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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-4116) Document metrics
Date Thu, 30 Jun 2016 16:02:10 GMT

    [ https://issues.apache.org/jira/browse/FLINK-4116?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15357336#comment-15357336
] 

ASF GitHub Bot commented on FLINK-4116:
---------------------------------------

Github user uce commented on a diff in the pull request:

    https://github.com/apache/flink/pull/2158#discussion_r69160791
  
    --- 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.
    --- End diff --
    
    I agree, but note that there are other components who also do it outside of `ConfigConstants`.


> Document metrics
> ----------------
>
>                 Key: FLINK-4116
>                 URL: https://issues.apache.org/jira/browse/FLINK-4116
>             Project: Flink
>          Issue Type: Improvement
>          Components: Documentation, Metrics
>    Affects Versions: 1.1.0
>            Reporter: Chesnay Schepler
>            Assignee: Chesnay Schepler
>             Fix For: 1.1.0
>
>
> The metric system is currently not documented, which should be fixed before the 1.1 release.



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