Java LLD: Designing a Kafka-Like Message Queue for Machine Coding Interviews

Designing a high-performance message queue is a frequent requirement in senior-level machine coding rounds. It tests your ability to balance thread safety with decoupled architecture while managing stateful consumer progress.

The mistake most candidates make

• Using a standard java.util.Queue that removes elements upon polling, which prevents multiple consumer groups from reading the same data.
• Coupling the Producer directly to the Consumer logic, violating the Pub-Sub principle and making the system brittle to scale.
• Failing to implement independent offset management, leading to data loss or duplicate processing when one consumer lags.

The right approach

• Core mental model: An immutable, append-only log where messages are persisted per topic, allowing consumers to track their own progress independently.
• Key entities: Topic, Message, Subscriber, ConsumerGroup, OffsetManager.
• Why it wins: It enables "replayability" and allows multiple heterogeneous systems to consume the same stream at different speeds without interference.

Implementation Insight: The Thread-Safe Log

public class Topic {
    private final List<Message> messages = new ArrayList<>();
    private final ReentrantLock lock = new ReentrantLock();

    public void addMessage(Message message) {
        lock.lock();
        try {
            messages.add(message);
        } finally {
            lock.unlock();
        }
    }

    public List<Message> getMessagesFrom(int offset) {
        return (offset >= messages.size()) ? List.of() : messages.subList(offset, messages.size());
    }
}

Key takeaways

• Decouple producers and consumers by using an append-only log structure instead of a destructive queue.
• Use ReentrantLock to ensure atomic appends to the message log in a multi-threaded producer environment.
• Manage consumer state via independent offsets, allowing each ConsumerGroup to process messages at its own pace.

Full working implementation with execution trace available at https://javalld.com/problems/message-queue