abhishekagarwal87 commented on a change in pull request #11398:
URL: https://github.com/apache/druid/pull/11398#discussion_r674498267
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File path:
server/src/main/java/org/apache/druid/segment/loading/SegmentCacheManager.java
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@@ -40,6 +40,34 @@
*/
File getSegmentFiles(DataSegment segment) throws SegmentLoadingException;
+ /**
+ * Tries to reserve the space for a segment on any location. When the space
has been reserved,
+ * {@link #getSegmentFiles(DataSegment)} should download the segment on the
reserved location or
+ * fail otherwise.
+ *
+ * This function is useful for custom extensions. Extensions can try to
reserve the space first and
+ * if not successful, make some space by cleaning up other segments, etc.
There is also improved
+ * concurrency for extensions with this function. Since reserve is a cheaper
operation to invoke
+ * till the space has been reserved. Hence it can be put inside a lock if
required by the extensions. getSegment
+ * can't be put inside a lock since it is a time-consuming operation, on
account of downloading the files.
+ *
+ * @param segment - Segment to reserve
+ * @return True if enough space found to store the segment, false otherwise
+ */
+ /*
+ * We only return a boolean result instead of a pointer to
+ * {@link StorageLocation} since we don't want callers to operate on {@code
StorageLocation} directly outside {@code SegmentLoader}.
+ * {@link SegmentLoader} operates on the {@code StorageLocation} objects in
a thread-safe manner.
+ */
+ boolean reserve(DataSegment segment);
+
+ /**
+ * Reverts the effects of {@link #reserve(DataSegment)} (DataSegment)} by
releasing the location reserved for this segment.
+ * @param segment - Segment to release the location for.
+ * @return - True if any location was reserved and released, false otherwise.
+ */
+ boolean release(DataSegment segment);
Review comment:
Sure. if `reserve` is not called, `getSegmentFiles` will `reserve` the
space. I will document this.
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