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.

Cacheable content

Cloud CDN caches only those responses that meet all the requirements in this section. Some of these requirements are specified by RFC 7234, and others are specific to Cloud CDN.

A response can be stored in Cloud CDN caches only if all of the following are true.

Attribute Requirement
Served by Backend service, backend bucket, or a custom origin with Cloud CDN enabled
In response to GET request
Status code 200, 203, 206, 300, 301, 302, 307, or 410
Cache directive Has a Cache-Control header with a public directive
Freshness Has a Cache-Control header with a max-age or s-maxage directive, or an Expires header with a timestamp in the future
Content

Contains a valid Content-Length, Content-Range, or Transfer-Encoding header

For example, a Content-Length header that correctly matches the size of the response
Size Less than or equal to the maximum size

For Cloud Storage backend buckets, following are several ways to satisfy these requirements:

  • Make your bucket publicly readable. This is the approach that we recommend for public content. With this setting, anyone on the internet can view and list your objects and their metadata, excluding ACLs. The recommended practice is to dedicate specific buckets for public objects. For more information, see Recommended bucket architecture.

  • Make the individual objects publicly readable. We don't recommend this approach.

By default, when the entire bucket is public or the individual objects are public and the individual objects don't specify cache-control metadata, Cloud Storage assigns the objects a Cache-Control: public, max-age=3600 header. You can set different values by using Cache-Control metadata.

For an example that shows how to configure an external HTTP(S) load balancer with a backend bucket, see Setting up Cloud CDN with a backend bucket.

Maximum size

Cloud CDN enforces a maximum size for each response. Any response with a body larger than the maximum size is not cached but is still delivered to the client.

The maximum size varies depending on whether the origin server supports byte range requests.

Origin server supports byte range requests Origin server does not support byte range requests
5 TB (5,497,558,138,880 bytes) 10 MB (10,485,760 bytes)

Nearly all modern web servers (including NGINX, Apache, and Varnish) support byte range requests.

Non-cacheable content

There are checks that block caching of responses.

A response isn't cached if it does not meet the requirements for Cacheable content, or if any of the following is true.

Attribute Requirement
Served by Backend service, backend bucket, or a custom origin that doesn't have Cloud CDN enabled
Cookie Has a Set-Cookie header
Response directive Response has a Cache-Control header with no-store, no-cache, or private directive
Request directive Request has a Cache-Control: no-store header
Size Larger than the maximum size

These requirements might be relaxed in the future, allowing Cloud CDN to cache additional responses.

If Cache-Control: no-store, no-cache, or private is present, but the content is still being cached, this is because URL signing is configured. The next section provides information about how to prevent responses from being cached.

Preventing caching

To prevent private information from being cached in Cloud CDN caches, do the following:

  1. Include a Cache-Control: private header in responses that should not be stored in Cloud CDN caches, or a Cache-Control: no-store header in responses that should not be stored in any cache, even a web browser's cache.
  2. Do not sign URLs that provide access to private information. When content is accessed by using a signed URL, it is potentially eligible for caching regardless of any Cache-Control directives in the response.

Cache keys

Each cache entry in a Cloud CDN cache is identified by a cache key. When a request comes into the cache, the cache converts the URI of the request into a cache key, and then compares it with keys of cached entries. If it finds a match, the cache returns the object associated with that key.

For backend services, Cloud CDN defaults to using the complete request URI as the cache key. For example, https://example.com/images/cat.jpg is the complete URI for a particular request for the cat.jpg object. This string is used as the default cache key. Only requests with this exact string match. Requests for http://example.com/images/cat.jpg or https://example.com/images/cat.jpg?user=user1 do not match.

You can change which parts of the URI are used in the cache key. While the filename and path must always be part of the key, you can include or omit any combination of protocol, host, or query string when customizing your cache key. Using cache keys describes how to customize your cache keys.

URI part Customization Example URLs that have the same cache key
Protocol Omit the protocol from the cache key.
  • https://example.com/images/cat.jpg
  • http://example.com/images/cat.jpg
Host Omit the host from the cache key.
  • https://example.com/images/cat.jpg
  • https://example2.com/images/cat.jpg
Query string

Omit the query string from the cache key.

