[jira] [Created] (KAFKA-7026) Sticky assignor could assign a partition to multiple consumers

["Vahid Hashemian (JIRA)" <ji...@apache.org>]

Vahid Hashemian created KAFKA-7026:
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             Summary: Sticky assignor could assign a partition to multiple consumers
                 Key: KAFKA-7026
                 URL: https://issues.apache.org/jira/browse/KAFKA-7026
             Project: Kafka
          Issue Type: Bug
          Components: clients
            Reporter: Vahid Hashemian
            Assignee: Vahid Hashemian


In the following scenario sticky assignor assigns a topic partition to two consumers in the group:
 # Create a topic {{test}} with a single partition
 # Start consumer {{c1}} in group {{sticky-group}} ({{c1}} becomes group leader and gets {{test-0}})
 # Start consumer {{c2}}  in group {{sticky-group}} ({{c1}} holds onto {{test-0}}, {{c2}} does not get any partition) 
 # Pause {{c1}} (e.g. using Java debugger) ({{c2}} becomes leader and takes over {{test-0}}, {{c1}} leaves the group)
 # Resume {{c1}}

At this point both {{c1}} and {{c2}} will have {{test-0}} assigned to them.

 

The reason is {{c1}} still has kept its previous assignment ({{test-0}}) from the last assignment it received from the leader (itself) and did not get the next round of assignments (when {{c2}} became leader) because it was paused. Both {{c1}} and {{c2}} enter the rebalance supplying {{test-0}} as their existing assignment. The sticky assignor code does not currently check for this duplication.

 


Note: This issue was originally reported on [StackOverflow|https://stackoverflow.com/questions/50761842/kafka-stickyassignor-breaking-delivery-to-single-consumer-in-the-group].



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