[GitHub] [druid] abhishekagarwal87 commented on a change in pull request #11398: Make SegmentLoader extensible and customizable

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

abhishekagarwal87 commented on a change in pull request #11398:
URL: https://github.com/apache/druid/pull/11398#discussion_r668635537



##########
File path: server/src/main/java/org/apache/druid/segment/loading/SegmentLoader.java
##########
@@ -29,10 +30,45 @@
  * Loading segments from deep storage to local storage.
  * Implementations must be thread-safe.
  */
+@UnstableApi
 public interface SegmentLoader
 {
   boolean isSegmentLoaded(DataSegment segment);
-  Segment getSegment(DataSegment segment, boolean lazy, SegmentLazyLoadFailCallback loadFailed) throws SegmentLoadingException;
+
+  /**
+   * Returns a {@link ReferenceCountingSegment} that will be added by the {@link org.apache.druid.server.SegmentManager}
+   * to the {@link org.apache.druid.timeline.VersionedIntervalTimeline}. This method can be called multiple times
+   * by the {@link org.apache.druid.server.SegmentManager}
+   *
+   * Returning a {@code ReferenceCountingSegment} will let custom implementations keep track of reference count for
+   * segments that the custom implementations are creating. That way, custom implementations can know when the segment
+   * is in use or not.
+   * @param segment - {@link DataSegment} segment to download
+   * @param lazy - whether the loading is lazy
+   * @param loadFailed - Callback to invoke if the loading fails
+   * @return Segment object wrapped inside {@link ReferenceCountingSegment}.
+   * @throws SegmentLoadingException
+   */
+  ReferenceCountingSegment getSegment(DataSegment segment, boolean lazy, SegmentLazyLoadFailCallback loadFailed) throws SegmentLoadingException;

Review comment:
       It can do either and the caller is not supposed to depend on that behavior. From the caller's perspective, it is going to get a segment object wrapped inside `ReferenceCountingSegment`. Implementations can have optimizations to save on repeated expensive work. 




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