You manage an ecommerce application that processes purchases from customers who can subsequently cancel or change those purchases. You discover that order volumes are highly variable and the backend o

Question

Options

• ASend the purchase and change requests over WebSockets to the backend.
• BSend the purchase and change requests as REST requests to the backend.
• CUse a Pub/Sub subscriber in pull mode and use a data store to manage ordering.
• DUse a Pub/Sub subscriber in push mode and use a data store to manage ordering.

• A
  11% (5)
• B
  2% (1)
• C
  80% (37)
• D
  7% (3)

Explanation

The key requirements are: (1) handle highly variable order volumes without dropping requests, (2) backend can only process one request at a time, and (3) requests must be processed in the sequence they were generated. Cloud Pub/Sub in pull mode is the correct solution. Pub/Sub acts as a durable, scalable message queue that absorbs traffic spikes - messages are buffered in the queue regardless of backend capacity. Pull mode allows the backend to fetch one message at a time at its own pace, preventing overload. A data store is used alongside Pub/Sub to persist ordering information, since Pub/Sub does not natively guarantee strict message ordering across all scenarios (though ordered delivery can be configured). Option D (push mode) has Pub/Sub push messages to the backend endpoint as fast as they arrive, which would overwhelm a backend that can only handle one request at a time. Options A (WebSockets) and B (REST) send requests directly to the backend with no buffering mechanism for traffic spikes, causing failures under high load.

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