Ich gehe davon aus, dass du eine Website entwickelst

Du musst einfach einen HTTP-Request in JavaScript machen.

Ich glaube dir ist nicht ganz klar was mit xmlHttp.onreadystatechange passiert. Es handelt sich da um eine Callback-Funktion. Diese wird aufgerufen, sobald die Antwort zum Request vom Server kommt. Du solltest NICHT die Skriptausführung blockieren solange du wartest! Dann wird potentiell der ganze Browser hängen oder zumindest der entsprechende Tab im Browser.

Achja noch ein Tipp: manche Browser (BIRNE) halten sich nicht an den Standard und fordern ein Explizites "Cache-Control: no-cache, must-revalidate"

Wofür?

13.2.2 Heuristic Expiration

Since origin servers do not always provide explicit expiration times,
HTTP caches typically assign heuristic expiration times, employing
algorithms that use other header values (such as the Last-Modified
time) to estimate a plausible expiration time. The HTTP/1.1
specification does not provide specific algorithms, but does impose
worst-case constraints on their results. Since heuristic expiration
times might compromise semantic transparency, they ought to used
cautiously, and we encourage origin servers to provide explicit
expiration times as much as possible.

13.1.1 Cache Correctness

A correct cache MUST respond to a request with the most up-to-date
response held by the cache that is appropriate to the request (see
sections 13.2.5, 13.2.6, and 13.12) which meets one of the following
conditions:
[…]
2. It is "fresh enough" (see section 13.2). In the default case,
this means it meets the least restrictive freshness requirement
of the client, origin server, and cache (see section 14.9); if
the origin server so specifies, it is the freshness requirement
of the origin server alone.

If a stored response is not "fresh enough" by the most
restrictive freshness requirement of both the client and the
origin server, in carefully considered circumstances the cache
MAY still return the response with the appropriate Warning
header (see section 13.1.5 and 14.46), unless such a response
is prohibited (e.g., by a "no-store" cache-directive, or by a
"no-cache" cache-request-directive; see section 14.9).

A response received with a status code of 200, 203, 206, 300, 301 or
410 MAY be stored by a cache and used in reply to a subsequent
request, subject to the expiration mechanism, unless a cache-control
directive prohibits caching. However, a cache that does not support
the Range and Content-Range headers MUST NOT cache 206 (Partial
Content) responses.

A response received with any other status code (e.g. status codes 302
and 307) MUST NOT be returned in a reply to a subsequent request
unless there are cache-control directives or another header(s) that
explicitly allow it. For example, these include the following: an
Expires header (section 14.21); a "max-age", "s-maxage", "must-
revalidate", "proxy-revalidate", "public" or "private" cache-control
directive (section 14.9).

Es handelt sich da um eine Callback-Funktion.