be a pain to find, so metrics-clojure uses "default" and "default" for those parts. This results in a name like "default.default.my-metric".

If you want to specify something other than "default" you can pass a collection of three strings instead of a single string:

(use '[metrics.timers :only (timer)])

(def response-time
  (timer ["webservice" "views" "response-time"]))

This will result in a name like "webservice.views.response-time".

Removing Metrics

You can remove metrics as long as you know their names:

(use '[metrics.core :only (remove-metric)])

(remove-metric "files-served")

You can use the sequence form of metric names here too, of course:

(remove-metric ["webservice" "views" "response-time"])

Reporting Metrics

Currently metrics-clojure only supports reporting metrics through the console (on standard error):

(use '[metrics.core :only (report-to-console)])

(report-to-console 10)

This will tell metrics to print all metrics to the console every ten seconds.

More reporting methods will be added in the future. Pull requests are welcome.

Side Effects and Agents

Pretty much everything metrics-clojure does causes side effects. If you're recording metrics inside of a dosync they may be recorded multiple times if the transaction is restarted.

This may or may not be what you want. If you're recording how long it takes to do something, and you do it twice, then it makes sense to record it each time. If you're recording how many responses get sent to a user, then you probably don't want to overcount them.

metrics-clojure doesn't try to decide for you. It leaves it up to you to handle the issue.

If you don't want to record something multiple times, an agent may be a good way to handle things:

(def thing-given-to-user (agent (counter "thing-given-to-user")))

(dosync
   (send inc! thing-given-to-user)
   (make-thing ...))

This works because the recording functions for counters, meters and histograms return the metric object.

This will not work with timers! time! returns the result of the work, so the agent's value will be replaced by that and it won't work more than once.

In practice this shouldn't be a problem though, because if you're timing work you'll probably want to time it every time it happens, right?

More Information

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