When a cache has a stale entry that it would like to use as a
response to a client's request, it first has to check with the origin
server (or possibly an intermediate cache with a fresh response) to
see if its cached entry is still usable. We call this "validating"
the cache entry. Since we do not want to have to pay the overhead of
retransmitting the full response if the cached entry is good, and we
do not want to pay the overhead of an extra round trip if the cached
entry is invalid, the HTTP/1.1 protocol supports the use of
conditional methods.
The key protocol features for supporting conditional methods are
those concerned with "cache validators." When an origin server
generates a full response, it attaches some sort of validator to it,
which is kept with the cache entry. When a client (user agent or
proxy cache) makes a conditional request for a resource for which it
has a cache entry, it includes the associated validator in the
request.
The server then checks that validator against the current validator
for the entity, and, if they match, it responds with a special status
code (usually, 304 (Not Modified)) and no entity-body. Otherwise, it
returns a full response (including entity-body). Thus, we avoid
transmitting the full response if the validator matches, and we avoid
an extra round trip if it does not match.
Note: the comparison functions used to decide if validators match
are defined in section 13.3.3.
In HTTP/1.1, a conditional request looks exactly the same as a normal
request for the same resource, except that it carries a special
header (which includes the validator) that implicitly turns the
method (usually, GET) into a conditional.
The protocol includes both positive and negative senses of cache-
validating conditions. That is, it is possible to request either that
a method be performed if and only if a validator matches or if and
only if no validators match.
Note: a response that lacks a validator may still be cached, and
served from cache until it expires, unless this is explicitly
prohibited by a Cache-Control directive. However, a cache cannot do
a conditional retrieval if it does not have a validator for the
entity, which means it will not be refreshable after it expires.
