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More explicit comments around Zookeeper race condition

This commit is contained in:
cheddar 2013-04-29 13:58:57 -05:00
parent cee15bb88a
commit 2630d75cd3
1 changed files with 13 additions and 2 deletions
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15
server/src/main/java/com/metamx/druid/master/LoadQueuePeon.java
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@ -44,7 +44,6 @@ import java.util.List;
import java.util.Set;
import java.util.concurrent.ConcurrentSkipListSet;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.atomic.AtomicLong;
/**
@ -249,7 +248,19 @@ public class LoadQueuePeon
if (stat == null) {
final byte[] noopPayload = jsonMapper.writeValueAsBytes(new SegmentChangeRequestNoop());
// Create a node and then delete it to remove the registered watcher
// Create a node and then delete it to remove the registered watcher. This is a work-around for
// a zookeeper race condition. Specifically, when you set a watcher, it fires on the next event
// that happens for that node. If no events happen, the watcher stays registered foreverz.
// Couple that with the fact that you cannot set a watcher when you create a node, but what we
// want is to create a node and then watch for it to get deleted. The solution is that you *can*
// set a watcher when you check to see if it exists so, we first create the node and then set a
// watcher on its existence. However, if already does not exist by the time the existence check
// returns, then the watcher that was set will never fire (nobody will ever create the node
// again) and thus lead to a slow, but real, memory leak. So, we create another node to cause
// that watcher to fire and delete it right away.
//
// We do not create the existence watcher first, because then it will fire when we create the
// node and we'll have the same race when trying to refresh that watcher.
curator.create().withMode(CreateMode.EPHEMERAL).forPath(path, noopPayload);
entryRemoved(path);