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# OpenTelemetry metrics export framework

The metrics world split between "pushed based" and "pull based" libraries and backends, and because
of this, the OpenTelemetry export framework needs to address all the possible combinations.

To achieve the support for "pushed based" and "pull based" libraries the OpenTelemetry defines two
interfaces that helps with this:
* MetricProducer - is the interface that a "pull based" library should implement in order to make
data available to OpenTelemetry exporters.
* MetricExporter - is an interface that every OpenTelemetry exporter should implement in order to
allow "push based" libraries to push metrics to the backend.

Here are some examples on how different libraries will interact with pull/push backends.

**Push backend:**

```java
import io.opentelemetry.sdk.metrics.export.MetricExporter;
import io.opentelemetry.sdk.metrics.export.MetricProducer;

/**
 * Simple implementation of the MetricExporter that pushes data to the backend.
 */
public final class PushMetricExporter implements MetricExporter {  
  @Override
  ResultCode export(Collection metrics) {
    // A "push based" library calls to export metrics
    return pushToBackend(metrics);
  }
}

/**
 * Class that periodically reads from all MetricProducers and pushes metrics using the
 * PushMetricExporter.
 */
public final class PushExporter {
  private final PushMetricExporter metricExporter;
  // IntervalMetricReader reads metrics from all producers periodically.
  private final IntervalMetricReader intervalMetricReader;

  public PushExporter(Collection producers) {
    metricExporter = new PushMetricExporter();
    intervalMetricReader =
        IntervalMetricReader.builder()
                    .readEnvironment() // Read configuration from environment variables
                    .readSystemProperties() // Read configuration from system properties
                    .setExportIntervalMillis(100_000) 
                    .setMetricExporter(metricExporter)
                    .setMetricProducers(Collections.singletonList(producers))
                    .build();
  }
  
  // Can be accessed by any "push based" library to export metrics.
  public MetricExporter getMetricExporter() {
    return metricExporter;
  }
}
```

**Pull backend:**

```java
import io.opentelemetry.sdk.metrics.data.MetricData;
import io.opentelemetry.sdk.metrics.export.MetricExporter;
import io.opentelemetry.sdk.metrics.export.MetricProducer;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;

/**
 * Simple implementation of the MetricExporter that stores data in memory and makes them available
 * via MetricProducer interface.
 */
public final class PullMetricExporter implements MetricExporter, MetricProducer {
  private final List metricsBuffer = new ArrayList<>();

  @Override
  synchronized ResultCode export(Collection metrics) {
    metricsBuffer.addAll(metrics);
    return ResultCode.SUCCESS;
  }
  
  synchronized Collection getAllMetrics() {
    List ret = metricsBuffer;
    metricsBuffer = new ArrayList<>();
    return ret;
  }
}

public final class PullExporter {
  private final PullMetricExporter metricExporter;
  private final Collection producers;

  public PushExporter(Collection producers) {
    metricExporter = new PullMetricExporter();
    producers = Collections.unmodifiableCollection(new ArrayList(producers));
  }

  // Can be accessed by any "push based" library to export metrics.
  public MetricExporter getMetricExporter() {
    return metricExporter;
  }

  private void onPullRequest() {
    // Iterate over all producers and the PullMetricExporter and export all metrics.
    for (MetricProducer metricProducer : producers) {
      Collection metrics = metricProducer.getAllMetrics();
      // Do something with metrics
    }
  }
}
```