You need a stateless web app with unpredictable traffic (idle days and large spikes), automatic scale up/down, and minimal cost. Which stack and deployment meet these requirements?

Cloud Run is ideal for stateless web applications with unpredictable traffic because it scales from zero to handle spikes and scales back to zero during idle periods, minimizing costs. Its pay-per-use model aligns perfectly with the "minimal cost" requirement. Firestore is a NoSQL, serverless database that scales automatically and handles variable loads efficiently, making it a good fit for unpredictable traffic and stateless applications. App Engine flexible environment would incur costs even when idle, as instances are always running. App Engine standard environment scales well but might not scale to zero as effectively as Cloud Run for all configurations, and Cloud SQL, while robust, is a relational database that requires more management and can be less cost-effective for highly variable, stateless workloads compared to Firestore. Compute Engine managed instance groups provide autoscaling but require more operational overhead than Cloud Run and don't scale to zero in the same cost-efficient manner.