Selectively omit or include portions of the query string.
  • https://example.com/images/cat.jpg?user=user1
  • https://example.com/images/cat.jpg?user=user2

In addition to including or omitting the entire query string, you can use portions of the query string by using include lists and exclude lists.

For backend buckets, the cache key consists of the URI without the protocol, host, or query string.

Thus, for a given backend bucket, the following URIs resolve to the same cached object:

  • http://example.com/images/cat.jpg
  • https://example.com/images/cat.jpg
  • https://example.com/images/cat.jpg?user=user1
  • http://example.com/images/cat.jpg?user=user1
  • https://example.com/images/cat.jpg?user=user2
  • https://media.example.com/images/cat.jpg
  • https://www.example.com/images/cat.jpg

Query string include list

You can selectively control which query string parameters Cloud CDN incorporates into cache keys. For example, if you create an include list of user, thenhttps://example.com/images/cat.jpg?user=user1&color=blue creates a cache key of https://example.com/images/cat.jpg?user=user1 that also matches https://example.com/images/cat.jpg?user=user1&color=red.

To use this option, you must include the query string, specify a non-empty include list, and not specify an exclude list.

Query string exclude list

You can selectively control which query string parameters Cloud CDN ignores by using an exclude list. For example, if you create an exclude list of user, all query string parameters except user are used in the cache key.

With the exclude list configured and an input of https://example.com/images/cat.jpg?user=user1&color=blue, Cloud CDN creates a cache key of https://example.com/images/cat.jpg?color=blue that also matches https://example.com/images/cat.jpg?user=user2&color=blue but not https://example.com/images/cat.jpg?user=user1&color=red.

To use this option, you must include the query string, specify a non-empty exclude list, and not specify an include list.

Vary headers

In addition to the request URI, Cloud CDN respects any Vary headers that origin servers include in responses. The Vary header indicates that the response varies depending on the client's request headers. For example, if a response specifies Vary: Accept, Cloud CDN uses one cache entry for requests that specify Accept: image/webp,image/*,*/*;q=0.8 and another for requests that specify Accept: */*.

Vary headers are sometimes used when serving compressed content. Cloud CDN does not compress or decompress responses itself, but it can serve responses that the origin server compressed. If your origin server chooses whether to serve compressed or uncompressed content based on the value of the Accept-Encoding request header, make sure that the response specifies Vary: Accept-Encoding.

Responses with Vary headers are cached only if the header has one of the values listed in Cacheable content.

Expiration times and validation requests

Each cache entry in a Cloud CDN cache has an expiration time defined by the Cache-Control: s-maxage, Cache-Control: max-age, and/or Expires headers in accordance with RFC 7234. If more than one is present, Cache-Control: s-maxage takes precedence over Cache-Control: max-age, and Cache-Control: max-age takes precedence over Expires.

When Cloud CDN receives a request, it looks up the corresponding cache entry and checks its age. If the cache entry exists and is fresh enough, the response can be served from the cache. However, if the expiration time has passed, Cloud CDN cannot serve a response without first contacting one of your backends.

Cloud CDN revalidates cached objects that are older than 30 days. This allows for automatic invalidation and eviction of stale, user-generated cached content. When a max-age or s-maxage value exceeds 30 days (2,592,000 seconds), Cloud CDN treats the value as if it were 2,592,000 seconds. Downstream clients still see the accurate values of max-age and s- maxage, even if they exceed 30 days.

If the previously cached response has Last-Modified and/or ETag headers, Cloud CDN can attempt to use the information in those headers to validate the cache entry with the backend. Cloud CDN performs this validation slightly differently depending on whether the response was cached by using byte range requests:

  • If the response was cached by using byte range requests, Cloud CDN initiates a separate validation request that includes If-Modified-Since and/or If-None-Match headers.
  • Otherwise, Cloud CDN adds If-Modified-Since and/or If-None-Match headers to the client request and forwards the modified request to the backend.

If the cached copy is still up to date, the backend can validate the existing cache entry by sending a 304 Not Modified response. In this case, the backend sends only the response headers, not the response body. Cloud CDN inserts the new response headers into the cache, updates the expiration time, and serves the new response headers and cached response body to the client.

