### 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](https://docs.cloud.google.com/appengine/docs/standard/python/how-requests-are-routed#region-id).
You can use various methods to communicate between your App Engine
services or with other services, including Google Cloud services and
external applications.
The simplest approach for communicating with your App Engine service is
to send targeted HTTP requests, where the URL includes the name or ID of a
resource. For example, you can include the ID of a service or version that you
want to target, in addition to the corresponding Google Cloud project ID:
`https://VERSION-dot-SERVICE-dot-PROJECT_ID.https://docs.cloud.google.com/appengine/docs/standard/communicating-between-services#appengine-urls.r.appspot.com`
Note that the combined length of
`VERSION-dot-SERVICE-dot-PROJECT_IDhttps://docs.cloud.google.com/appengine/docs/standard/communicating-between-services#appengine-urls`, where
`VERSION` is the name of your version, `SERVICE
` is the name of your service, and `PROJECT_ID` is your
project ID, cannot be longer than 63 characters and cannot start or end with
a hyphen. If the combined length is
longer than 63 characters, you might see Error `DNS address could not be
found. `
Learn more about requests in App Engine:
- [How requests are
handled](https://docs.cloud.google.com/appengine/docs/standard/how-requests-are-handled): Learn how your application receives requests and sends responses.
- [How requests are routed](https://docs.cloud.google.com/appengine/docs/standard/how-requests-are-routed): Learn how to target your services, including how to define HTTPS URLs.
- Learn how to authorize the requests between your services and other Google Cloud services:
- [Provide credentials for Application Default Credentials](https://docs.cloud.google.com/docs/authentication/provide-credentials-adc)
- [Using user-managed service accounts](https://docs.cloud.google.com/appengine/docs/standard/user-managed-service-accounts)
Your App Engine services can also communicate using
[Pub/Sub](https://docs.cloud.google.com/pubsub/docs), which provides reliable asynchronous
many-to-many messaging between processes, including App Engine. These
processes can be individual instances of your application, services, or even
external applications.
To share data across databases and your App Engine app or some other
external application, see
[Understanding Data and File Storage](https://docs.cloud.google.com/appengine/docs/standard/storage-options).
If you use the legacy bundled services, you can also pass requests between services and
from services to external endpoints using the
[URL Fetch](https://docs.cloud.google.com/appengine/docs/legacy/standard/python/issue-requests) API.
Additionally, services in the standard environment that reside within the same
Google Cloud project can also use one of the App Engine APIs for the
following tasks:
- Share a single [memcache](https://docs.cloud.google.com/appengine/docs/legacy/standard/python/memcache) instance.
- Collaborate by assigning work between services through [Task Queues](https://docs.cloud.google.com/appengine/docs/legacy/standard/python/taskqueue).
## Private Communication
### Communication between services in the same project
You can allow an App Engine standard service to communicate with
another App Engine service in the same project without having to expose
the destination service to the public internet.
To allow communication between services in the same project:
1. Configure ingress controls by adjusting the destination service's
[ingress settings](https://docs.cloud.google.com/appengine/docs/standard/ingress-settings)
to allow "internal" traffic only.
The "internal" setting allows requests from the project's VPC
networks only. This includes App Engine resources from a client app on
the same network when egress traffic is routed over a connector.
All other traffic from the internet or other Google Cloud projects,
including other App Engine services, is blocked.
> [!NOTE]
> **Note:** The "internal" traffic setting blocks all traffic from additional Google Cloud services that do not use the VPC network, such as URLFetch, Cloud Tasks, and Pub/Sub, even within the same project.
2. Route the traffic over a [Serverless VPC Access connector](https://docs.cloud.google.com/vpc/docs/configure-serverless-vpc-access#appengine):
1. For each App Engine version sending private traffic to other app
endpoints, attach the version to a Serverless VPC Access connector
belonging to one of the Google Cloud project's own networks, not a
Shared VPC network.
2. Ensure Private Google Access is enabled for the subnet used by the
Serverless VPC Access connector.
3. Configure one of the following:
- Configure client requests to use the `private.googleapis.com` IP range
by adding a DNS entry for the destination hostname. Follow
[DNS configuration](https://docs.cloud.google.com/vpc/docs/configure-private-google-access#config-domain)
to add the DNS hostname, but be sure to configure the private zone
to be for `appspot.com` rather than `googleapis.com`. Also ensure that
traffic is directed to the destination app's `appspot.com` address,
not a custom domain. Your app can only be reached on the
`private.googleapis.com` IP range using this `appspot.com` domain.
- Configure the client app to
[send](https://docs.cloud.google.com/appengine/docs/standard/connecting-vpc#manage)
`all-traffic` through the Serverless VPC Access connector,
instead of configuring requests to use the `private.googleapis.com` IP
range.
### Communication between services in different projects
You can have private access between Google Cloud projects when apps running
in projects belong to a Shared VPC network that is
configured to invoke an app running in the Shared VPC network's host project.
To use this pattern, follow the previous steps for
[communicating between services in the same project](https://docs.cloud.google.com/appengine/docs/standard/communicating-between-services#between-services).
In the standard environment, attach each client version to a
Serverless VPC Access connector on the Shared VPC network.
Other methods of communication between projects using internal access are not
possible in App Engine.
## Reserved URL paths
It is not possible to use the following URL paths:
- Paths ending with `/eventlog`
- Paths starting with `/_ah/`
- Some paths ending with `z`