Package org.apache.hadoop.metrics2

Metrics 2.0

See:
Description

Interface Summary

MetricsBuilder	The metrics builder interface
MetricsPlugin	A fairly generic plugin interface
MetricsRecord	An immutable snapshot of metrics with a timestamp
MetricsSink	The metrics sink interface
MetricsSource	The metrics source interface
MetricsSystem	The metrics system interface
MetricsSystem.Callback	The metrics system callback interface
MetricsSystemMXBean	The JMX interface to the metrics system
MetricsVisitor	A visitor interface for metrics

Class Summary

Metric	The immutable metric
MetricCounter<T extends Number>	A generic immutable counter metric type
MetricGauge<T extends Number>	A generic immutable gauge metric
MetricsFilter	The metrics filter interface
MetricsRecordBuilder	The metrics record builder interface
MetricsSystem.AbstractCallback	Convenient abstract class for implementing callback interface
MetricsTag	Immutable tag for metrics (for grouping on host/queue/username etc.)

Exception Summary

MetricsException

A general metrics exception wrapper

Package org.apache.hadoop.metrics2 Description

Metrics 2.0

• Overview
• Getting Started
• Metrics Filtering
• Metrics Instrumentation Strategy
• Migration from previous system

Overview

This package provides a framework for metrics instrumentation and publication.

The instrumentation of metrics just need to implement the simple MetricsSource interface with a single getMetrics method; The consumers of metrics just need to implement the simple MetricsSink interface with a putMetrics method along with the init and flush methods. Producers register the metrics sources with a metrics system, while consumers register the sinks. A default metrics system is provided to marshal metrics from sources to sinks based on (per source/sink) configuration options. Metrics from getMetrics would also be published and queryable via the standard JMX mechanism. This document targets the framework users. Framework developers could consult the design document for architecture and implementation notes.

Sub-packages

org.apache.hadoop.metrics2.impl

Implementation classes of the framework for interface and/or abstract classes defined in the top-level package. Sink plugin code usually does not need to reference any class here.

org.apache.hadoop.metrics2.lib

Convenience classes for implementing metrics sources, including the MetricMutable[Gauge*|Counter*|Stat] and MetricsRegistry.

org.apache.hadoop.metrics2.filter

Builtin metrics filter implementations include the GlobFilter and RegexFilter.

org.apache.hadoop.metrics2.source

Builtin metrics source implementations including the JvmMetricsSource.

org.apache.hadoop.metrics2.sink

Builtin metrics sink implementations including the FileSink.

Getting started

Here is a simple MetricsSource:

    class MyMetrics implements MetricsSource {
      public void getMetrics(MetricsBuilder builder, boolean all) {
        builder.addRecord("myRecord").setContext("myContext")
               .addGauge("myMetric", "My metrics description", 42);
      }
    }

In this example there are three names:

myContext

The optional context name typically identifies either the application, or a group of modules within an application or library.

myRecord

The record name generally identifies some entity for which a set of metrics are to be reported. For example, you could have a record named "cacheStat" for reporting a number of statistics relating to the usage of some cache in your application.

myMetric

This identifies a particular metric. In this case the metric is a "gauge" type, which means it can vary in both directions, compared with a "counter" type, which can only increase or stay the same. The 42 here is the value of the metric, which can be substituted a variable or a non-blocking method call returning a number.

Note, the boolean argument all, if true, means that the source should send all the metrics it defines, even if the metrics are unchanged since last getMetrics call. This enable an optimization for less copying for metrics that don't change much (total capacity of something etc. which only changes when new resources (nodes or disks) are being added.)

Here is a simple MetricsSink:

    public class MySink implements MetricsSink {
      public void putMetrics(MetricsRecord record) {
        System.out.print(record);
      }
      public void init(SubsetConfiguration conf) {}
      public void flush() {}
    }

In this example there are three additional concepts:

record

This object corresponds to the record created in metrics sources e.g., the "myRecord" in previous example.

conf

The configuration object for the sink instance with prefix removed. So you can get any sink specific configuration using the usual get* method.

flush

This method is called for each update cycle, which may involve more than one record. The sink should try to flush any buffered metrics to its backend upon the call. But it's not required that the implementation is synchronous.

In order to make use our MyMetrics and MySink, they need to be hooked up to a metrics system. In this case (and most cases), the DefaultMetricsSystem would suffice.

    DefaultMetricsSystem.initialize("test"); // called once per application
    DefaultMetricsSystem.INSTANCE.register("MyMetrics", "my metrics description",
                                           new MyMetrics());

Sinks are usually specified in a configuration file, say, "hadoop-metrics2-test.properties", as:

    test.sink.mysink0.class=com.example.hadoop.metrics.MySink

The configuration syntax is:

    [prefix].[source|sink|jmx|].[instance].[option]

In the previous example, test is the prefix and mysink0 is an instance name. DefaultMetricsSystem would try to load hadoop-metrics2-[prefix].properties first, and if not found, try the default hadoop-metrics2.properties in the class path. Note, the [instance] is an arbitrary name to uniquely identify a particular sink instance. The asterisk (*) can be used to specify default options.

