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From tillrohrmann <...@git.apache.org>
Subject [GitHub] flink pull request #2158: [FLINK-4116] Metrics documentation
Date Mon, 27 Jun 2016 11:56:06 GMT
Github user tillrohrmann commented on a diff in the pull request:

    https://github.com/apache/flink/pull/2158#discussion_r68563905
  
    --- 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.
    +
    +### Reporter
    +
    +Metrics can be exposed to an external system by configuring a reporter in the `conf/flink-conf.yaml`.
    +
    +- `metrics.reporter.class`: The class of the reporter to use.
    +  - Example: org.apache.flink.metrics.reporter.JMXReporter)
    +- `metrics.reporter.arguments`: A list of named parameters that are passed to the reporter.
    +  - Example: --host localhost --port 9010)
    +- `metrics.reporter.interval`: The interval between reports.
    +  - Example: 10 SECONDS)
    +
    +By default Flink uses JMX to expose metrics.
    --- End diff --
    
    JMX should also get a sub heading.


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