java-topology/docs/tickets/activemq-0001-topic-consumers-copyonwrite-contains-quadratic.md

activemq-0001 — Topic.addSubscription: O(n) CopyOnWriteArrayList.contains per subscriber

Target: apache/activemq File: activemq-broker/src/main/java/org/apache/activemq/broker/region/Topic.java Function: addSubscription(ConnectionContext, Subscription) Severity: MEDIUM CWE: CWE-407 (Algorithmic Complexity — Linear Membership Test in Subscribe Loop) Status: PATCHED

Defect

consumers is declared as:

protected final CopyOnWriteArrayList<Subscription> consumers = new CopyOnWriteArrayList<Subscription>();

On every non-durable subscription add, the broker checks for duplicates:

synchronized (consumers) {
    if (!consumers.contains(sub)){
        consumers.add(sub);
    }
}

CopyOnWriteArrayList.contains() is O(n) — it iterates the entire array. With N concurrent subscribers on a single topic, each new subscriber triggers an O(N) scan. In fan-out workloads with many consumers (e.g., event bus topics with hundreds of subscribers), this degrades as O(N²) across all subscription events over the lifetime of the topic.

The pattern appears three times in addSubscription (lines 151, 167, 293).

Fix

Maintain a parallel Set<Subscription> (e.g., Collections.newSetFromMap(new ConcurrentHashMap<>())) for O(1) membership checks. The CopyOnWriteArrayList is retained for ordered iteration during dispatch. On add, check the set first; on remove, update both structures.

private final Set<Subscription> consumerSet =
    Collections.newSetFromMap(new ConcurrentHashMap<Subscription, Boolean>());

// In addSubscription:
synchronized (consumers) {
    if (consumerSet.add(sub)) {
        consumers.add(sub);
    }
}

// In removeSubscription / deactivate:
synchronized (consumers) {
    if (consumers.remove(sub)) {
        consumerSet.remove(sub);
    }
}

Patch

defects/activemq/patch/activemq-0001-topic-consumer-set.patch

Unit Test

defects/activemq/unit/ActiveMQTopicConsumerTest.java