Github user tillrohrmann commented on a diff in the pull request:
https://github.com/apache/flink/pull/2158#discussion_r69093837
--- Diff: docs/apis/common/index.md ---
@@ -1350,3 +1350,397 @@ 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 gathering and exposing 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.
+
+### Metric types
+
+Flink supports `Counters`, `Gauges` and `Histograms`.
+
+#### Counter
+
+A `Counter` is used to count something. The current value can be in- or
decremented using `inc()/inc(long n)` or `dec()/dec(long n)`.
+You can create and register a `Counter` by calling `counter(String name)`
on a MetricGroup.
+
+{% 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 %}
+
+Alternatively you can also use your own `Counter` implementation:
+
+{% highlight java %}
+
+public class MyMapper extends RichMapFunction<String, Integer> {
+ ...
+
+ @Override
+ public void open(Configuration config) {
+ // register a custom counter
+ this.counter =
getRuntimeContext().getmetricGroup().counter("myCustomCounter", new
CustomCounter());
+ ...
+ }
+ ...
+}
+
+{% endhighlight %}
+
+#### Gauge
+
+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.
+
+{% highlight java %}
+
+public class MyMapper extends RichMapFunction<String, Integer> {
+ private int valueToExpose;
+
+ @Override
+ public void open(Configuration config) {
+ // register the gauge
+ getRuntimeContext().getmetricGroup().gauge("MyGauge", new
Gauge<Integer>() {
+ @Override
+ public Integer getValue() {
+ return valueToExpose;
+ }});
+ ...
+ }
+ ...
+}
+
+{% endhighlight %}
+
+#### Histogram
+
+A Histogram measure the distribution of long values.
+You can register one by calling histogram(String name, Histogram
histogram) on a MetricGroup.
+
+{% highlight java %}
+public class MyMapper extends RichMapFunction<Long, Integer> {
+ private Histogram histogram;
+
+ @Override
+ public void open(Configuration config) {
+ // create and register a counter
+ this.histogram =
getRuntimeContext().getMetricGroup().histogram("myHistogram", new
MyHistogram());
+ ...
+ }
+
+ @public Integer map(Long value) throws Exception {
+ this.histogram.update(value);
+ ...
+ }
+}
+{% endhighlight %}
+
+Flink only provides an interface for Histograms, but offers a Wrapper that
allows usage of Codahale/DropWizard Histograms.
(org.apache.flink.dropwizard.metrics.DropWizardHistogramWrapper)
+This wrapper is contained in the `flink-metrics-dropwizard` module.
+
+### Scope
+
+Every registered metric has an automatically assigned scope which
represents 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 either
`MetricGroup#addGroup(String name)` or `MetricGroup#addGroup(int name)`.
+
+{% highlight java %}
+
+counter =
getRuntimeContext().getMetricGroup().addGroup("MyMetrics").counter("myCounter");
+
+{% 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
+ - Applied to all metrics that were scoped to a jobmanager.
+- `metrics.scope.jm.job`
+ - Default: \<host>.jobmanager.\<job_name>
+ - Applied to all metrics that were scoped to a jobmanager and job.
+- `metrics.scope.tm`
+ - Default: \<host>.taskmanager.\<tm_id>
+ - Applied to all metrics that were scoped to a taskmanager.
+- `metrics.scope.tm.job`
+ - Default: \<host>.taskmanager.\<tm_id>.\<job_name>
+ - Applied to all metrics that were scoped to a taskmanager and job.
+- `metrics.scope.tm.task`
+ - Default:
\<host>.taskmanager.\<tm_id>.\<job_name>.\<task_name>.\<subtask_index>
+ - Applied to all metrics that were scoped to a task.
+- `metrics.scope.tm.operator`
+ - Default:
\<host>.taskmanager.\<tm_id>.\<job_name>.\<operator_name>.\<subtask_index>
+ - Applied to all metrics that were scoped to an operator.
+
+Note that for metrics for which multiple formats may apply (like jm and
jm.job) the most specific format takes precedence,
+in this case jm.job.
+
+The hierarchical orders are as follows:
+
+jm < jm.job
+
+tm < tm.job < tm.task < tm.operator
+
+This hierarchy also defines which variables may be accessed. The
`tm.operator` format may contain variables for jobs, whereas `tm.job` may not
contain
+variables for operators. There is no restriction on order of variables.
+
+There is no restriction on the order of variables.
+
+The default scope for operator metrics will result in a metric name akin
to `localhost.taskmanager.1234.MyJob.MyOperator.0`
+
+If you also want to include the task name, but omit the taskmanager
information you can specify the following format:
+
+`metrics.scope.tm.operator:
\<host>.\<job_name>.\<task_name>.\<operator_name>.\<subtask_index>`
--- End diff --
I used Chrome to look at the docs. Maybe it depends on the browser.
Replacing them by `<` and `>` sounds like the right way to go.
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