E‑commerce system where order updates must be applied in original sequence and backend processes one request at a time with variable load. Which approach ensures ordered, scalable processing?

The correct approach is to use a Pub/Sub subscriber in pull mode with a data store to manage ordering. Google Cloud Pub/Sub ensures messages are delivered at least once, and pull mode allows the consumer to control the rate of message processing, preventing overload during variable loads. A data store is necessary to explicitly manage the sequence of order updates, as Pub/Sub does not guarantee strict message order across all subscribers, only within a single publisher-to-subscriber connection. Sending requests over WebSockets or as REST requests directly to the backend would not inherently guarantee ordered processing or provide the necessary buffering and scalability for variable loads. While a Pub/Sub subscriber in push mode could be used, pull mode offers more control over processing rates, which is crucial for a backend that processes one request at a time with variable load.