Apache Pulsar Advanced Skill — v1.0

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# Apache Pulsar Advanced Skill — v1.0

---

## 1. Architecture Mental Model

```java
Pulsar Cluster
  ├── Broker(s)        — stateless, route messages; can scale horizontally
  ├── BookKeeper       — durable message storage (write-ahead log)
  └── ZooKeeper        — metadata, broker coordination

Topic naming:
  persistent://  {tenant} / {namespace} / {topic-name}
  Example:       persistent://orders/payments/card-transactions

  persistent://  → messages survive broker restart (backed by BookKeeper)
  non-persistent:// → messages lost if broker restarts (high throughput, low durability)
```java

### Key Concepts vs Kafka

| Concept | Kafka | Pulsar |
|---------|-------|--------|
| Message storage | Broker (stateful) | BookKeeper (separate) |
| Consumer model | Consumer Group | Subscription (4 types) |
| Rebalancing | Required on membership change | Not needed (per-message ack) |
| Multi-tenancy | Manual (naming convention) | Built-in (tenant/namespace) |
| Schema | Schema Registry (separate) | Built-in Schema Registry |
| Geo-replication | MirrorMaker2 | Built-in |

---

## 2. Producers

### Java Producer (Spring for Apache Pulsar)

```java
import org.springframework.pulsar.core.PulsarTemplate;

/**
 * Publishes order events to the Pulsar order-events topic.
 *
 * <p>Uses Spring's {@link PulsarTemplate} — the Spring-managed, lifecycle-aware
 * wrapper around the Pulsar client. Always prefer PulsarTemplate over creating
 * PulsarClient directly in Spring apps.</p>
 */
@Service
@RequiredArgsConstructor
@Slf4j
public class OrderEventPublisher {

    private final PulsarTemplate<OrderEvent> pulsarTemplate;

    // Topic name — use a constant to avoid typos across the codebase
    private static final String ORDER_EVENTS_TOPIC = "persistent://orders/default/order-events";

    /**
     * Publishes an order created event.
     *
     * <p>This is a fire-and-forget send — the method returns immediately.
     * Use {@link #publishAndWait} if you need confirmation before continuing.</p>
     *
     * @param event the order event to publish
     * @throws PulsarClientException if the broker rejects the message
     */
    public void publish(OrderEvent event) {
        pulsarTemplate.sendAsync(ORDER_EVENTS_TOPIC, event)
            .thenAccept(messageId ->
                log.info("OrderEvent published. MessageId: {}, OrderId: {}",
                         messageId, event.getOrderId()))
            .exceptionally(ex -> {
                log.error("Failed to publish OrderEvent for order {}: {}",
                          event.getOrderId(), ex.getMessage(), ex);
                // Re-throw or handle based on your reliability requirements
                throw new EventPublishException("Failed to publish order event", ex);
            });
    }

    /**
     * Publishes an event and waits for broker acknowledgment before returning.
     *
     * <p>Use this when the caller must know the message was persisted
     * before proceeding (e.g. within a database transaction boundary).</p>
     *
     * @param event the event to publish
     * @return the Pulsar MessageId assigned to this message
     */
    public MessageId publishAndWait(OrderEvent event) {
        return pulsarTemplate.send(ORDER_EVENTS_TOPIC, event);
    }
}
```java

### Producer with Custom Configuration

```java
/**
 * Sends payment events with batching and compression enabled.
 *
 * <p>Batching groups multiple messages into one network round trip —
 * dramatically improves throughput at the cost of a small latency increase.</p>
 */
@Bean
public PulsarTemplate<PaymentEvent> paymentEventTemplate(PulsarClient pulsarClient) {
    ProducerBuilder<PaymentEvent> builder = pulsarClient
        .newProducer(Schema.JSON(PaymentEvent.class))
        .topic("persistent://payments/default/payment-events")
        .producerName("payment-service-producer")
        .batchingMaxMessages(500)           // batch up to 500 messages
        .batchingMaxPublishDelay(10, TimeUnit.MILLISECONDS)  // or send after 10ms
        .compressionType(CompressionType.LZ4)               // fast compression
        .sendTimeout(5, TimeUnit.SECONDS);  // fail if broker doesn't ack in 5s

    return new PulsarTemplate<>(new DefaultPulsarProducerFactory<>(pulsarClient, builder));
}
```java

