About container instance autoscaling

In Cloud Run, each revision is automatically scaled to the number of container instances needed to handle all incoming requests. When a revision does not receive any traffic, by default it is scaled in to zero container instances. However, if desired, you can change this default to specify an instance to be kept idle or "warm" using the minimum instances setting.

In addition to the rate of incoming requests, the number of instances scheduled is impacted by:

In some cases you may want to limit the total number of container instances that can be started, for cost control reasons, or for better compatibility with other resources used by your service. For example, your Cloud Run service might interact with a database that can only handle a certain number of concurrent open connections.

About maximum container instances

You can use the maximum container instances setting to limit the total number of instances that can be started in parallel, as documented in Setting a maximum number of container instances.

Exceeding maximum instances

Under normal circumstances, your revision scales out by creating new instances to handle incoming traffic load. But when you set a maximum instances limit, in some scenarios there will be insufficient instances to meet that traffic load. In that case, incoming requests can be queued for up to 60 seconds. During this 60 second window, if an instance finishes processing requests, it becomes available to process queued requests. If no instances become available during the 60 second window, the request fails with a 429 error code on Cloud Run.

Scaling guarantees

The maximum instances limit is an upper limit. Setting a high limit does not mean that your revision will scale out to the specified number of container instances. It only means that the number of container instances at any point in time should not exceed the limit.

Traffic spikes

In some cases, such as rapid traffic surges, Cloud Run may, for a short period of time, create slightly more container instances than the specified max instances value. If your service cannot tolerate this temporary behavior, you may want to factor in a safety margin and set a lower max instances value.

Deployments

When you deploy a new revision, Cloud Run gradually migrates traffic from the old revision to the new one. Because maximum instances limits are set for each revision, you may temporarily exceed the specified limit during the period after deployment.

Idle instances and minimizing cold starts

Cloud Run does not immediately shut down instances once they have handled all requests. To minimize the impact of cold starts, Cloud Run may keep some instances idle for a maximum of 15 minutes. These instances are ready to handle requests in case of a sudden traffic spike.

For example, when a container instance has finished handling requests, it may remain idle for a period of time in case another request needs to be handled. An idle container instance may persist resources, such as open database connections. Note that CPU is only allocated during request processing unless you explicitly configure your service to have CPU always allocated.

To keep idle instances permanently available, use the min-instance setting. Note that using this feature will incur cost even when the service is not actively serving requests.

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