java-topology/defects/kafka/patch/kafka-0005-stickassignor-topics-subscription-hashset.md

UNDF: UNDF-2026-000000436

kafka-0004: AbstractStickyAssignor — consumerSubscription.topics().contains() in prepopulateCurrentAssignments

Defect ID

kafka-0004

File:Line

clients/src/main/java/org/apache/kafka/clients/consumer/internals/AbstractStickyAssignor.java:1052

Description

In prepopulateCurrentAssignments(), an outer loop iterates over all consumers (C) and their current assignment. An inner loop iterates over partitions (P per consumer). At line 1052:

} else if (!consumerSubscription.topics().contains(partition.topic()) || ...)

consumerSubscription.topics() returns List<String>, so .contains() is O(T). The loop is O(C × P × T) total. Note: kafka-0001/0002 fixed consumer2AllPotentialTopics and currentAssignment.get(consumer) in a separate code path — this is a distinct occurrence in prepopulateCurrentAssignments().

With C=200 consumers, P=500 partitions/consumer, T=50 topics → 5M O(T) calls = 250M ops.

Complexity

• Slow: O(C × P × T) — List.contains() = O(T)
• Fast: O(C × P) — HashSet.contains() = O(1)

Severity

HIGH

Speedup Estimate

~T× improvement = 50x at T=50 topics.

Fix

Convert consumerSubscription.topics() to a HashSet<String> before the inner partition loop, or use the pre-built consumer2AllPotentialTopics (which kafka-0002 already converts to Set<String>) as the subscription check.