image caching and image deferment control
Date: Thu, 21 Oct 93 21:18:55 EDT
Original-From: hotsand!ellson (John Ellson)
Message-id: <9310220118.AA15940@hotsand.dacsand>
Subject: image caching and image deferment control
I'm presently putting together an httpd server program that produces an
up-to-the-minute graph of the http access statistics of the server. 
I'm using ncsa-httpd, a shell script in the htbin directory to collect
and process the data, gnuplot to generate the graph and pbmplus to convert
the result to xbm.  The final result is returned to the client,
xmosaic-2.0pre6, as an inline image.

Two issues arise from this use (abuse?) of the inline image feature:

1. Each time you click on this link the server updates the statistics
and the graph image should change, therefore it would be useful to be able to
explicitly disable caching for these images, either in the URL, or
better with some sort of "valid-lifetime" parameter in the response
which could be set to zero. (At the moment I'm generating a unique
filename in each response, which works, but which leaves known useless
images in the client's cache.)

2. The graph is the information bearing response to an explicit action
by the user.  It makes no sense to defer image loading even if the browser
is in a dil mode for other graphics.  There should be a way to over-ride
dil mode for individual images either in the URL or in the response.

Specific proposals:  Problem 1. should be handled with a valid-lifetime
parameter because the server is in the best position to know the 
life of the data.  Problem 2. should be controlled by a flag in the
html because the html document author is the best judge of the display
context.  I suggest "<img no_dil src=xxx.xbm>"

Sound ok?