CNDB-15995 The default memtable configuration for CC4 compatibility should be an empty map, not a map containing "class : default"

src/java/org/apache/cassandra/schema/MemtableParams.java

@@ -73,8 +73,9 @@ public Memtable.Factory factory()
      * Returns a map representation of the memtable configuration for backward compatibility with CC 4.0.
      * This is used when outputting schema in a format compatible with CC 4.0.
      *
-     * For the "default" configuration key, we output {'class': 'default'} to ensure round-trip
-     * consistency.
+     * For the "default" configuration key, we output an empty map {} to let each Cassandra version
+     * interpret "default" according to its own configuration. This ensures backward compatibility
+     * with CC 4.0 which uses an empty map to represent the default memtable configuration.
      *
      * For other configurations, CC 4.0 accepts both short class names (e.g., 'TrieMemtable') and
      * fully qualified names (e.g., 'org.apache.cassandra.db.memtable.TrieMemtable'). For standard
@@ -84,7 +85,7 @@ public Memtable.Factory factory()
     public Map<String, String> toMapForCC4()
     {
         if ("default".equals(configurationKey))
-            return ImmutableMap.of("class", "default");
+            return ImmutableMap.of();
 
         ParameterizedClass definition = CONFIGURATION_DEFINITIONS.get(configurationKey);
         if (definition != null && definition.class_name != null)

test/unit/org/apache/cassandra/db/SchemaCQLHelperTest.java

@@ -773,8 +773,9 @@ public void testSchemaBackwardCompatibilityCc40()
             Assertions.assertThat(cql).doesNotContain("incremental_backups");
             // Should contain other properties
             Assertions.assertThat(cql).contains("bloom_filter_fp_chance");
-            // Should use map format for memtable (CC 4.0 compatible) with short class name
-            Assertions.assertThat(cql).contains("memtable = {'class': 'default'}");
+            // Should use empty map for default memtable (CC 4.0 compatible)
+            // This lets each Cassandra version interpret "default" according to its own configuration
+            Assertions.assertThat(cql).contains("memtable = {}");
             // Should NOT contain fully qualified class name
             Assertions.assertThat(cql).doesNotContain("org.apache.cassandra.db.memtable");
 
@@ -931,12 +932,13 @@ public void testToMapForCC4OutputFormat()
             Assertions.assertThat(cql5).contains("memtable = {'class': 'SkipListMemtable'");
             Assertions.assertThat(cql5).doesNotContain("org.apache.cassandra.db.memtable.SkipListMemtable");
 
-            // Test that tables created with memtable = 'default' output 'default' in CC 4.0 mode
+            // Test that tables created with memtable = 'default' output empty map in CC 4.0 mode
+            // This ensures backward compatibility with CC 4.0 which uses {} for default memtable
             String tableName6 = createTable(keyspace, "CREATE TABLE %s (id int PRIMARY KEY, value text) WITH memtable = 'default'");
             ColumnFamilyStore cfs6 = Keyspace.open(keyspace).getColumnFamilyStore(tableName6);
             String cql6 = SchemaCQLHelper.getTableMetadataAsCQL(cfs6.metadata(), cfs6.keyspace.getMetadata());
-            // Should output 'default' as the class name
-            Assertions.assertThat(cql6).contains("memtable = {'class': 'default'");
+            // Should output empty map to let each version interpret "default" according to its own configuration
+            Assertions.assertThat(cql6).contains("memtable = {}");
 
             // Test that custom memtables from other packages preserve fully qualified class names
             // Note: We can't easily test this without adding a custom memtable configuration to cassandra.yaml,