Increase the number of replicas for the microservice deployment in GKE Autopilot by updating the replicas field in the deployment configuration. Configure Horizontal Pod Autoscaling (HPA) based on Pub/Sub subscription lag metrics using a custom metric in Cloud Monitoring. Set up an alert in Cloud Monitoring to notify the on-call team if lag exceeds a threshold.

Manually increase the CPU and memory resources for the microservice’s pods in GKE Autopilot. Export Pub/Sub logs to BigQuery and analyze processing delays daily to identify bottlenecks. Notify customers via email if their orders are affected by stock discrepancies.

Create a new GKE Standard cluster and redeploy the microservice with higher resource limits. Use Cloud Functions to process Pub/Sub messages in parallel and update the warehousing system. Implement a manual review process to cancel out-of-stock orders.

Switch the microservice to use Cloud Run instead of GKE Autopilot. Configure Cloud Run to scale based on HTTP request volume. Use Cloud Scheduler to periodically check Pub/Sub message backlog and trigger additional instances if needed.