[jira] [Created] (KAFKA-8332) Regression in handling of JoinGroupRequest disallows deterministic protocol selection based on order of preference

["Konstantine Karantasis (JIRA)" <ji...@apache.org>]

Konstantine Karantasis created KAFKA-8332:
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             Summary: Regression in handling of JoinGroupRequest disallows deterministic protocol selection based on order of preference
                 Key: KAFKA-8332
                 URL: https://issues.apache.org/jira/browse/KAFKA-8332
             Project: Kafka
          Issue Type: Bug
          Components: core
            Reporter: Konstantine Karantasis
            Assignee: Konstantine Karantasis


When a group of Kafka clients includes more than one embedded protocol in its {{JoinGroupRequest}} along with its metadata, the group membership protocol defines that the protocol which is supported by all the members of a group is selected, and if more than one protocols are supported by all the members the protocol is selected based on the order of preference as defined in the {{JoinGroupRequest}}. 

A recent change from type {{List}} to type {{Set}} for storing the set of supported embedded protocols in the {{JoinGroupRequest}} combined with the old type of handling with implicit types in the scala code, has introduced non-determinism in the selection of the embedded protocol by the {{GroupCoordinator}}, even though the underlying type of the Set in use is a variant of LinkedHashSet (it respects order). 

The relevant code is: 
{code:java}
// KafkaApis.scala
val protocols = joinGroupRequest.data().protocols().asScala.map(protocol =>
	(protocol.name, protocol.metadata)).toList
{code}
 



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