Consult the metrics instrumentation in JvmMetricsSource, RpcInstrumentation, etc. for more examples.

Metrics Filtering

One of the features of the default metrics system is metrics filtering configuration by source, context, record/tags and metrics. The least expensive way to filter out metrics would be at the source level, e.g., filtering out source named "MyMetrics". The most expensive way would be per metric filtering.

Here are some examples:

    test.sink.file0.class=org.apache.hadoop.metrics2.sink.FileSink
    test.sink.file0.context=foo

In this example, we configured one sink instance that would accept metrics from context foo only.

    *.source.filter.class=org.apache.hadoop.metrics2.filter.GlobFilter
    test.*.source.filter.include=foo
    test.*.source.filter.exclude=bar

In this example, we specify a source filter that includes source foo and excludes bar. When only include patterns are specified, the filter operates in the white listing mode, where only matched sources are included. Likewise, when only exclude patterns are specified, only matched sources are excluded. Sources that are not matched in either patterns are included as well when both patterns are present. Note, the include patterns have precedence over the exclude patterns.

Similarly, you can specify the record.filter and metrics.filter options, which operate at record and metric level, respectively. Filters can be combined to optimize the filtering efficiency.

Metrics instrumentation strategy

In previous examples, we showed a minimal example to use the metrics framework. In a larger system (like Hadoop) that allows custom metrics instrumentation, we recommend the following strategy:

    class MyMetrics extends MyInstrumentation implements MetricsSource {

      final MetricsRegistry registry = new MetricsRegistry("myRecord");
      final MetricMutableGaugeInt gauge0 =
          registry.newGauge("myGauge", "my gauge description", 0);
      final MetricMutableCounterLong counter0 =
          registry.newCounter("myCounter", "my metric description", 0L);
      final MetricMutaleStat stat0 =
          registry.newStat("myStat", "my stat description", "ops", "time");

      @Override public void setGauge0(int value) { gauge0.set(value); }
      @Override public void incrCounter0() { counter0.incr(); }
      @Override public void addStat0(long elapsed) { stat0.add(elapsed); }

      public void getMetrics(MetricsBuilder builder, boolean all) {
        registry.snapshot(builder.addRecord(registry.name()), all);
      }

    }
    

Note, in this example we introduced the following:

MyInstrumentation

This is usually an abstract class (or interface) to define an instrumentation interface (incrCounter0 etc.) that allows different implementations. This is a mechanism to allow different metrics systems to be used at runtime via configuration.

MetricsRegistry

This is a library class for easy creation and manipulation of mutable metrics.

MetricMutable[Gauge*|Counter*|Stat]

These are library classes to manage mutable metrics for implementations of metrics sources. They produce immutable gauge and counters (Metric[Gauge*|Counter*]) for downstream consumption (sinks) upon snapshot. The MetricMutableStat in particular, provides a way to measure latency and throughput of an operation. In this particular case, it produces a long counter "myStat_num_ops" and double gauge "myStat_avg_time" when snapshotted.

Migration from previous system

Users of the previous metrics system would notice the lack of context prefix in the configuration examples. The new metrics system decouples the concept for context (for grouping) with the implementation where a particular context object does the updating and publishing of metrics, which causes problems when you want to have a single context to be consumed by multiple backends. You would also have to configure an implementation instance per context, even if you have a backend that can handle multiple contexts (file, gangalia etc.):

Before	After
context1.class=org.hadoop.metrics.file.FileContext context2.class=org.hadoop.metrics.file.FileContext ... contextn.class=org.hadoop.metrics.file.FileContext	myprefix.sink.file.class=org.hadoop.metrics2.sink.FileSink

In the new metrics system, you can simulate the previous behavior by using the context option in the sink options like the following:

Before	After
context0.class=org.hadoop.metrics.file.FileContext context0.fileName=context0.out context1.class=org.hadoop.metrics.file.FileContext context1.fileName=context1.out ... contextn.class=org.hadoop.metrics.file.FileContext contextn.fileName=contextn.out	myprefix.sink.*.class=org.apache.hadoop.metrics2.sink.FileSink myprefix.sink.file0.context=context0 myprefix.sink.file0.filename=context1.out myprefix.sink.file1.context=context1 myprefix.sink.file1.filename=context1.out ... myprefix.sink.filen.context=contextn myprefix.sink.filen.filename=contextn.out

to send metrics of a particular context to a particular backend. Note, myprefix is an arbitrary prefix for configuration groupings, typically they are the name of a particular process (namenode, jobtracker, etc.)