add comment and remove redundant code

Signed-off-by: Kaituo Li <[email protected]>

src/main/java/org/opensearch/ad/ml/ADColdStart.java

@@ -13,7 +13,6 @@
 
 import java.time.Clock;
 import java.time.Duration;
-import java.util.ArrayList;
 import java.util.List;
 
 import org.apache.logging.log4j.LogManager;
@@ -222,13 +221,11 @@ protected List<Sample> trainModelFromDataSegments(
         // use build instead of new TRCF(Builder) because build method did extra validation and initialization
         ThresholdedRandomCutForest trcf = rcfBuilder.build();
 
-        List<Sample> imputed = new ArrayList<>();
         for (int i = 0; i < pointSamples.size(); i++) {
             Sample dataSample = pointSamples.get(i);
             double[] dataValue = dataSample.getValueList();
             // We don't keep missing values during cold start as the actual data may not be reconstructed during the early stage.
             trcf.process(dataValue, dataSample.getDataEndTime().getEpochSecond());
-            imputed.add(new Sample(dataValue, dataSample.getDataStartTime(), dataSample.getDataEndTime()));
         }
 
         entityState.setModel(trcf);

src/main/java/org/opensearch/ad/transport/ADHCImputeTransportAction.java

@@ -35,6 +35,33 @@
 
 import com.amazon.randomcutforest.parkservices.ThresholdedRandomCutForest;
 
+/**
+ * This class manages the broadcasting mechanism and entity data processing for
+ * the HC detector. The system broadcasts a message after processing all records
+ * in each interval to ensure that each node examines its hot models in memory
+ * and determines which entity models have not received data during the current interval.
+ *
+ * "Hot" entities refer to those models actively loaded in memory, as opposed to
+ * "cold" models, which are not loaded and remain in storage due to limited memory resources.
+ *
+ * Upon receiving the broadcast message, each node checks whether each hot entity
+ * has received new data. If a hot entity has not received any data, the system
+ * assigns a NaN value to that entity. This NaN value signals to the model that no
+ * data was received, prompting it to impute the missing value based on previous data,
+ * rather than using current interval data.
+ *
+ * The system determines which node manages which entities based on memory availability.
+ * The coordinating node does not immediately know which entities are hot or cold;
+ * it learns this during the pagination process. Hot entities are those that have
+ * recently received data and are actively maintained in memory, while cold entities
+ * remain in storage and are processed only if time permits within the interval.
+ *
+ * For cold entities whose models are not loaded in memory, the system does not
+ * produce an anomaly result for that interval due to insufficient time or resources
+ * to process them. This is particularly relevant in scenarios with short intervals,
+ * such as one minute, where an underscaled cluster may cause processing delays
+ * that prevent timely anomaly detection for some entities.
+ */
 public class ADHCImputeTransportAction extends
     TransportNodesAction<ADHCImputeRequest, ADHCImputeNodesResponse, ADHCImputeNodeRequest, ADHCImputeNodeResponse> {
     private static final Logger LOG = LogManager.getLogger(ADHCImputeTransportAction.class);

src/main/java/org/opensearch/timeseries/util/ActionListenerExecutor.java

@@ -20,7 +20,8 @@ private ActionListenerExecutor() {}
 
     /**
      * Wraps the provided response and failure handlers in an ActionListener that executes the
-     * response handler asynchronously using the provided ExecutorService.
+     * response handler asynchronously using the provided ExecutorService. This would allow
+     * us to execute listener code using AD thread pool for example.
      *
      * @param <Response> the type of the response
      * @param onResponse a CheckedConsumer that handles the response; it can throw an exception