pkuwm commented on a change in pull request #1109:
URL: https://github.com/apache/helix/pull/1109#discussion_r443974192
##########
File path:
zookeeper-api/src/main/java/org/apache/helix/zookeeper/zkclient/ZkClient.java
##########
@@ -984,11 +997,51 @@ private void fireAllEvents() {
protected List<String> getChildren(final String path, final boolean watch) {
long startT = System.currentTimeMillis();
+
try {
List<String> children = retryUntilConnected(new Callable<List<String>>()
{
+ private int connectionLossRetryCount = 0;
+
@Override
public List<String> call() throws Exception {
- return getConnection().getChildren(path, watch);
+ try {
+ return getConnection().getChildren(path, watch);
+ } catch (ConnectionLossException e) {
+ ++connectionLossRetryCount;
+ // Allow retrying 3 times before checking stat checking number of
children,
+ // because there is a higher possibility that connection loss is
caused by other
+ // factors such as network connectivity, connected ZK node could
not serve
+ // the request, session expired, etc.
+ if (connectionLossRetryCount >= 3) {
+ // Issue: https://github.com/apache/helix/issues/962
+ // Connection loss might be caused by an excessive number of
children.
+ // Infinitely retrying connecting may cause high GC in ZK server
and kill ZK server.
+ // This is a workaround to check numChildren to have a chance to
exit retry loop.
+ // TODO: remove this check once we have a better way to exit
infinite retry
+ Stat stat = getStat(path);
+ if (stat != null) {
+ if (stat.getNumChildren() > NUM_CHILDREN_LIMIT) {
+ LOG.error("Failed to get children for path {} because number
of children {} "
+ + "exceeds limit {}, aborting retry.", path,
stat.getNumChildren(),
+ NUM_CHILDREN_LIMIT);
+ // There is not an accurate KeeperException for the purpose.
+ // MarshallingErrorException could represent transport error,
+ // so use it to exit retry loop and tell that zk is not able
to
+ // transport the data because packet length is too large.
+ throw new KeeperException.MarshallingErrorException();
Review comment:
OK. Let me just remove that line of comment to avoid confusion :)
Considering backward compatibility, if a `KeeperException` is thrown for the
operation, it will be converted to `ZkException`. If user catches `ZkException`
and handles it, there is extra impact, their app still runs.
If we throw a ZkException (we don't throw HelixException since we've
separate zk module from helix), it will be converted to a RuntimeException,
which may cause ZkClient users terminated if they don't catch RuntimeException.
Or an option is: we change `retryUntilConnected` to also catch
`ZkException`. With this change, we could throw `ZkException` with a message
indicating "Large children error" and so don't have to log and throw exception.
I am not sure if we would like to change `retryUntilConnected` considering this
is a workaround and later we remove the logic. What do you think about this
option? @jiajunwang
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