From 27d51518bbc34d6fc78344a669b8c7cb2ea2185f Mon Sep 17 00:00:00 2001
From: blambov <branimir.lambov@ibm.com>
Date: Mon, 18 May 2026 12:31:01 +0300
Subject: [PATCH] Expose UnifiedCompactionStrategy.Level stats

---
 .../compaction/UnifiedCompactionStrategy.java | 32 +++++++++++++++++--
 .../BaseCompactionStrategyTest.java           |  1 +
 .../UnifiedCompactionStrategyTest.java        | 13 ++++++++
 3 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/src/java/org/apache/cassandra/db/compaction/UnifiedCompactionStrategy.java b/src/java/org/apache/cassandra/db/compaction/UnifiedCompactionStrategy.java
index 5b9a67ce4112..13ff07dc6e56 100644
--- a/src/java/org/apache/cassandra/db/compaction/UnifiedCompactionStrategy.java
+++ b/src/java/org/apache/cassandra/db/compaction/UnifiedCompactionStrategy.java
@@ -1512,6 +1512,34 @@ public int getIndex()
             return index;
         }
 
+        // The stats below are set up by getLevels and useful for diagnostics.
+        public int getFanout()
+        {
+            return fanout;
+        }
+
+        public int getThreshold()
+        {
+            return threshold;
+        }
+
+        public double getMinDensity()
+        {
+            return min;
+        }
+
+        public double getMaxDensity()
+        {
+            return max;
+        }
+
+        public double getAverageSSTableSize()
+        {
+            return avg;
+        }
+
+        // We don't expose max overlap as it's not valid until getCompactionAggregates gets called.
+
         void add(CompactionSSTable sstable)
         {
             this.sstables.add(sstable);
@@ -1528,8 +1556,8 @@ void complete()
         }
 
         private List<CompactionAggregate.UnifiedAggregate> getOversizeShardsAggregates(Arena arena,
-                                                                               Controller controller,
-                                                                               ShardManager shardManager)
+                                                                                       Controller controller,
+                                                                                       ShardManager shardManager)
         {
             List<CompactionAggregate.UnifiedAggregate> aggregates = new ArrayList<>();
             double shardThreshold = fanout * controller.getMaxSstablesPerShardFactor();
diff --git a/test/unit/org/apache/cassandra/db/compaction/BaseCompactionStrategyTest.java b/test/unit/org/apache/cassandra/db/compaction/BaseCompactionStrategyTest.java
index 8272e3455008..218137e77ca2 100644
--- a/test/unit/org/apache/cassandra/db/compaction/BaseCompactionStrategyTest.java
+++ b/test/unit/org/apache/cassandra/db/compaction/BaseCompactionStrategyTest.java
@@ -237,6 +237,7 @@ SSTableReader mockSSTable(int level,
         when(ret.isSuitableForCompaction()).thenReturn(true);
         when(ret.getSSTableLevel()).thenReturn(level);
         when(ret.onDiskLength()).thenReturn(bytesOnDisk);
+        when(ret.onDiskComponentsSize()).thenReturn(bytesOnDisk);
         when(ret.uncompressedLength()).thenReturn(bytesOnDisk); // let's assume no compression
         when(ret.hotness()).thenReturn(hotness);
         when(ret.getMaxTimestamp()).thenReturn(timestamp);
diff --git a/test/unit/org/apache/cassandra/db/compaction/UnifiedCompactionStrategyTest.java b/test/unit/org/apache/cassandra/db/compaction/UnifiedCompactionStrategyTest.java
index bae0828f0545..d85c2403162a 100644
--- a/test/unit/org/apache/cassandra/db/compaction/UnifiedCompactionStrategyTest.java
+++ b/test/unit/org/apache/cassandra/db/compaction/UnifiedCompactionStrategyTest.java
@@ -375,11 +375,24 @@ private void testGetMultipleBucketsOneArenaNonOverlappingAggregates(Map<Integer,
         {
             List<UnifiedCompactionStrategy.Level> levels = entry.getValue();
             assertEquals(expectedLevels, levels.size());
+            double expectedMin = 0.0;
 
             for (int i = 0; i < expectedLevels; i++)
             {
                 UnifiedCompactionStrategy.Level level = levels.get(i);
                 assertEquals(i, level.getIndex());
+                assertEquals(expectedMin, level.getMinDensity(), 1.0);
+                expectedMin = level.getMaxDensity();
+                assertEquals(level.scalingParameter >= 0 ? level.scalingParameter + 2 : 2, level.getThreshold());
+                assertEquals(2 + Math.abs(level.scalingParameter), level.getFanout());
+                if (level.getSSTables().size() > 0)
+                    assertEquals(level.getSSTables()
+                                      .stream()
+                                      .mapToLong(CompactionSSTable::onDiskLength)
+                                      .summaryStatistics()
+                                      .getAverage(),
+                                 level.getAverageSSTableSize(),
+                                 1.0);
 
                 Collection<CompactionAggregate.UnifiedAggregate> compactionAggregates =
                     level.getCompactionAggregates(entry.getKey(), controller, shardManager, dataSetSizeBytes);