Question

i’m issuing a request using MSXML’s XmlHttpRequest object:

IXMLHttpRequest http = new XmlHttpRequest();
http.open("GET", "http://www.bankofcanada.ca/stat/fx-xml.xml", False, "", "");
http.send();

And the send succeeds, and i get my xml data.

Except that XmlHttpRequest didn’t actually hit the network (i can see there no actual http request issued). And Process Monitor shows the file is actually being served from my cache:

So i want to instruct the XmlHttpRequest user agent that any cached content older than 0 seconds is too old. The standards way to do this is to add a request header:

Cache-Control: max-age=0

to the send request:

http = new XmlHttpRequest();
http.open("GET", "http://www.bankofcanada.ca/stat/fx-xml.xml", False, "", "");
http.setRequestHeader("Cache-Control", "max-age=0");
http.send();

And the send succeeds, and i get my xml data.

Except that XmlHttpRequest didn’t actually hit the network (i can see there no actual http request issued). And Process Monitor shows the file is actually being served from my cache.

So what is wrong? Is max-age not doing what i think it does?

From RFC 2616 – Hypertext Transfer Protocol, Part 14: Header Field Definitions:

Other directives allow a user agent to
modify the basic expiration mechanism.
These directives MAY be specified on a
request:

max-age
Indicates that the client is
willing to accept a response whose age
is no greater than the specified time
in seconds. Unless max- stale
directive is also included, the client
is not willing to accept a stale
response.

Which exactly what i want.

Is Cache-Control: max-age=0 not exactly what i want, or is MSXML’s XmlHttpRequest object buggy?

Update One

This is the MSXML XmlHttpRequest COM object:

• CLSID: {88d96a0a-f192-11d4-a65f-0040963251e5}
• ProgID: Msxml2.XMLHTTP.6.0

Update Two

The max-age directive is added by the client for all cache’s to adhere to. From RFC:

The Cache-Control general-header field
is used to specify directives that
MUST be obeyed by all caching
mechanisms along the request/response
chain. The directives specify behavior
intended to prevent caches from
adversely interfering with the request
or response. These directives
typically override the default caching
algorithms. Cache directives are
unidirectional in that the presence of
a directive in a request does not
imply that the same directive is to be
given in the response.

Max-age is not for the server; it makes no sense for a server. It is intended for all caching systems between the user and the server.

Update Three

From W3C XmlHttpRequest:

If the user agent implements a HTTP cache it should respect
Cache-Control request headers set by
the setRequestHeader() (e.g.,
Cache-Control: no-cache bypasses the
cache). It must not send Cache-Control
or Pragma request headers
automatically unless the end user
explicitly requests such behavior
(e.g. by reloading the page).

Following their example, i tried using the no-cache directive:

http = new XmlHttpRequest();
http.open("GET", "http://www.bankofcanada.ca/stat/fx-xml.xml", False, "", "");
http.setRequestHeader("Cache-Control", "no-cache");
http.send();

And the XmlHttpRequest client still services requests completely from the cache, without querying the server at all.

The W3C says that if there is a cache, it must honor Cache-Control if it is set through setRequestHeader. Microsoft’s XmlHttpRequest doesn’t seem to honor that requirement.

Answer 1

Unfortunately the XMLHttpRequest object was designed this way, because it is based on WinInet. Also, it is not recommend to be used from the server side. You should use ServerXMLHttpRequest, which has the same functionality, but depends on WinHTTP instead. See the FAQ for more information. A description from the ServerXMLHttp documentation states that:

The HTTP client stack offers longer
uptimes. WinInet features that are not
critical for server applications, such
as URL caching, auto-discovery of
proxy servers, HTTP/1.1 chunking,
offline support, and support for
Gopher and FTP protocols are not
included in the new HTTP subset.

This means that rather than using XmlHttpRequest:

IXMLHTTPRequest http = CreateComObject("Msxml2.XMLHTTP.6.0");     http.open("GET", "http://www.bankofcanada.ca/stat/fx-xml.xml", False, "", "");
http.setRequestHeader("Cache-Control", "max-age=0");
http.send();

you can use ServerXmlHttpRequest:

IXMLHTTPRequest http = CreateComObject("Msxml2.ServerXMLHTTP");
http.open("GET", "http://www.bankofcanada.ca/stat/fx-xml.xml", False, "", "");
http.setRequestHeader("Cache-Control", "max-age=0");
http.send();

or WinHttpRequest:

IWinHttpRequest http = CreateComObject("WinHttp.WinHttpRequest.5.1");
http.open("GET", "http://www.bankofcanada.ca/stat/fx-xml.xml", False, "", "");
http.setRequestHeader("Cache-Control", "max-age=0");
http.send();