leventov commented on a change in pull request #6041: Synchronize scheduled
poll() calls in SQLMetadataSegmentManager
URL: https://github.com/apache/incubator-druid/pull/6041#discussion_r204922700
##########
File path:
server/src/main/java/io/druid/metadata/SQLMetadataSegmentManager.java
##########
@@ -96,9 +94,18 @@
private final AtomicReference<ConcurrentHashMap<String, DruidDataSource>>
dataSourcesRef;
private final SQLMetadataConnector connector;
- private volatile ListeningScheduledExecutorService exec = null;
- private volatile ListenableFuture<?> future = null;
- private volatile boolean started;
+ /** The number of times this SQLMetadataSegmentManager was started. */
+ private long startCount = 0;
+ /**
+ * Equal to the current {@link #startCount} value, if the
SQLMetadataSegmentManager is currently started; -1 if
+ * currently stopped.
+ *
+ * This field is used to implement a simple stamp mechanism instead of just
a boolean "started" flag to prevent
+ * the theoretical situation of two tasks scheduled in {@link #start()}
calling {@link #poll()} concurrently, if
+ * the sequence of {@link #start()} - {@link #stop()} - {@link #start()}
actions occurs quickly.
+ */
Review comment:
Please cross-reference those things in both classes. Like I changed one
class and didn't know that there is another one that demands a similar change.
You knew that, but this knowledge should be in code for any code developer.
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