Github user eribeiro commented on a diff in the pull request:
https://github.com/apache/zookeeper/pull/632#discussion_r224208545
--- Diff:
zookeeper-server/src/main/java/org/apache/zookeeper/server/DataTree.java ---
@@ -154,6 +160,26 @@
private final ReferenceCountedACLCache aclCache = new
ReferenceCountedACLCache();
+ // The maximum number of tree digests that we will keep in our history
+ public static final int DIGEST_LOG_LIMIT = 1024;
+
+ // Dump digest every 128 txns, in hex it's 80, which will make it
easier
+ // to align and compare between servers.
+ public static final int DIGEST_LOG_INTERVAL = 128;
+
+ // If this is not null, we are actively looking for a target zxid that
we
+ // want to validate the digest for
+ private ZxidDigest digestFromLoadedSnapshot;
+
+ // The digest associated with the highest zxid in the data tree.
+ private volatile ZxidDigest lastProcessedZxidDigest;
+
+ // Will be notified when digest mismatch event triggered.
+ private List<DigestWatcher> digestWatchers = new ArrayList<>();
--- End diff --
tip: when you are dealing with Observer pattern in Java where the list of
observers can potentially change concurrently with the traversal of the list
for notification this can lead to `ConcurrentModificationException`. It's
better to use `CopyOnWriteArrayList` or `CopyOnWriteArraySet`, given some
important constraints specified in the javadocs, because they guarantee that a
traversal over the collection will never throw
`ConcurrentModificationException`.
Besides that, it would be nice to use a `Set` instead of a `List` because
then there's no risk of bloating the collection by adding duplicate observers.
So, my suggestion would be to put:
`private Set<DigestWatcher> digestWatchers = new CopyOnWriteArraySet<>();`
---
