Use external backends (also called custom origins) for Cloud CDN (Content Delivery Network) when content is hosted on-premises or in another cloud, and you want to deliver the content over Google's high performance, distributed edge caching infrastructure.
Terminology
The following terms are sometimes used interchangeably because they have the same or similar meanings:
- external backend: A backend that resides outside of Google Cloud and is reachable across the internet. The endpoint in an internet NEG.
- internet network endpoint group (NEG): The Google Cloud API resource that you use to specify an external backend.
- external endpoint: Same as an external backend.
To maintain consistency with the load balancing documentation, this document uses the term external backend except when referring to the internet NEG API resource.
Supported backend types for Cloud CDN
Cloud CDN works with an external Application Load Balancer to deliver content to your users. The external Application Load Balancer provides the frontend IP addresses and ports that receive requests. Cloud CDN content can be sourced from various types of backends:
- Instance groups
- Zonal network endpoint groups (NEGs)
- Serverless NEGs: One or more App Engine, Cloud Run, or Cloud Run functions services
- Internet NEGs for external backends
- Buckets in Cloud Storage
External backends can be hosted within an on-premises infrastructure or origins provided by third-party providers. The following sections discuss external backends in more detail.
Hybrid and multi-cloud architectures
As you move your services to Google Cloud, you might need to do so in phases. Sometimes certain content can't immediately be moved to a cloud environment and might need to stay on-premises. In other cases, the content might be hosted in another cloud. Cloud CDN support for external backends lets you use Google's globally distributed edge caching infrastructure for such content.
In the diagram, images content resides in Google Cloud, while video
resides in a Tokyo data center, which could be on-premises or in another cloud.
With external backends, origins in the Tokyo data center can be
the backend source of the video content with Cloud CDN and the
external Application Load Balancer delivering the content to users.
Using URL maps, this deployment can
direct origin pull requests for video traffic to the external backend in Tokyo.
This mapping is determined based on request URL: /video.
For images (determined based on request URL: /images), content is sourced
from Google Cloud and is delivered by the Cloud CDN edge
infrastructure.
Specifying an external backend
Similar to configuring Cloud CDN with your endpoints deployed in Google Cloud, you can use the network endpoint groups (NEGs) API to add your server as the external backend for Cloud CDN.
To specify the external backend, use an internet NEG. An internet NEG has one of the endpoint types shown in the following table.
| Endpoint address | Type | Definition | When to use |
|---|---|---|---|
| Hostname and an optional port | INTERNET_FQDN_PORT |
A publicly resolvable fully qualified domain name (FQDN), and an
optional port, for example backend.example.com:443 (default ports:
80 for HTTP and 443 for HTTPS) |
Use this endpoint when your external backend can be resolved by using an FQDN with public DNS. |
| IP address and an optional port | INTERNET_IP_PORT |
A publicly accessible IP address and an optional port, for example
192.0.2.8 or 192.0.2.8:443 (default ports:
80 for HTTP and 443 for HTTPS) |
Use this endpoint to specify a publicly accessible IP address and a port to connect to. |
The best practice is to create the internet NEG with the INTERNET_FQDN_PORT
endpoint type and an FQDN value as an origin hostname value. This insulates the
Cloud CDN configuration from IP address changes in the origin
infrastructure. Network endpoints that are defined by using FQDNs are resolved
through public DNS. Make sure that the configured FQDN is resolvable through
Google Public DNS.
After you create the internet NEG, the type cannot be changed between
INTERNET_FQDN_PORT and INTERNET_IP_PORT. You need to create a new internet
NEG and change your backend service to use the new internet NEG.
When using an external backend that expects a particular value for the HTTP
request's Host header, you must configure the backend service to set the
Host header to that expected value. If you don't configure a user-defined
request header, a backend service preserves the Host header that the client
used to connect to the Google Cloud external Application Load Balancer. For general
information about custom headers, see
Create custom headers in backend services.
For a specific example, see Setting up Cloud CDN with an external
backend.
Using external backends and Google Cloud-based origins
The following figure shows an internet NEG used to deploy an external backend with an external Application Load Balancer and Cloud CDN.
What's next
- To set up an external backend, see Set up an external backend with an internet NEG.
- To learn about what content is cached, see Caching overview.
- To resolve issues, see Troubleshooting external backend and internet NEG issues.