caching images served by ASP

[Evertjan.]

Tim Williams wrote on 28 jun 2003 in
microsoft.public.inetserver.asp.general:

> It is pretty common to append a querystring to an image if you're
> always using the same src and the <image> is changing, but this only
> works if the querystring contains a random parameter value.

Random does not prevent a repeating of a value.

What it needs is a nonrepeating value,
which could be simply an increasing value.

In practice a simple pseudorandom value is easier on the web,
because it precludes the storing of such value.
That this goes wrong one in say 10,000 not many complaits will come in.
Perhaps not good enoug for a banking site?

> - the browser looks at the whole url and
> [should] only re-fetch the image if the url changes (though
> it may always do so, depending on the client-settings).

Not exactly if I read you well. The querystring is part of the url in the
desision whether to re-fetch or not-to-fetch. Otherwise this would go
Hamlettian wrong:

<img src="/getpicture.asp?picnr=7"><br>
<img src="/getpicture.asp?picnr=14"><br>

--
Evertjan.
The Netherlands.
(Please change the x'es to dots in my emailaddress)

[Evertjan.]

Michael King wrote on 29 jun 2003 in
microsoft.public.inetserver.asp.general:

> imgs = new Array();
> imgs[0] = new Image();
> imgs[0].src = "binimage.asp?id=100"
> imgs[1] = new Image();
> imgs[1].src = "binimage.asp?id=101"
> imgs[2] = new Image();
> imgs[2].src = "binimage.asp?id=102"
> ...
>
> This time, the when the prev function assigns an image from the array
> into the IMG element, the browser re-requests the image from the
> server.

I never use a DB that way, so I cannot test it.

Perhaps binimage.asp introduses a noncatching header ??

[Or needs a catching header?]

--
Evertjan.
The Netherlands.
(Please change the x'es to dots in my emailaddress)

[Michael King]

Dear all

Please accept my apology for this question - I have now sorted the issue out
(thanks to a TCP/IP trace).

It was due to invalid headers returned by my binimage.asp script which
prevented the browser from caching the response.

The fact the the browser still displays "downloading image ..." didn't help
!

"Michael King" <mzk_at_btopenworld.com> wrote in message
news:On3pAqkPDHA.2244@TK2MSFTNGP11.phx.gbl...

> I am not sure if I'm asking the right question, but the problem can be
> demonstrated as follows:
>
> If I use client-side JavaScript to preload static images and allow the

user

> to browse through them, it works as expected:
>
> imgs = new Array();
> imgs[0] = new Image();
> imgs[0].src = "static0.png"
> imgs[1] = new Image();
> imgs[1].src = "static1.png"
> imgs[2] = new Image();
> imgs[2].src = "static2.png"
> ...
> function prev() {
> if (i > 0) {
> i--;
> oImg.src = imgs[i].src;
> }
> }
>
> The activation of the prev function moves an image referenced in the imgs
> array into the IMG element, without requesting the image data from the
> server.
>
> However, when I use exactly the same code, but with images read from a DB,
> it doesn't work:
>
> imgs = new Array();
> imgs[0] = new Image();
> imgs[0].src = "binimage.asp?id=100"
> imgs[1] = new Image();
> imgs[1].src = "binimage.asp?id=101"
> imgs[2] = new Image();
> imgs[2].src = "binimage.asp?id=102"
> ...
>
> This time, the when the prev function assigns an image from the array into
> the IMG element, the browser re-requests the image from the server.
>
> I have tried setting the expires and cachecontrol properties in the
> binimage.asp script, but the images still don't cache !!
>
> If anyone can confirm that this is behaviour is by design, then please put
> me out of my misery ;)
>
>