This document clarifies the critical difference between two acknowledgment strategies in Pub/Sub, based on your confirmed architecture.

### ✅ Your Architecture: Acknowledge on Receipt (At-Most-Once Delivery)

You have confirmed that your system uses an **"Acknowledge on Receipt"** model. This is a valid, but less common, pattern that prioritizes throughput over guaranteed delivery.

**Flow:**
1.  Your `ScheduleService` receives a message.
2.  It **immediately** calls `consumer.ack()`. Pub/Sub now considers the message successfully processed and deletes it.
3.  It then calls the Backend API.
4.  If the Backend API call fails, **the message is lost forever.**

**Code Example (Your Confirmed Pattern):**
```java
MessageReceiver receiver = (message, consumer) -> {
    // Acknowledge immediately upon receipt.
    consumer.ack();
    log.info("Acked message {} immediately.", message.getMessageId());

    try {
        // Proceed with business logic after acknowledging.
        callBackendAPI(message.getData().toStringUtf8());
    } catch (Exception e) {
        // The message is already gone.
        // Rely on application-level logging/monitoring to handle this failure.
        log.error("API call failed for message {}, but it was already acked and is now lost.", message.getMessageId(), e);
    }
};
```

---

### The Standard Pattern: Acknowledge After Processing (At-Least-Once Delivery)

For context, it's important to understand the more common and reliable pattern, which ensures no messages are lost if processing fails.

**Flow:**
1.  Your `ScheduleService` receives a message.
2.  It calls the Backend API.
3.  The Backend API returns a success response.
4.  **Only then** does the service call `consumer.ack()`.

If the API call fails or the service crashes, the `ack()` is never sent, and Pub/Sub will redeliver the message. This guarantees the work is completed **at least once**.

---

### Summary Table: Your Ack Position vs. Standard

| Ack Position | Your Model: **BEFORE** API Call | Standard Model: **AFTER** API Call |
| :--- | :--- | :--- |
| **Reliability** | **Lower** | **Higher** |
| **Guarantee** | At-Most-Once | At-Least-Once |
| **Use Case** | High throughput where individual message loss is acceptable. | Standard for all reliable business-critical processing. |
| **Failure Impact** | **Message is lost forever.** | Message is retried until successful. |

**Conclusion:** Your system is intentionally designed for high throughput with the trade-off of potential data loss on processing failure. All documentation has been updated to reflect this "Acknowledge on Receipt" model.