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1. Public interface to emit the preview data:

   Code Block

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1. language

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1. java

   public interface PreviewEmitter<V>

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1. { /**

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interface PreviewEmitter {	
	void emit(T record);
}

class DefaultPreview implements PreviewContext, PreviewEmitter {
   // implementation for methods, emit, getEmittedRecords, getLogs and getMetrics.
}
 

@Singleton
class PreviewHelper {
	Map<PreviewStageId, DefaultPreview> previewStageIdToDefaultPreview;
 
	PreviewEmitter getPreviewEmitter(PreviewStageId identifier) {
		// create DefaultPreview for identifier in previewStageIdToDefaultPreview and return if doesn't exist already, else get and return.	
	}
 
	PreviewContext getPreviewContext(PreviewStageId identifier) {
		// create DefaultPreview for identifier in previewStageIdToDefaultPreview and return if doesn't exist already, else get and return.
	}
}

class PreviewStageId {
	NamespaceId namespace;
	String previewId; // this could be runId ?
	String stageId;
}

Implementation:

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1. * Emit 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 emit(String key, Map<String, List<V>> 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, V propertyValue); }

2. 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 class MyMapReduce extends AbstractMapReduce { private PreviewEmitter<String> emitter;   @Override public void initialize() throws Exception { emitter.emit("MyMapReduce.initialize", "logical.start.time", getContext().getLogicalStartTime().toString()); emitter.emit("MyMapReduce.initialize", "actual.start.time", System.currentTimeMillis().toString()); }   public MyMapper extends Mapper<byte[], Text, Text, Text> { @Override public void map(byte[] key, Text value, Context context) throws IOException { if (value.toString().startsWith("Product") { emitter.emit("MapReduce.map", "map.product", value.toString()); } } } }

3. How will CDAP get data from the preview?

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   /** * Represents the state of the preview. */ public class PreviewState { 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 PreviewInfo {   /** * Get the state of the preview represented by previewId. */ PreviewState getStatus(PreviewId previewId);   /** * Get the data associated with the preview represented by previewId. */ Map<String, Map<String, List<V>> 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. */ String getLogs(PreviewId previewId); }   class PreviewId { NamespaceId namespace; String preview; }

SDK:

Preview Execution Isolation:

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