Support for HTTP response headers

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This topic describes how Edge handles HTTP/1.1 caching headers when you're using the ResponseCache policy. Apigee Edge currently supports a subset of the HTTP/1.1 caching headers and directives (unsupported features are listed in this topic) received from backend target (origin) servers.

In addition, with certain headers Edge takes action based on their directives. In some cases, these HTTP/1.1 cache headers override whatever behavior is specified in the ResponseCache policy. For example, if the Cache-Control header is returned from a backend server, you can have the header's s-maxage directive potentially override other expiration settings in the policy.

Header	Support
Cache-Control	Supported on responses returned from backend origin servers, but not client requests. Edge supports a subset of directives.
Expires	Supported. Can be overridden.
Entity Tags (ETags)	Specific behavior for If-Match and If-None-Match.
If-Modified-Since	On GET requests, header is passed to the origin server even if a valid cache entry exists.
Accept-Encoding	Edge sends either compressed or uncompressed responses depending on the incoming headers.

Cache-Control

Apigee Edge supports the Cache-Control header only on responses returned from backend origin servers (the HTTP/1.1 spec allows Cache-Control headers in both client requests and origin server responses). Origin servers can include both target endpoints defined in an Apigee Edge API proxy and those created using TargetServer API calls.

Cache-Control support limitations

Apigee Edge supports a subset of Cache-Control response header capabilities defined in the HTTP/1.1 specification. Please note the following:

Apigee Edge does not support Cache-Control headers that arrive with inbound client requests.
Apigee Edge supports only the notion of public caches. (According to the HTTP specification, Cache-Control can either be public (shared) or private (single user).)
Apigee Edge supports only a subset of Cache-Control response directives in the HTTP/1.1 specification. See Support for Cache-Control response header directives for details.

Support for Cache-Control response header directives

Apigee supports a subset directives from the HTTP/1.1 specification on responses from origin servers. The following table describes Apigee Edge support for HTTP Cache-Control response header directives.

For more detailed information on the directives listed here, see Cache-Control in the HTTP/1.1 specification.

Cache-Control directive	How Apigee Edge processes the directive
`cache-extension`	Not supported.
`max-age`	If your ResponseCache policy sets the `<UseResponseCacheHeaders>` element to `true`, the response can be cached for the number of seconds specified by this directive. This directive is overridden by the `s-maxage` directive and overrides the `Expires` header. It can also be overridden by the policy's `<ExpirySettings>` element. For more, see "Setting cache entry expiration" and `<UseResponseCacheHeaders>` in Response Cache policy. Note: Content cached with the ResponseCache policy will retain the max-age value set by its origin. This means that when retrieved from the cache, the content might be older than its max-age value suggests (older than the client knows).
`must-revalidate`	Not supported. All cache entries are deleted by Apigee Edge as soon as they expire.
`no-cache`	Edge caches the origin response, but it must be revalidated with the origin server before being used to satisfy any subsequent client requests. This rule allows the origin to return a 304 Not Modified response in order to indicate that the response should be returned from cache, thus saving the processing required to return the entire response. If the origin server returns a full response, it replaces the existing cache entry. Any field names specified with this directive are ignored. Note: The HTTP/1.0 header, `Pragma: no-cache` is treated as equivalent to `Cache-Control: no-cache`.
`no-store`	Not supported.
`no-transform`	Not supported.
`private`	Not supported. If this directive is received, the origin response is not cached. Any field names are ignored.
`proxy-revalidate`	Not supported. All cache entries are deleted by Apigee Edge as soon as they expire.
`public`	Edge caches the origin response, even when other directives indicate otherwise. Per the HTTP/1.1 specification, the only exception to this rule is if the response includes an Authorization header.
`s-maxage`	If your ResponseCache policy sets the `<UseResponseCacheHeaders>` element to `true`, the response can be cached for the number of seconds specified by this directive. This directive is overrides the `max-age` directive and the `Expires` header. It can be overridden by the policy's `<ExpirySettings>` element. For more, see "Setting cache entry expiration" and `<UseResponseCacheHeaders>` in Response Cache policy. Note: Content cached with the ResponseCache policy will retain the s-maxage value set by its origin. This means that when retrieved from the cache, the content might be older than its s-maxage value suggests (older than the client knows).

