[jetty-users] How to check if the client dropped the connection

[Andrea Aime <andrea.aime@xxxxxxxxx>]

Hi,
I'm working on the development of an open source application, GeoServer,
implementing the Web Map Service specification.
The specification allows a client to request maps using simple GET
requests like:

http://sigma.openplans.org:8080/geoserver/wms?WIDTH=431&SRS=EPSG%3A4326&LAYERS=tiger-ny&HEIGHT=550&STYLES=&FORMAT=image%2Fpng&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&EXCEPTIONS=application%2Fvnd.ogc.se_inimage&BBOX=-74.01697805908182,40.69808217724622,-73.99567744140603,40.72526393994153

By spec we're forced to use a plain request and response approach, but
we're experiencing a problem with clients making lots of request as the
user zooms/pans around: basically a request is made, but the user keeps
on moving, the client drops the request and makes another.
Unfortunately the older requirest is still running, drawing a map takes
time and a lot of memory, for example the above request, which is a
small one btw, allocates a BufferedImage of 700KB. The memory is
consumed up until the image is encoded out to the stream, which is also
the moment we finally figure out the client dropped the connection
(since writing to the servlet output stream fails).

This is very sub-optimal. Servers like Apache with cgi do kill the cgi
process the moment the connection is dropped, significantly reducing the
server load both in terms of CPU and memory consumption.

Is there any way to check if the client connection is still open using
only the standard servlet API?
If not, is there any J2EE server specific approach that might work instead?
I've tried already to monitor the request input stream in the hopes
that a dropped connection would cause any operation against it
to throw an IOException, but that does not happen.
The output side is not helpful either, as I can start writing only
when the image is fully drawn.

As I said above the WMS is an international standard so I cannot touch
neither the protocol, and of course I have no control over what the
clients do either.
	
If there is no solution that can be applied at the general servlet
api level, do you know of any Jetty specific approach one could use?
e.g., casting the HttpServletResponse to some Jetty specific class
and get some connection information status there. I've looked a bit
but did not find anything.

Cheers
Andrea

--
Andrea Aime
OpenGeo - http://opengeo.org
Expert service straight from the developers.