Cloud CDN (Content Delivery Network) uses Google's global edge network to serve content closer to users, which accelerates your websites and applications.
Cloud CDN works with the global external HTTP(S) load balancer or the global external HTTP(S) load balancer (classic) to deliver content to your users. The external HTTP(S) load balancer provides the frontend IP addresses and ports that receive requests and the backends that respond to the 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 Functions services
- Internet NEGs for external backends
- Buckets in Cloud Storage
In Cloud CDN, these backends are also called origin servers. Figure 1 illustrates how responses from origin servers that run on virtual machine (VM) instances flow through an external HTTP(S) load balancer before being delivered by Cloud CDN. In this situation, the Google Front End (GFE) comprises Cloud CDN and the external HTTP(S) load balancer.
How Cloud CDN works
When a user requests content from an external HTTP(S) load balancer, the request arrives at a GFE that is at the edge of Google's network as close as possible to the user.
If the load balancer's URL map routes traffic to a backend service or backend bucket that has Cloud CDN configured, the GFE uses Cloud CDN.
Cache hits and cache misses
A cache is a group of servers that stores and manages content so that future requests for that content can be served faster. The cached content is a copy of cacheable content that is stored on origin servers.
If the GFE looks in the Cloud CDN cache and finds a cached response to the user's request, the GFE sends the cached response to the user. This is called a cache hit. When a cache hit occurs, the GFE looks up the content by its cache key and responds directly to the user, shortening the round-trip time and saving the origin server from having to process the request.
A partial hit occurs when a request is served partially from cache and partially from a backend. This can happen if only part of the requested content is stored in a Cloud CDN cache, as described in Support for byte range requests.
The first time that a piece of content is requested, the GFE determines that it can't fulfill the request from the cache. This is called a cache miss. When a cache miss occurs, the GFE forwards the request to the external HTTP(S) load balancer. The load balancer then forwards the request to one of your origin servers. When the cache receives the content, the GFE forwards the content to the user.
If the origin server's response to this request is cacheable, Cloud CDN stores the response in the Cloud CDN cache for future requests. Data transfer from a cache to a client is called cache egress. Data transfer to a cache is called cache fill.
Figure 2 shows a cache hit and a cache miss:
- Origin servers running on VM instances send HTTP(S) responses.
- The external HTTP(S) load balancer distributes the responses to Cloud CDN.
- Cloud CDN delivers the responses to end users.
For costs related to cache hits and cache misses, see Pricing.
Cache hit ratio
The cache hit ratio is the percentage of times that a requested object is served from the cache. If the cache hit ratio is 60%, it means that the requested object is served from the cache 60% of the time and must be retrieved from the origin 40% of the time.
For information about how cache keys can affect the cache hit ratio, see Using cache keys. For troubleshooting information, see Cache hit ratio is low.
View the cache hit ratio for a small time period
To view the cache hit ratio for a small time period (the last few minutes):
In the Google Cloud console, go to the Cloud CDN page.
For each origin, see the Cache hit ratio column.
n/a means that the load-balanced content isn't currently cached or hasn't been requested recently.
View the cache hit ratio for a longer time period
To view the cache hit ratio for a time period from 1 hour to 30 days:
- In the Google Cloud console, go to the Cloud CDN page.
- In the Origin name column, click the origin name.
- Click the Monitoring tab.
- Optional: select a specific backend.
The CDN hit rate is one of the available monitoring graphs. A graph that displays n/a means that the content isn't cached or hasn't been requested in the displayed time range.
You can adjust the time period by selecting a different time range. The following image is an example of time ranges that you can select:
Inserting content into the cache
Caching is reactive in that an object is stored in a particular cache if a request goes through that cache and if the response is cacheable. An object stored in one cache does not automatically replicate into other caches; cache fill happens only in response to a client-initiated request. You cannot preload caches except by causing the individual caches to respond to requests.
When the origin server supports byte range requests, Cloud CDN can initiate multiple cache fill requests in reaction to a single client request.
