guozhangwang commented on a change in pull request #10345:
URL: https://github.com/apache/kafka/pull/10345#discussion_r598954254
##########
File path:
clients/src/main/java/org/apache/kafka/clients/consumer/internals/ConsumerCoordinator.java
##########
@@ -198,6 +198,7 @@ public ConsumerCoordinator(GroupRebalanceConfig
rebalanceConfig,
Collections.sort(supportedProtocols);
protocol = supportedProtocols.get(supportedProtocols.size() - 1);
+ log.debug("Using rebalance protocol {}", protocol);
Review comment:
nit: Initializing rebalance protocol to?
##########
File path:
clients/src/main/java/org/apache/kafka/clients/consumer/internals/ConsumerCoordinator.java
##########
@@ -356,6 +357,23 @@ protected void onJoinComplete(int generation,
if (assignor == null)
throw new IllegalStateException("Coordinator selected invalid
assignment protocol: " + assignmentStrategy);
+ log.debug("{} assignor selected by the group coordinator", assignor);
+
+ List<RebalanceProtocol> assignorSupportedProtocols =
assignor.supportedProtocols();
+ Collections.sort(assignorSupportedProtocols);
+ RebalanceProtocol newProtocol =
assignorSupportedProtocols.get(assignorSupportedProtocols.size() - 1);
+ if (newProtocol.id() < protocol.id()) {
+ log.error("Latest commonly supported rebalance protocol is {}
which is lower than the current commonly supported rebalance protocol {}. "
Review comment:
I think a more common error scenario is when a new member joining a
group with ONLY old assignor in the configs, while all the existing members in
the current generation have both old and new assignors. In this case the old
would have to be selected.
So in the error message, we should also clarify this case, e.g. saying
something that "once a new protocol is selected, even though everyone still
supports the old protocol it cannot be downgraded anymore --- even if a new
member with only the old protocol joins the group"
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