Expires

When the UseResponseCacheHeaders flag in the ResponseCache policy is set to true, Edge can use the Expires header to determine the time to live (TTL) of a cached entry. This header specifies a date/time after which a response's cache entry is considered stale. This header allows servers to signal when it's okay to return a cached value based on a time stamp.

Note: The Cache-Control header directives max-age and s-maxage take precedence over and override the Expires header. For more, see "Setting cache entry expiration" and <UseResponseCacheHeaders> in Response Cache policy.

Acceptable date formats for the Expires header are described in the HTTP/1.1 specification. For example:

Expires: Thu, 01 Dec 1994 16:00:00 GMT

For detailed information on HTTP date/time formats, see Date/Time Formats in the HTTP/1.1 specification.

Note: While section 14.21 of the specification indicates that an Expires value of more than one year in the future means that a cache entry never expires, Apigee interprets such a value to mean that the entry should be cached until the specified date and time.

For more information on Expires header, see Header Field Definitions in the HTTP/1.1 specification.

ETag

An entity tag (ETag) is an identifier associated with a requested resource. Using an ETag, a server can determine if the requested resource and the associated cached resource match. For example, the server could re-cache the response if it doesn't match what's currently cached. It could return the cached resource if the ETags match.

When a target endpoint sends a response back to Edge with an ETag, Edge caches the ETag along with the response.

You can read more about Entity Tags in Protocol Parameters in the HTTP/1.1 specification.

If-Match

With the If-Match request header, a cached entity is current if the ETag in the header matches the cached ETag. Any requests other than GET that specify an If-Match header are passed on to the origin server to ensure that any origin caching facilities have a chance to process the request.

You can read more about If-Match in Header Field Definitions in the HTTP/1.1 specification.

If Edge receives an inbound GET request from a client that includes an If-Match header:

If	Then
The `If-Match` header specifies one or more ETags	Apigee Edge retrieves any unexpired cache entries for the specified resource and compares any strong ETags on those cached entries with those specified in the `If-Match` header. If a match is found, the cache entry is returned. If not, the request is passed to the origin server.
The `If-Match` header specifies "*"	The request is passed on to the origin server to ensure that any origin caching facilities have a chance to process the request
A cache entry with the same request URI is found, but it contains only weak ETags	The entry must be revalidated by the origin server before being returned to the client
The ETags comes from the origin server.	The ETag is returned unchanged to the client

If-None-Match

With the If-None-Match header, a cached entity is current if the ETag in the header does not match the cached ETag. Requests other than a GET that contain this header are passed to the origin server.

If Edge receives an inbound GET request with this header:

If	Then
The `If-None-Match` header specifies one or more ETags	Apigee Edge retrieves any unexpired cache entries for the specified URI and compares any strong ETags on those cached entries with those specified in the `If-None-Match` header. If a match is found, Edge returns a 304 Not Modified status. If no match is found, Edge passes the request to the origin server.
The `If-None-Match` header specifies "*" and an unexpired cached entry for the requested URI exists	Edge returns a 304 Not Modified status
A cache entry with the same request URI is found but contains only weak ETags	The entry must be revalidated by the origin server before Edge returns it to the client
Edge receives an ETag from an origin server	The ETag is returned unchanged to the client

If-Modified-Since

If Apigee Edge receives an If-Modified-Since header in a GET request, it is passed along to the origin server even if a valid cache entry exists.

This ensures that any updates to a resource that did not pass through Apigee Edge are accounted for. If the origin server returns a new entity, then Edge replaces the existing cache entry with the new value. If the server returns a 304 Not Modified status, Edge returns the response value if the cached response's Last-Modified header indicates that it has not changed.

Accept-Encoding

When an incoming request includes the header Accept-Encoding with values of gzip, deflate or compress, the origin server responds with compressed data. When subsequent requests come without the Accept-Encoding headers, they expect an uncompressed response. Apigee's response caching mechanism is capable of sending both compressed and uncompressed responses depending on the incoming headers without going back to the origin server.

You can have Accept header values appended to cache keys to make the keys more meaningful for each cached item. For more details, see "Configuring a cache key" in Response Cache policy.