Known issues in the App Engine flexible environment

Region ID

The REGION_ID is an abbreviated code that Google assigns based on the region you select when you create your app. The code does not correspond to a country or province, even though some region IDs may appear similar to commonly used country and province codes. For apps created after February 2020, REGION_ID.r is included in App Engine URLs. For existing apps created before this date, the region ID is optional in the URL.

Learn more about region IDs.

For a full list of known issues or to report a new issue, see the issue tracker.

  • After you deploy your application with gcloud app deploy, you might need to wait 1-2 minutes before your application starts serving at Until then, you might see HTTP 503 errors.

  • If there is an organization policy on your project that restricts access to external IPs, you won't be able to deploy an App Engine flexible environment app with external IP addresses. For example, the organization policy could look as follows:

    • The effective policy for constraints/compute.vmExternalIpAccess is set to DENY_ALL.
    • The effective policy for constraints/compute.vmExternalIpAccess is set to allow only specific VM instances.

    These constraints are not automatically detected, and deployments might time out and fail. You can check the organization policy for your project by running the command gcloud beta resource-manager org-policies describe compute.vmExternalIpAccess --project=my-project --effective. You can also override the organizational policy for a specific project.

    However, even with such organization policies set, you can deploy a private App Engine flexible environment app that uses only its internal IP address.

  • After you deploy a new version of an existing service in the App Engine flexible environment with gcloud app deploy, the "Count/sec" metric shown in the "Summary" graph of the App Engine dashboard may decrease significantly. The metric will gradually return to the expected request count over the next 5-10 minutes.

    This does not mean that your application is serving fewer requests. When you deploy a new version of your application, there is a delay between the time the new version is ready to serve requests and the time that the metrics for new instances become available.

    To ensure that this metric is unaffected by a new version deployment:

    1. Deploy your new version with gcloud app deploy --no-promote.
    2. Wait 15 minutes after the deployment completes.
    3. Migrate traffic to the new version.

    If you deploy with --no-promote but allocate any amount of traffic to the new version before the 15 minute window after the deployment completes, this metric may be impacted.

  • It is not possible in the App Engine flexible environment to configure app.yaml so that your app automatically redirects requests to always use HTTPS. This differs from the App Engine standard environment, where you can use the secure setting.

    As an alternative, you can handle the redirect inside your application code by parsing the value of X-Forwarded-Proto header. You can also encourage clients to use the Strict-Transport-Security header.

  • If you assign a user-managed service account to an App Engine flexible environment version, your project may be billed for metrics. Normally, these agent metrics are not charged to your project. We recommend that you continue to use the App Engine default service account until this issue is resolved.

  • You can't establish an SSH connection to a VM Instance using IAP.

Unexpected reduction in number of instances

  • In rare events, your application could see an unexpected reduction in the number of instances due to zone failures, or if an entire group of instances stop responding. To prevent this, Google recommends overprovisioning your application to prevent your system from dropping below the minimum number of instances. You can set your App Engine flexible environment application's min_num_instances size when deploying it. Some events that may affect App Engine flexible environment minimum number of instances are:

    1. Rolling out updates to flexible environment instances
    2. Zonal failure (Stockout issues, such as when your region is at capacity for your selected CPU, etc.)

    App Engine flexible environment uses 3 zones to distribute your instances and in such a configuration, we recommend provisioning 50% more instances than required.