If the previously cached response does not have a Last-Modified or an ETag header, Cloud CDN ignores the expired cache entry and forwards the client request to the backend unmodified.

A cache entry's expiration time is an upper bound on how long the cache entry remains valid. There is no guarantee that a cache entry remains in the cache until it expires. Cache entries for unpopular content can be evicted to make room for new content. Regardless of the specified expiration time, cache entries that aren't accessed for 30 days are automatically removed.

For more information, see Eviction and expiration.

Support for byte range requests

A response that satisfies the following criteria indicates that the origin server supports byte range requests:

  • Status code: 200 OK or 206 Partial Content
  • Header: Accept-Ranges: bytes
  • Header: Content-Length and/or Content-Range
  • Header: ETag with a strong validator
  • Header: Last-Modified

Cloud Storage supports byte range requests for most objects. However, Cloud Storage does not support byte range requests for objects with Content-Encoding: gzip metadata unless the client request includes an Accept- Encoding: gzip header. If you have Cloud Storage objects larger than 10 MB, make sure that they do not have Content-Encoding: gzip metadata. For information about how to edit object metadata, see Viewing and editing object metadata.

Popular web server software also supports byte range requests. Consult your web server software's documentation for details about how to enable support. For more information about byte range requests, see the HTTP specification.

When an origin server supports byte range requests, a Cloud CDN cache declines to store an otherwise cacheable response the first time it is requested if either of the following is true:

  • The response body is incomplete because the client requested only part of the content.
  • The response body is larger than 1 MB (1,048,576 bytes).

When this happens and the response would otherwise satisfy the normal cacheability requirements, the cache records that the origin server supports byte range requests for that cache key and forwards the origin server's response to the client.

On a cache miss, the cache checks whether the origin server is known to support byte range requests. If byte range requests are known to be supported for the cache key, the cache does not forward the client request to the external HTTP(S) load balancer. Instead, the cache initiates its own byte range cache fill requests for the missing parts of the content. If your origin server returns the requested byte range in a 206 Partial Content response, the cache can store that range for future requests.

A cache stores a 206 Partial Content response only when it is received in response to a byte range request that the cache initiated. Because a cache doesn't initiate a byte range request unless it had previously recorded that the origin server supports byte range requests for that cache key, a given cache doesn't store content that's larger than 1 MB until the second time that content is accessed.

Requests initiated by Cloud CDN

When your origin server supports byte range requests, Cloud CDN can send multiple requests to your origin server in reaction to a single client request. Cloud CDN can initiate two types of requests: validation requests and byte range requests.

If the response that indicated that your origin server supported byte range requests for a particular cache key has expired, Cloud CDN initiates a validation request to confirm that the content hasn't changed and that your origin server still supports range requests for the content. If your origin server responds with a 304 Not Modified response, Cloud CDN proceeds to serve the content by using byte ranges. Otherwise, Cloud CDN forwards your origin server's response to the client. You control expiration times by using the Cache-Control and Expires response headers.

On a cache miss, Cloud CDN initiates cache fill requests for a set of byte ranges that overlap the client request. If some ranges of the content requested by the client are present in the cache, Cloud CDN serves whatever it can from the cache and sends byte range requests for only the missing ranges to your origin server.

Each byte range request initiated by Cloud CDN specifies a range that begins at an offset that's a multiple of 2,097,136 bytes. With the possible exception of the final range, each range is also 2,097,136 bytes. If the content isn't a multiple of that size, the final range is smaller. The size and offsets used in byte range requests might change in the future.

As an example, consider a client request for bytes 1,000,000 through 3,999,999 of content that is not present in the cache. In this example, Cloud CDN could initiate two GET requests, one for the first 2,097,136 bytes of the content and another for the second 2,097,136 bytes. This results in 4,194,272 bytes of cache fill even though the client requested only 3,000,000 bytes.

When you use a Cloud Storage bucket as your origin, each GET request is billed as a separate Class B operation. You are charged for all GET requests processed by Cloud Storage, including any requests initiated by Cloud CDN. When a response is served entirely from a Cloud CDN cache, no GET requests are sent to Cloud Storage, and you are not charged for any Cloud Storage operations.

When Cloud CDN initiates a validation request or byte range request, it does not include client-specific headers such as Cookie or User-Agent.

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