Google Cloud Professional Cloud Developer Practice Test

Using a Pub/Sub subscriber in pull mode with a datastore is an effective approach to ensure that order update requests are processed in the sequence they are generated in a highly variable order volume application. This methodology allows for greater control over how messages are handled and processed.

In pull mode, the subscriber requests messages from the Pub/Sub service and processes them at its own pace. This means that the subscriber can control the flow of data and ensure that it processes messages in a specific order. The subscriber can implement its logic to prioritize or delay processing based on the needs of the application, making it adept at handling variable order volumes without losing the sequence of updates.

Utilizing a datastore along with this setup facilitates the storage of the state of order updates, enhancing reliability. If a message needs to be retried or if there are dependencies between messages, the pull model allows managing these scenarios more effectively compared to push mode, where messages are automatically delivered to subscribers; this might lead to out-of-order processing under high load situations.

In contrast to other methods, sending requests over WebSockets (the first choice) or REST calls (the second choice) may not inherently manage order as effectively because they may not provide built-in mechanisms for sequencing. WebSockets are suitable for real-time