java-topology/defects/kafka/unit/KafkaStickyAssignorTopicsTest.java

package unit;

import java.util.*;

/**
 * kafka-0004: AbstractStickyAssignor prepopulateCurrentAssignments —
 * consumerSubscription.topics().contains(partition.topic()) where topics() returns List<String>.
 *
 * Inside a double for loop (consumers x partitions), this gives O(C * P * T).
 * Fix: convert subscription topics to HashSet<String> before the inner loop.
 */
public class KafkaStickyAssignorTopicsTest {

    // Simulate the slow path: subscription.topics() returns List<String>
    // Returns total number of element comparisons performed
    static long slowPrepopulate(
            Map<String, List<String>> consumerToTopics,   // consumer -> topic subscription list
            Map<String, List<String>> consumerToPartitions // consumer -> assigned partitions (topic names)
    ) {
        long ops = 0;
        for (Map.Entry<String, List<String>> entry : consumerToPartitions.entrySet()) {
            String consumer = entry.getKey();
            List<String> subTopics = consumerToTopics.get(consumer); // returns List<String>
            for (String partitionTopic : entry.getValue()) {
                // List.contains: O(T) scan
                for (String t : subTopics) {
                    ops++;
                    if (t.equals(partitionTopic)) break;
                }
            }
        }
        return ops;
    }

    // Simulate the fast path: subscription topics converted to HashSet<String>
    static long fastPrepopulate(
            Map<String, Set<String>> consumerToTopicsSet,
            Map<String, List<String>> consumerToPartitions
    ) {
        long ops = 0;
        for (Map.Entry<String, List<String>> entry : consumerToPartitions.entrySet()) {
            String consumer = entry.getKey();
            Set<String> subTopics = consumerToTopicsSet.get(consumer);
            for (String partitionTopic : entry.getValue()) {
                ops++; // O(1) HashSet lookup counts as 1 op
                subTopics.contains(partitionTopic);
            }
        }
        return ops;
    }

    public static void main(String[] args) {
        int passed = 0;
        int total = 0;

        // Test 1: basic op-count comparison
        {
            total++;
            int C = 20;  // consumers
            int P = 100; // partitions per consumer
            int T = 30;  // topics per consumer (worst-case: partition topic is last in list)

            Map<String, List<String>> consumerToTopics = new HashMap<>();
            Map<String, Set<String>> consumerToTopicsSet = new HashMap<>();
            Map<String, List<String>> consumerToPartitions = new HashMap<>();

            for (int c = 0; c < C; c++) {
                String consumerId = "consumer-" + c;
                List<String> topics = new ArrayList<>();
                Set<String> topicsSet = new HashSet<>();
                for (int t = 0; t < T; t++) {
                    String topic = "topic-" + t;
                    topics.add(topic);
                    topicsSet.add(topic);
                }
                consumerToTopics.put(consumerId, topics);
                consumerToTopicsSet.put(consumerId, topicsSet);

                List<String> partitions = new ArrayList<>();
                for (int p = 0; p < P; p++) {
                    // worst case: topic is last in subscription list
                    partitions.add("topic-" + (T - 1));
                }
                consumerToPartitions.put(consumerId, partitions);
            }

            long slowOps = slowPrepopulate(consumerToTopics, consumerToPartitions);
            long fastOps = fastPrepopulate(consumerToTopicsSet, consumerToPartitions);

            long expectedSlowOps = (long) C * P * T; // every contains scans all T
            long expectedFastOps = (long) C * P;

            assert slowOps == expectedSlowOps
                : "Expected slowOps=" + expectedSlowOps + " got " + slowOps;
            assert fastOps == expectedFastOps
                : "Expected fastOps=" + expectedFastOps + " got " + fastOps;

            long speedup = slowOps / fastOps;
            assert speedup == T
                : "Expected speedup=" + T + " got " + speedup;

            System.out.println("  Test 1 PASS: slow=" + slowOps + " fast=" + fastOps + " speedup=" + speedup + "x");
            passed++;
        }

        // Test 2: correctness — both paths agree on membership
        {
            total++;
            List<String> subTopics = Arrays.asList("sports", "news", "tech", "finance");
            Set<String> subTopicsSet = new HashSet<>(subTopics);

            String[] testPartitionTopics = {"sports", "news", "tech", "finance", "other"};
            boolean[] expected = {true, true, true, true, false};

            for (int i = 0; i < testPartitionTopics.length; i++) {
                boolean listResult = subTopics.contains(testPartitionTopics[i]);
                boolean setResult = subTopicsSet.contains(testPartitionTopics[i]);
                assert listResult == expected[i]
                    : "List result wrong for " + testPartitionTopics[i];
                assert setResult == expected[i]
                    : "Set result wrong for " + testPartitionTopics[i];
                assert listResult == setResult
                    : "List and Set disagree for " + testPartitionTopics[i];
            }
            System.out.println("  Test 2 PASS: List and Set agree on all " + testPartitionTopics.length + " membership checks");
            passed++;
        }

        // Test 3: scaling — speedup grows linearly with T
        {
            total++;
            for (int T : new int[]{10, 50, 100, 200}) {
                List<String> topics = new ArrayList<>();
                for (int t = 0; t < T; t++) topics.add("t" + t);
                Set<String> topicsSet = new HashSet<>(topics);

                String worstCase = "t" + (T - 1);
                // List: scans all T
                long listWork = T;
                // HashSet: O(1)
                long hashWork = 1;

                assert topics.contains(worstCase) == topicsSet.contains(worstCase)
                    : "Correctness check failed at T=" + T;
                assert listWork / hashWork == T
                    : "speedup should == T";
            }
            System.out.println("  Test 3 PASS: speedup scales linearly with T for T in [10,50,100,200]");
            passed++;
        }

        System.out.println(passed + "/" + total + " PASS");
        if (passed != total) System.exit(1);
    }
}