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Public interface to emit the preview data:

Code Block

language	java

public interface PreviewEmitter {
	
	/**
 	 * put the Map of properties corresponding to the given key.
 	 * @param key the key under which properties are stored
	 * @param propertyValues the map of property values to be stored under the given key
 	 */
	 void put(String key, Map<String, List<Object>> propertyValues);
 
	/**
 	 * Emit the property specified by name and value for the given key.
 	 * @param key the key under which properties are stored
     * @param propertyName the the name of the property
	 * @param propertyValue the value associated with the property
 	 */
	 void emit(String key, String propertyName, Object propertyValue);
	
}

2. Preview Context API

Code Block

public interface PreviewContext() {
	/**
	 * boolean flag to indicate if preview is enabled or not.
	 */
	boolean isPreviewEnabled();
 
	/**
	 * get PreviewEmitter, PreviewEmitter can be used to emit objects collected by key and field names.
	 */
    PreviewEmitter getPreviewEmitter();
}

3. How the application will get access to the PreviewEmitter? Similar to Metric and @UseDataSet, instance of the PreviewEmitter will be injected by CDAP in the application.

Code Block

language	java

public static class ETLMapper extends Mapper implements ProgramLifecycle<MapReduceTaskContext<Object, Object>> {

  private TransformRunner<Object, Object> transformRunner;
  // injected by CDAP
  @SuppressWarnings("unused")
  private Metrics mapperMetrics;
  
  private PreviewEmitter previewEmitter;

  @Override
  public void initialize(MapReduceTaskContext<Object, Object> context) throws Exception {
    // get source, transform, sink ids from program properties
    Map<String, String> properties = context.getSpecification().getProperties();
    if (Boolean.valueOf(properties.get(Constants.STAGE_LOGGING_ENABLED))) {
      LogStageInjector.start();
    }
    transformRunner = new TransformRunner<>(context, mapperMetrics);
	if (context.isPreviewEnabled()) {
		previewEmitter = context.getPreviewEmitter();
	}
  }

  @Override
  public void map(Object key, Object value, Mapper.Context context) throws IOException, InterruptedException {
    try {
      transformRunner.transform(key, value, previewEmitter);
    } catch (Exception e) {
      Throwables.propagate(e);
    }
  }
...
}
 
TrackedTransform.java
 
public TrackedTransform(Transformation<IN, OUT> transform, StageMetrics metrics,
                        @Nullable String metricInName, @Nullable String metricOutName, String stageName, @Nullable PreviewEmitter previewEmitter) {
  this.transform = transform;
  this.metrics = metrics;
  this.metricInName = metricInName;
  this.metricOutName = metricOutName;
  this.stageName = stageName;
  this.previewEmitter = previewEmitter;
}


@Override
public void transform(IN input, Emitter<OUT> emitter) throws Exception {
  if (metricInName != null) {
    metrics.count(metricInName, 1);
  }
  if (previewEmitter != null) {
	// emitting input data to preview
	previewEmitter.emit(stageName, "inputData", input); 
  }
  transform.transform(input, metricOutName == null ? emitter : 
						new TrackedEmitter<>(emitter, metrics, metricOutName, stageName, previewEmitter));
}


 
... TrackedEmitter.java
 
public TrackedEmitter(Emitter<T> delegate, StageMetrics stageMetrics, String emitMetricName, String stageName, PreviewEmitter previewEmitter) {
  this.delegate
= delegate;
  this.stageMetrics = stageMetrics;
  this.emitMetricName = emitMetricName;
  this.stageName = stageName;
  this.previewEmitter = previewEmitter;
}

@Override
public void emit(T value) {
  delegate.emit(value);
  stageMetrics.count(emitMetricName, 1);
  //emitting output data for preview
  previewEmitter.emit(stageName, "outputData", value);
}

@Override
public void emitError(InvalidEntry<T> value) {
  delegate.emitError(value);
  stageMetrics.count("records.error", 1);
  // emitting error data for preview
  previewEmitter.emit(stageName, "errorData", value);
}

PreviewContext implementation will use previewId to create a preview emitter which can be obtained using getPreviewEmitter by programs. Programs can use isPreviewEnabled to check if preview is enabled before emitting.

4 How will CDAP get data from the preview?

Code Block

/**
 * Represents the state of the preview.
 */
public class PreviewStatus {
  public enum Status {
	RUNNING,
	COMPLETED,
	DEPLOY_FAILED,
	RUNTIME_FAILED	
  };
 
  Status previewStatus;
  @Nullable	
  String failureMessage;			
}
 
// This is internal interface which will be used by REST handlers
// to retrieve the preview information.
public interface PreviewManager {
 
    /**
	 * Get the state of the preview represented by previewId.
     */
	PreviewStatus getStatus(PreviewId previewId);
 
	/**
	 * Get the data associated with the preview represented by previewId.
	 */
    Map<String, Map<String, List<Object>> getData(PreviewId previewId);
 
	/**
	 * Get all metrics associated with the preview represented by previewId.
	 */
	Collection<MetricTimeSeries> getMetrics(PreviewId previewId);
  
 	/**
	 * Get all logs associated with the preview represented by previewId.
	 */
	List<LogEntry> getLogs(PreviewId previewId);
}
 
class PreviewId extends EntityId implements NamespaceId, ParentId<NamespaceId> {
	NamespaceId namespace;
    String preview;
}

...

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