---

## 3. Consumers — The Four Subscription Types

This is the most important concept in Pulsar. Choose the wrong subscription type and
you get duplicate processing, missed messages, or ordering violations.

```java
// Type 1: EXCLUSIVE
// → Only ONE consumer active at a time. If it dies, the next consumer takes over.
// → Use for: ordered processing, single-instance workers
// → Risk: no parallelism; one slow consumer blocks everything

// Type 2: SHARED (Round-Robin)
// → Many consumers, messages distributed round-robin
// → Use for: high-throughput parallel processing where order doesn't matter
// → Risk: no ordering guarantee; a slow consumer can hold up acks

// Type 3: FAILOVER
// → One ACTIVE consumer, others are standbys. Hot standby — instant failover.
// → Use for: ordered processing with high availability
// → Ordered within each partition

// Type 4: KEY_SHARED
// → Like SHARED but all messages with the same key always go to the same consumer
// → Use for: parallel processing WITH per-key ordering (e.g. all events for customer 42 in order)
// → Best of both worlds for keyed workloads
```java

### Spring for Apache Pulsar Consumer

```java
/**
 * Listens to the order-events topic and processes each order.
 *
 * <p>Uses SHARED subscription so multiple instances can process in parallel.
 * Each message is individually acknowledged — if processing fails, only
 * that message is redelivered, not the whole batch.</p>
 */
@Service
@Slf4j
public class OrderEventConsumer {

    /**
     * Processes an incoming order event.
     *
     * <p>Spring calls this method for each message. The framework handles
     * acknowledging the message when the method returns normally, and
     * negative-acknowledging (nack) when it throws.</p>
     *
     * @param event the deserialized order event
     */
    @PulsarListener(
        topics     = "persistent://orders/default/order-events",
        subscriptionName = "order-processor-subscription",
        subscriptionType = SubscriptionType.Shared,
        schemaType  = SchemaType.JSON
    )
    public void onOrderEvent(OrderEvent event) {
        log.info("Processing order event: orderId={}, type={}", event.getOrderId(), event.getType());

        // If this throws, Spring Pulsar will nack the message
        // → Pulsar will redeliver after the nack delay
        orderProcessingService.process(event);

        log.info("Order event {} processed successfully", event.getOrderId());
        // Method returns normally → Spring Pulsar auto-acks the message
    }
}
```java

### KEY_SHARED Consumer (Ordered Per Customer)

```java
/**
 * Processes payment events in order per customer.
 *
 * <p>KEY_SHARED ensures all payments for customer 42 go to the same consumer
 * instance, preserving order for that customer. Different customers are
 * processed in parallel across multiple instances.</p>
 */
@PulsarListener(
    topics           = "persistent://payments/default/payment-events",
    subscriptionName = "payment-processor-subscription",
    subscriptionType = SubscriptionType.Key_Shared,
    schemaType       = SchemaType.JSON
)
public void onPaymentEvent(Message<PaymentEvent> message) {
    // The key is the customer ID — set by the producer
    String customerId = message.getKey();
    PaymentEvent event = message.getValue();

    log.info("Processing payment for customer {} (key={}) : paymentId={}",
             customerId, message.getKey(), event.getPaymentId());

    paymentService.process(event);
}
```java

