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From tillrohrmann <...@git.apache.org>
Subject [GitHub] flink pull request #2158: [FLINK-4116] Metrics documentation
Date Mon, 27 Jun 2016 11:46:21 GMT
Github user tillrohrmann commented on a diff in the pull request:

    --- 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 %}
    +Flink exposes a metric system that allows users gather and expose metrics to external
    +### 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
    +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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