HTTP support for caching & replication

Conditional requests Server executes conditional request. Responds with a message body only if the condition is true. Condition false: 304 Not Modified Servers provide meta-information through last-modified and ETag. Example ETag: MD5

Conditional Headers (caching) If-modified since (execute request if requested object updated since last- modified date). IMS requests. If-none-match (execute request if ETag of requested object is different).

Conditional Headers (replication) Conditional headers to be used with update requests. If-unmodified-since (execute request if the object’s last-modified date matches the supplied date). If-match (execute request if the object’s ETag matches the supplied ETag). If-match ≠ if-not-match, if-modified-since ≠ if-unmodified-since.

Age and Expiration of cached objects HTTP server: explicit time to live with expires and max-age headers. Expires: data up to which the cached object may be considered valid. Max-age: max age the object may reach before validation. Expires: absolute value of TTL. Max-age: specifies the TTL relative to the time the object left.

Request redirection 300 Multiple Choices (choices are provided in entity body). 301 Moved permanently (location header) 302 Found (location header) 303 See other (location header) 305 use proxy (location header) 307 Temporary redirect (location header)

Range Requests Range header: byte range of the entity being retrieved. Status line: 206 Partial content

Cache-control header The cache-control header contains multiple directives that control the use of all the caches between the client and the origin server. Cache-control: present in requests and responses (general header).

Cache-control in requests No-cache: cached objects cannot be used to satisfy the request. No-store: the response cannot be stored in a cache. Max-age: only younger cached objects can be used to satisfy the request. Min-fresh: only cached objects that will not expire for a specified time can be used. Max-stale: cached objects that expired up to the specified time ago can be used. No-transform: only the precise response as provided by the origin server can be user. Only-if-cached: a proxy should not forward the request on a cache miss.

Cache-control in responses No-cache: response cannot be cached. No-store: response cannot be stored in any client. Private: the response can be reused only for the client that originally requested it. Public: the response can be cached and shared among different clients. Must-revalidate: a cache must always validate the cached object. Proxy-revalidation: same as must-revalidate but applies to proxy caches only. Max-age: a cache must validate this object before serving it once the object age reaches specified value.