---

## 4. Schema Registry

Pulsar has a built-in schema registry. Always use it — it prevents consumers from
receiving messages they can't deserialize.

```java
// ── Define the schema (the message contract) ─────────────────────────────

/**
 * Represents an order event published to Pulsar.
 *
 * <p>Schema is registered automatically in Pulsar's schema registry
 * when the first producer or consumer connects with this type.</p>
 */
@Data
@Builder
@NoArgsConstructor
@AllArgsConstructor
public class OrderEvent {
    private Long     orderId;
    private Long     customerId;
    private String   eventType;    // CREATED, UPDATED, CANCELLED, SHIPPED
    private BigDecimal totalAmount;
    private Instant  occurredAt;
}

// ── Producer registers the schema automatically ───────────────────────────
PulsarTemplate<OrderEvent> template = ...;
// Spring Pulsar registers Schema.JSON(OrderEvent.class) on first send

// ── Consumer validates against the registered schema ─────────────────────
@PulsarListener(
    topics     = "...",
    schemaType = SchemaType.JSON   // Pulsar validates incoming messages against the schema
)
public void onOrderEvent(OrderEvent event) { ... }
```java

### Schema Evolution Rules

```json
BACKWARD COMPATIBLE (safe):
  ✅ Add a new optional field (with default value)
  ✅ Delete a field (consumers ignore unknown fields)

NOT BACKWARD COMPATIBLE (breaking):
  ❌ Change a field's type (e.g. String → Integer)
  ❌ Rename a required field
  ❌ Remove a field that consumers depend on
```java

---

## 5. Dead Letter Topics

When a message fails to process after all retries, Pulsar sends it to a
Dead Letter Topic (DLT) automatically. Always monitor your DLTs.

```java
/**
 * Consumer with Dead Letter Topic configuration.
 *
 * <p>If processing fails 3 times (maxRedeliverCount), the message is
 * automatically moved to the DLT. A separate DLT consumer handles
 * alerting and manual review.</p>
 */
@PulsarListener(
    topics           = "persistent://orders/default/order-events",
    subscriptionName = "order-processor-subscription",
    subscriptionType = SubscriptionType.Shared,
    deadLetterPolicy = @DeadLetterPolicy(
        maxRedeliverCount    = 3,
        deadLetterTopic      = "persistent://orders/default/order-events-DLT",
        retryLetterTopic     = "persistent://orders/default/order-events-RETRY"
    ),
    // Negative ack redelivery: wait 5 seconds before retrying a failed message
    negativeAckRedeliveryDelay = "5000"
)
public void onOrderEvent(OrderEvent event) {
    orderProcessingService.process(event);
}


/**
 * Separate consumer for the Dead Letter Topic.
 *
 * <p>Messages here have failed all retries and need manual intervention.
 * This consumer logs, alerts, and stores them for review.</p>
 */
@PulsarListener(
    topics           = "persistent://orders/default/order-events-DLT",
    subscriptionName = "order-dlt-handler",
    subscriptionType = SubscriptionType.Shared
)
public void onDeadLetter(Message<OrderEvent> deadMessage) {
    log.error("Message moved to DLT after {} redeliveries. OrderId: {}. " +
              "MessageId: {}. Payload: {}",
              deadMessage.getRedeliveryCount(),
              deadMessage.getValue().getOrderId(),
              deadMessage.getMessageId(),
              deadMessage.getValue());

    // Save to a review table for human intervention
    deadLetterRepository.save(new DeadLetterRecord(
        deadMessage.getMessageId().toString(),
        deadMessage.getValue(),
        deadMessage.getRedeliveryCount()
    ));

    // Alert the on-call engineer
    alertingService.sendAlert("DLT message received for order: " + deadMessage.getValue().getOrderId());
}
```java

---

## 6. Spring for Apache Pulsar — application.yml

```yaml
spring:
  pulsar:
    client:
      service-url: pulsar://localhost:6650     # broker URL
      # For TLS:
      # service-url: pulsar+ssl://pulsar.prod.example.com:6651

    producer:
      send-timeout: 5s                         # fail if not acked in 5 seconds
      batching-enabled: true
      batching-max-messages: 500
      compression-type: lz4

    consumer:
      subscription-initial-position: earliest  # start from beginning of topic if new subscription
      receiver-queue-size: 1000                # how many messages to pre-fetch
      ack-timeout: 10s                         # if not acked in 10s, redeliver

    listener:
      observation-enabled: true                # integrates with Micrometer for metrics
      schema-type: json                        # default schema for all listeners
```java

