apurtell commented on a change in pull request #1945:
URL: https://github.com/apache/hbase/pull/1945#discussion_r445734505
##########
File path:
hbase-zookeeper/src/main/java/org/apache/hadoop/hbase/zookeeper/ZKWatcher.java
##########
@@ -79,8 +86,22 @@
// listeners to be notified
private final List<ZKListener> listeners = new CopyOnWriteArrayList<>();
+ // Single threaded executor pool that processes event notifications from
Zookeeper. Events are
+ // processed in the order in which they arrive (pool backed by an unbounded
fifo queue). We do
+ // this to decouple the event processing from Zookeeper's ClientCnxn's
EventThread context.
+ // EventThread internally runs a single while loop to serially process all
the events. When events
+ // are processed by the listeners in the same thread, that blocks the
EventThread from processing
+ // subsequent events. Processing events in a separate thread frees up the
event thread to continue
+ // and further prevents deadlocks if the process method itself makes other
zookeeper calls.
+ // It is ok to do it in a single thread because the Zookeeper ClientCnxn
already serializes the
+ // requests using a single while loop and hence there is no performance
degradation.
+ private final ExecutorService zkEventProcessor =
+
Executors.newSingleThreadExecutor(Threads.getNamedThreadFactory("zk-event-processor"));
Review comment:
We should definitely avoid dependencies on Guava. Hadoop, HBase, and
Phoenix do not stay in sync (Hadoop 3 is up on 21 and that will be painful for
the rest!) and Google regularly breaks compatibility among Guava major
versions.
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