[GitHub] [druid] jihoonson commented on a change in pull request #11828: Refactor ResponseContext

[GitBox <gi...@apache.org>]

jihoonson commented on a change in pull request #11828:
URL: https://github.com/apache/druid/pull/11828#discussion_r759799673



##########
File path: processing/src/main/java/org/apache/druid/query/context/ResponseContext.java
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@@ -30,93 +33,334 @@
 import org.apache.druid.java.util.common.IAE;
 import org.apache.druid.java.util.common.ISE;
 import org.apache.druid.java.util.common.NonnullPair;
-import org.apache.druid.java.util.common.jackson.JacksonUtils;
 import org.apache.druid.query.SegmentDescriptor;
+import org.apache.druid.utils.CollectionUtils;
 import org.joda.time.Interval;
 
 import javax.annotation.Nullable;
+
 import java.io.IOException;
 import java.util.ArrayList;
-import java.util.Collection;
-import java.util.Collections;
 import java.util.Comparator;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.ConcurrentSkipListMap;
-import java.util.function.BiFunction;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.Function;
+import java.util.stream.Collectors;
 
 /**
  * The context for storing and passing data between chains of {@link org.apache.druid.query.QueryRunner}s.
  * The context is also transferred between Druid nodes with all the data it contains.
- */
+ * <p>
+ * The response context consists of a set of key/value pairs. Keys are those defined in
+ * the {@code Keys} registry. Keys are indexed by key instance, not by name. The
+ * key defines the type of the associated value, including logic to merge values and
+ * to deserialize JSON values for that key.
+ *
+ * <h4>Structure</h4>
+ * The context has evolved to perform multiple tasks. First, it holds two kinds
+ * of information:
+ * <ul>
+ * <li>Information to be returned in the query response header.
+ * (These are values tagged as {@code HEADER}.)</li>
+ * <li>Values passed within a single server. These are tagged with
+ * visibility {@code NONE}.)</li>
+ * </ul>
+ * Second, it performs multiple tasks:
+ * <ul>
+ * <li>Registers the keys to be used in the header. But, since it also holds
+ * internal information, the internal information also needs keys, though the
+ * corresponding values are never serialized.</li>
+ * <li>Gathers information for the query as a whole.</li>
+ * <li>Merges information back up the query tree: from multiple segments,
+ * from multiple servers, etc.</li>
+ * <li>Manages headers size by dropping fields when the header would get too
+ * large.</li>
+ * </ul>
+ *
+ * A result is that the information the context, when inspected by a calling
+ * query, may be incomplete if some of it was previously dropped by the
+ * called query.
+ *
+ * <h4>API</h4>
+ *
+ * The query profile needs to obtain the full, untruncated information. To do this
+ * it piggy-backs on the set operations to obtain the full value. To ensure this
+ * is possible, code that works with standard values should call the set (or add)
+ * functions provided which will do the needed map update.
+  */
 @PublicApi
 public abstract class ResponseContext
 {
   /**
    * The base interface of a response context key.
    * Should be implemented by every context key.
    */
-  public interface BaseKey
+  public interface Key

Review comment:
       It sounds good to me :+1: 




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