egor-ryashin commented on a change in pull request #6041: Synchronize scheduled 
poll() calls in SQLMetadataSegmentManager
URL: https://github.com/apache/incubator-druid/pull/6041#discussion_r296711525
 
 

 ##########
 File path: 
server/src/main/java/io/druid/metadata/SQLMetadataSegmentManager.java
 ##########
 @@ -85,20 +85,38 @@
   private static final Interner<DataSegment> DATA_SEGMENT_INTERNER = 
Interners.newWeakInterner();
   private static final EmittingLogger log = new 
EmittingLogger(SQLMetadataSegmentManager.class);
 
-  // Use to synchronize start() and stop(). These methods should be 
synchronized to prevent from being called at the
-  // same time if two different threads are calling them. This might be 
possible if a druid coordinator gets and drops
-  // leadership repeatedly in quick succession.
-  private final Object lock = new Object();
+  /**
+   * Use to synchronize {@link #start()}, {@link #stop()}, {@link #poll()}, 
and {@link #isStarted()}. These methods
+   * should be synchronized to prevent from being called at the same time if 
two different threads are calling them.
+   * This might be possible if a druid coordinator gets and drops leadership 
repeatedly in quick succession.
+   */
+  private final ReentrantReadWriteLock readWriteLock = new 
ReentrantReadWriteLock();
 
 Review comment:
   I wonder if this should be the responsibility of the Lifecycle to 
synchronize start/stop of objects ?

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