Github user tillrohrmann commented on a diff in the pull request:
https://github.com/apache/flink/pull/2158#discussion_r68562801
--- Diff: docs/apis/common/index.md ---
@@ -1350,3 +1350,211 @@ You may specify program arguments before the job is
executed. The plan visualiza
the execution plan before executing the Flink job.
{% top %}
+
+Metrics
+-------------------
+
+Flink exposes a metric system that allows users gather and expose metrics
to external systems.
+
+### Registering metrics
+
+You can access the metric system from any user function that extends
[RichFunction]({{ site.baseurl }}/apis/common/index.html#rich-functions) by
calling getRuntimeContext().getMetricGroup().
+This method returns a MetricGroup object on which you can create and
register new metrics.
+
+If you want to count the number of records your user function has received
you could use a Counter like this:
+
+{% highlight java %}
+
+public class MyMapper extends RichMapFunction<String, Integer> {
+ private Counter counter;
+
+ @Override
+ public void open(Configuration config) {
+ // create and register a counter
+ this.counter =
getRuntimeContext().getMetricGroup().counter("myCounter");
+ ...
+ }
+
+ @public Integer map(String value) throws Exception {
+ // increment counter
+ this.counter.inc();
+ ...
+ }
+}
+
+{% endhighlight %}
+
+### Metric types
+
+Flink supports Counters, Gauges and Histograms.
+
+A Counter is used to count something. The current value can be in- or
decremented using `inc([long n])` or `dec([long n])`.
+You can create and registers a Counter by calling `counter(String name)`
on a MetricGroup. Alternatively, you can pass an instance of your own Counter
implementation along with the name.
+
+A Gauge provides a value of any type on demand. In order to use a Gauge
you must first create a class that implements the
`org.apache.flink.metrics.Gauge` interface.
+There is not restriction for the type of the returned value.
+You can register a gauge by calling `gauge(String name, Gauge gauge)` on a
MetricGroup.
+
+A Histogram measure the distribution of long values.
+You can register one by calling histogram(String name, Histogram
histogram) on a MetricGroup.
+
+Flink only provides an interface for Histograms, but offers a Wrapper that
allows usage of Codahale/DropWizard Histograms.
(org.apache.flink.dropwizard.metrics.DropWizardHistogramWrapper)
--- End diff --
I think it would be better to present the different metric types either as
a descriptive list similar to the description environment of latex or by using
sub headings. It could also be more descriptive to give for each metric type a
short code snippet how to register and interacting with it. That would explain
their interface which is important for the user.
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