Github user tillrohrmann commented on a diff in the pull request:
https://github.com/apache/flink/pull/2158#discussion_r68563783
--- 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)
+
+### Scope
+
+Every registered metric has an automatically assigned scope which
represent the entities it is tied to. By default a metric that is registered in
a user function will be scoped to the Operator in which the function runs, the
Task/Job it belongs to and the TaskManager/Host it is executed on. This is
referred to as the "system scope".
+
+You can define an additonal "user scope" by calling the
MetricGroup#addGroup((int/String) name) method.
+
+{% highlight java %}
+
+counter2 =
getRuntimeContext().getMetricGroup().addGroup("MyMetrics").counter("myCounter2");
+
+{% endhighlight %}
+
+The name under which a metric is exported is based on both scopes and the
name passed in the `counter() call. The order is always
\<system_scope>\<user_scope>\<name>.
+
+The system scope allows the reported name to contain contextual
information like the name of job it was registered in without requiring the
user to pass this information manually.
+
+How the system scope affects the reported name for a metric can be
modified by setting the following keys in the flink-conf.yaml.
+Each of these keys expect a format string that may contain constants (e.g.
"taskmanager") and variables (e.g. "\<task_id>") which will be replaced at
runtime.
+
+- `metrics.scope.jm`
+ - Default: \<host>.jobmanager
+- `metrics.scope.jm.job`
+ - Default: \<host>.jobmanager.\<job_name>
+- `metrics.scope.tm`
+ - Default: \<host>.taskmanager.\<tm_id>
+- `metrics.scope.tm.job`
+ - Default: \<host>.taskmanager.\<tm_id>.\<job_name>
+- `metrics.scope.task`
+ - Default:
\<host>.taskmanager.\<tm_id>.\<job_name>.\<task_name>.\<subtask_index>
+- `metrics.scope.operator`
+ - Default:
\<host>.taskmanager.\<tm_id>.\<job_name>.\<operator_name>.\<subtask_index>
+
+The following is a list of all variables available:
+
+- JobManager: \<host>
+- TaskManager: \<host>, \<tm_id>
+- Job: \<job_id>, \<job_name>
+- Task: \<task_id>, \<task_name>, \<task_attempt_id>, \<task_attempt_num>,
\<subtask_index>
+- Operator: \<operator_name>, \<subtask_index>
+
+There is no restriction on the order of variables.
+
+Which format is applied to a metric depends on which entities it was
scoped to.
+The `metrics.scope.operator` format will be applied to all metrics that
were scoped to an Operator.
+The `metrics.scope.tm.job` format will be applied to all metrics that were
only scoped to a Job and a TaskManager.
+
+This allows you to define explicitly how metrics on every level should be
reported.
--- End diff --
Giving a small example could be helpful.
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