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 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.
|Served by||Backend service, backend bucket, or a custom origin with Cloud CDN enabled|
|In response to||
|Cache directive||Has a
Contains a valid
For example, a
|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
Cache-Control: public, max-age=3600 header. You can set different
values by using Cache-Control
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.
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.
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.
|Served by||Backend service, backend bucket, or a custom origin that doesn't have Cloud CDN enabled|
|Response directive||Response has a
|Request directive||Request has a
|Size||Larger than the maximum size|
These requirements might be relaxed in the future, allowing Cloud CDN to cache additional responses.
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
To prevent private information from being cached in Cloud CDN caches, do the following:
- Include a
Cache-Control: privateheader in responses that should not be stored in Cloud CDN caches, or a
Cache-Control: no-storeheader in responses that should not be stored in any cache, even a web browser's cache.
- 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-Controldirectives in the response.
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
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.||
|Host||Omit the host from the cache key.||
Omit the query string from the cache key.
Selectively omit or include portions of the query string.
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:
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
creates a cache key of
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
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
https://example.com/images/cat.jpg?user=user2&color=blue but not
To use this option, you must include the query string, specify a non-empty exclude list, and not specify an include list.
In addition to the request URI, Cloud CDN respects any
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
and another for requests that specify
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 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: max-age, and/or
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
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
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
maxage, even if they exceed 30 days.
If the previously cached response has
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
- Otherwise, Cloud CDN adds
If-None-Matchheaders 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
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:
206 Partial Content
ETagwith a strong validator
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
Encoding: gzip header. If you have Cloud Storage objects larger than
10 MB, make sure that they do not have
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
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