Caching overview

A cacheable response is an HTTP response that Cloud CDN can store and quickly retrieve, thus allowing for faster load times. Not all HTTP responses are cacheable. Cloud CDN uses HTTP response headers that your origin server sends to determine how to cache content at the edge. You can use standard Cache-Control headers to manage caching universally, or use targeted CDN-Cache-Control headers to apply specific caching rules to Cloud CDN without affecting how end-user browsers cache your content.

Cache modes

With cache modes, you can control the factors that determine whether Cloud CDN caches your content.

Cloud CDN offers three cache modes, which define how responses are cached, whether Cloud CDN respects cache directives sent by the origin, and how cache TTLs are applied.

The available cache modes are shown in the following table:

Cache mode Behavior

Cache mode	Behavior
`CACHE_ALL_STATIC`	Automatically caches successful responses with static content that isn't otherwise non-cacheable. Origin responses that set valid caching directives are also cached. This behavior is the default for Cloud CDN-enabled backends created by using the Google Cloud CLI or the REST API.
`USE_ORIGIN_HEADERS`	Requires successful origin responses to set valid cache directives and valid caching headers. Successful responses without these directives are forwarded from the origin.
`FORCE_CACHE_ALL`	Unconditionally caches successful responses, overriding any cache directives set by the origin. This mode might not be appropriate if the backend serves private, per-user (user identifiable) content, such as dynamic HTML or API responses. Note: When private bucket access is enabled on a bucket, you must set the `FORCE_CACHE_ALL` cache mode to cache private content from that bucket.

CACHE_ALL_STATIC

Automatically caches successful responses with static content that isn't otherwise non-cacheable. Origin responses that set valid caching directives are also cached.

This behavior is the default for Cloud CDN-enabled backends created by using the Google Cloud CLI or the REST API.

USE_ORIGIN_HEADERS Requires successful origin responses to set valid cache directives and valid caching headers. Successful responses without these directives are forwarded from the origin.

FORCE_CACHE_ALL

Unconditionally caches successful responses, overriding any cache directives set by the origin. This mode might not be appropriate if the backend serves private, per-user (user identifiable) content, such as dynamic HTML or API responses.

Note: When private bucket access is enabled on a bucket, you must set the FORCE_CACHE_ALL cache mode to cache private content from that bucket.

Error responses may be cached even in the absence of valid cache directives.

Before you set the cache mode to FORCE_CACHE_ALL, consider the following behaviors:

For signed URLs or signed cookies, FORCE_CACHE_ALL overrides the maximum age specified through the Cache entry maximum age setting in the Google Cloud console or the gcloud --signed-url-cache-max-age option.
FORCE_CACHE_ALL changes the time to live (TTL) of any previously cached content. This change can cause some entries that were previously considered fresh (due to having longer TTLs from origin headers) to be considered stale, and it can cause some entries that were previously considered stale to be considered fresh.
FORCE_CACHE_ALL overrides cache directives (Cache-Control and Expires) but does not override other origin response headers. In particular, a Vary header might suppress caching even if the cache mode is FORCE_CACHE_ALL. For more information, see Vary headers.

For setup instructions, see Setting the cache mode.

Static content

Static content is content that is always the same, even when accessed by different users. The CSS that you use to style your site, JavaScript to provide interactivity, video, and image content typically don't change for each user for a given URL (cache key), and thus benefit from being cached across Cloud CDN's global edge network.

When you set the cache mode to CACHE_ALL_STATIC, and a response does not have explicit caching directives in Cache-Control or Expires headers, Cloud CDN automatically caches that response for the following:

Web Assets, including CSS (text/css), JavaScript (application/javascript) and all web fonts, including WOFF2 (font/woff2)
Images, including JPEG (image/jpg) and PNG (image/png)
Videos, including H.264, H.265, and MP4 (video/mp4)
Audio files, including MP3 (audio/mpeg) and MP4 (audio/mp4)
Formatted documents, including PDF (application/pdf)

The following table provides a summary.

Category	MIME types
Web assets	`text/css text/ecmascript text/javascript application/javascript`
Fonts	Any `Content-Type` matching `font/*`
Images	Any `Content-Type` matching `image/*`
Videos	Any `Content-Type` matching `video/*`
Audio	Any `Content-Type` matching `audio/*`
Formatted document types	`application/pdf` and `application/postscript`

Cloud CDN inspects the Content-Type HTTP response header, which reflects the MIME type of the content being served.

Note the following:

Your origin's web server software must set the Content-Type for each response. Many web servers automatically set the Content-Type header, including NGINX, Varnish, and Apache.
Cloud Storage sets the Content-Type header automatically when you use the Google Cloud console or the Google Cloud CLI to upload content.
Cloud Storage always provides a Cache-Control header to Cloud CDN. If no value is explicitly chosen, it sends a default value. As a result, all successful Cloud Storage responses are cached according to Cloud Storage default values, unless you explicitly adjust the cache control metadata for objects in Cloud Storage or use FORCE_CACHE_ALL mode to override the values sent by Cloud Storage.
If you want to cache text/html and application/json content types, you must set explicit Cache-Control headers in the response, being careful not to accidentally cache one user's data and serve it to all users.

If a response is cacheable based on its MIME type but has a Cache-Control response header of private or no-store, or a Set-Cookie header, it isn't cached. To learn more, see cacheability rules.

Other content types, such as HTML (text/html) and JSON (application/json), are not cached by default for successful responses. These types of responses are typically dynamic (per user). Examples include shopping carts, product pages with user personalization, and authenticated API responses. Negative caching, if enabled, can still cause them to be cached for certain status codes, however.

Cloud CDN doesn't use file extensions in the URL path to determine whether a response is cacheable because many valid cacheable responses aren't reflected in URLs.

Cache policy configuration methods

Depending on how much control you need over caching behavior, you can configure caching behavior for Cloud CDN at the backend service, backend bucket, or at a more granular level in URL maps.

Backend service or backend bucket cache policy

Configure a cache policy on a backend service or backend bucket to apply a single caching policy to all requests that are routed to that backend.

Cache policies in URL maps

You can configure Cloud CDN cache policies at various levels of the URL map. This provides granular control over caching policies based on criteria like hostname, URL path, HTTP headers, and query parameters for specific routes.

Setting cache policies at different levels of the URL map, such as root, path matchers, path rules, and route rules, gives you granular control over caching different types of content served by the same backend.

For example, you can configure path rules to:

Cache static images in the /images/* path for 24 hours.
Cache HTML pages in the /pages/* path for 5 minutes.

You can configure cache policies in URL maps when:

a single backend serves different types of content
different paths require different caching behavior
you want to enable caching for specific routes only

For details on how to configure cache policies in URL map, see Configure a Cloud CDN cache policy.

Default values for caching

For caching parameters, Cloud CDN uses the following default values:

Parameter	Default value	Description
Cache mode	`CACHE_ALL_STATIC`	Automatically caches successful responses for common static content types.
Client TTL	`3600s`	Sets a 1-hour `max-age` for the client's browser cache.
Default TTL	`3600s`	Sets a 1-hour cache duration if the origin provides no headers.
Include Host	`true`	The request host is included in the cache key.
Include Protocol	`true`	HTTP and HTTPS requests are cached as separate objects.
Include Query String	`true`	The entire query string is part of the cache key.
Max TTL	`86400s`	The absolute maximum time (24 hours) an object remains in the cache.
Negative Caching