Serving content from a cache
After you enable Cloud CDN, caching happens automatically for all cacheable content. Your origin server uses HTTP headers to indicate which responses are cached. You can also control cacheability by using cache modes.
When you use a backend bucket, the origin server is Cloud Storage. When you use VM instances, the origin server is the web server software that you run on those instances.
Cloud CDN uses caches in numerous locations around the world. Because of the nature of caches, it is impossible to predict whether a particular request is served out of a cache. You can, however, expect that popular requests for cacheable content are served from a cache most of the time, yielding significantly reduced latencies, reduced cost, and reduced load on your origin servers.
For more information about what Cloud CDN caches and for how long, see the Caching overview.
To see what Cloud CDN is serving from a cache, you can view logs.
Removing content from the cache
To remove an item from a cache, you can invalidate cached content. For more information, see:
To bypass Cloud CDN, you can request an object directly from a Cloud Storage bucket or a Compute Engine VM. For example, a URL for a Cloud Storage bucket object looks like this:
Eviction and expiration
For content to be served from a cache, it must have been inserted into the cache, it must not be evicted, and it must not be expired.
Eviction and expiration are two different concepts. They both affect what gets served, but they don't directly affect each other.
If you are testing content caching with a small number of requests, you might notice that the content gets evicted.
Every cache has a limit on how much it can hold. However, Cloud CDN adds content to caches even after they're full. To insert content into a full cache, the cache first removes something else to make room. This is called eviction. Caches are usually full, so they are constantly evicting content. They generally evict content that hasn't recently been accessed, regardless of the content's expiration time. The evicted content might be expired, and it might not be. Setting an expiration time doesn't affect eviction.
Unpopular content means content that hasn't been accessed in a while. A while and unpopular are both relative to the bulk of other items in the cache. Multiple Google Cloud projects share a common pool of cache space because the projects are served from the same set of GFEs. The relative popularity of content is compared across multiple projects, not only within a single project.
As caches receive more traffic, they also evict more cached content.
As with all large-scale caches, content can be evicted unpredictably, so no particular request is guaranteed to be served from the cache.
Content in HTTP(S) caches can have a configurable expiration time. The expiration time informs the cache not to serve old content, even if the content hasn't been evicted.
For example, consider a picture-of-the-hour URL. Its responses should be set to expire in under one hour. Otherwise, the served content might be an old picture from a cache.
For information about fine tuning expiration times, see Using TTL settings and overrides.
Requests initiated by Cloud CDN
When your origin server supports byte range requests, Cloud CDN can send multiple requests to the origin server in reaction to a single client request. As described in Support for byte range requests, Cloud CDN can initiate two types of requests: validation requests and byte range requests.
Data location settings of other Cloud Platform Services
Using Cloud CDN means that data may be stored at serving locations outside of the region or zone of your origin server. This is normal and how HTTP caching works on the internet. Under the Service Specific Terms of the Google Cloud Platform Terms of Service, the Data Location Setting that is available for certain Cloud Platform Services does not apply to Core Customer Data for the respective Cloud Platform Service when used with other Google products and services (in this case the Cloud CDN service). If you do not want this outcome, do not use the Cloud CDN service.
Support for Google-managed SSL certificates
You can use Google-managed certificates when Cloud CDN is enabled.
Google Cloud Armor with Cloud CDN
Google Cloud Armor with Cloud CDN features two types of security policies:
- Edge security policies. These policies can be applied to your Cloud CDN-enabled origin servers. They apply to all traffic, before CDN lookup.
- Backend security policies. These policies are enforced only for requests for dynamic content, cache misses, or other requests that are destined for your origin server.
For more information, see the Google Cloud Armor documentation.
- To enable Cloud CDN for your HTTP(S) load balanced instances and storage buckets, see Using Cloud CDN.
- To use Cloud CDN with Google Kubernetes Engine, see Configure Cloud CDN through Ingress.
- To find GFE points of presence, see Cache locations.