---

## 7. Python Producer and Consumer

```python
import pulsar
from pulsar.schema import JsonSchema
from dataclasses import dataclass


@dataclass
class OrderEvent:
    """Schema for order events in Pulsar.

    Attributes:
        order_id: Unique identifier for the order.
        customer_id: The customer who placed the order.
        event_type: What happened (CREATED, UPDATED, CANCELLED).
        total_amount: Order total in the order currency.
    """
    order_id: int
    customer_id: int
    event_type: str
    total_amount: float


# ── Producer ──────────────────────────────────────────────────────────────
client   = pulsar.Client("pulsar://localhost:6650")
producer = client.create_producer(
    topic           = "persistent://orders/default/order-events",
    schema          = JsonSchema(OrderEvent),
    send_timeout_millis = 5000,    # fail after 5 seconds
    compression_type    = pulsar.CompressionType.LZ4,
)

def publish_order_event(event: OrderEvent) -> None:
    """Publishes an order event to Pulsar.

    Args:
        event: The order event to publish.

    Raises:
        pulsar.exceptions.PulsarException: If the broker rejects the message.
    """
    message_id = producer.send(event)
    print(f"Published order {event.order_id}, messageId={message_id}")


# ── Consumer ──────────────────────────────────────────────────────────────
consumer = client.subscribe(
    topic             = "persistent://orders/default/order-events",
    subscription_name = "python-order-processor",
    schema            = JsonSchema(OrderEvent),
    consumer_type     = pulsar.ConsumerType.Shared,
)

def consume_events() -> None:
    """Continuously consumes and processes order events.

    Acknowledges messages on success, negative-acknowledges on failure.
    Failed messages are redelivered after the nack delay.
    """
    while True:
        message = consumer.receive(timeout_millis=5000)
        if message is None:
            continue

        try:
            event: OrderEvent = message.value()
            print(f"Processing order {event.order_id}")
            process_order(event)                       # your processing logic
            consumer.acknowledge(message)              # success → ack
        except Exception as e:
            print(f"Processing failed for order {event.order_id}: {e}")
            consumer.negative_acknowledge(message)     # failure → nack → redeliver
```java

---

## 8. Debugging Pulsar Issues

### Common Problems and Fixes

| Problem | Symptom | Investigation | Fix |
|---------|---------|---------------|-----|
| Messages not received | Consumer shows 0 messages | Check `pulsar-admin topics stats` — is backlog growing? | Verify subscription name matches publisher's topic exactly |
| Message redelivered forever | Same message keeps appearing | Check `redelivery_count` header; check DLT | Add DLT config; fix the root cause in the processor |
| Schema incompatibility | `IncompatibleSchemaException` on connect | Check schema registry via admin API | Add field as optional with default; bump schema version |
| Consumer lag growing | Backlog increasing | `pulsar-admin topics stats --subscriptions` | Scale consumers; increase `receiverQueueSize`; check processing time |
| Messages out of order | Order violation on keyed data | Check subscription type is `Key_Shared`, not `Shared` | Use `Key_Shared`; ensure producer sets message key |

### Useful Admin Commands

```bash
# Check topic status and consumer lag
pulsar-admin topics stats persistent://orders/default/order-events

# List all subscriptions on a topic
pulsar-admin topics subscriptions persistent://orders/default/order-events

# Check DLT backlog
pulsar-admin topics stats persistent://orders/default/order-events-DLT

# Skip a stuck message (use carefully — message is lost)
pulsar-admin topics skip \
  --count 1 \
  --subscription order-processor-subscription \
  persistent://orders/default/order-events

# Peek at messages without consuming them
pulsar-admin topics peek-messages \
  --count 5 \
  --subscription order-processor-subscription \
  persistent://orders/default/order-events
```java