Can I cancel a running server page from the browser?

[Larry Woods]

I have a server page that has served an HTML page with a "Cancel" button on
it, BUT this server page has not completed and is running a 15-30 second
process before it completes. I want the browser user to be able to
terminate the page before the server process has finished. What I tried was
an onClick event that issued a "location.href..." in hopes that this would
load this referenced page, etc. but I have found that the browser waits
until the server process had compled before executing the "location.href".
Is there some way that I can "signal" the server to stop processing so that
I can go to the "location.href" page; or is there some other way?

TIA,

Larry Woods

 

[Dave Anderson]

I have a server page that has served an HTML page with a "Cancel"
button on it, BUT this server page has not completed and is running
a 15-30 second process before it completes. I want the browser user
to be able to terminate the page before the server process has
finished...
...Is there some way that I can "signal" the server to stop
processing so that I can go to the "location.href" page; or is there
some other way?

You cannot directly cancel the request. The best you can do is have the ASP
process constantly monitor some independent entity (a session variable or a
record in a DB, for example), continuing only if some prearranged condition
is met. The cancel request would still be independent of the original
request, but this should be possible in principle.


--
Dave Anderson

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question is worth asking, it's worth posting.

 

[Larry Woods]

Thanks, Dave.

What would be the best way to set a Session variable for the browser? A
popup window ( 0 x 0 )?

TIA,

Larry

 

[Aaron Bertrand - MVP]

Use a hidden frame.
http://www.aspfaq.com/2001
http://www.aspfaq.com/2281

 

[Michael D. Kersey]

I have a server page that has served an HTML page with a "Cancel" button on
it, BUT this server page has not completed and is running a 15-30 second
process before it completes.

Long-running processes are not good for web servers. You should write
this application a different way. See
http://www.google.com/groups?hl=en&lr=&ie=UTF-8&selm=#eae9UZoCHA.1612@TK2MSFTNGP09
for a better way.

I want the browser user to be able to
terminate the page before the server process has finished.

Educate the user about the browser's "Stop" button. Alternatively the
user can click on another URL, select a bookmark, or enter a new URL.
When the Stop button is pressed, the TCP connection is reset and the
connection closed between client browser and server. Any queued output
will be discarded by whichever party (client, server or proxy agent) is
holding it.

Good Luck,
Michael D. Kersey

 

[Larry Woods]

The application is searching the Web using a second server; i.e., the ASP
server XMLHTTPs to a second server which is performing the searches.
USUALLY the second server responds in a fairly short time, but once in a
while it will be quite a while. It is possible that the user will want to
"bail out". Within the ASP server, I can abort the XMLHTTP session with the
second server if I know that I am to do it. We have implemented some
safeguards such as having the second server time out, and also having the
ASP server monitor the situation but there is STILL the possibility that the
user wants to terminate the "Please Wait" page.

Larry

 

[Larry Woods]

That is a good idea, Aaron, but this solution is "IE only" and even though I
wish that I could depend on that this site is a public site so I can't
assume IE. Any other suggestions?

TIA,

Larry

 

[Larry Woods]

I'll look into the Google suggestion.

I want the user to terminate the page, but not the present session. In
another response to my question I have explained my application. The delay
is a possible looooong Web search by the server.

TIA,

Larry

 

[Bob Barrows]

Well, you can use xmlhttp to run a second server-side page that sets a
session cancel variable to true can't you?

Bob

 

[Aaron Bertrand - MVP]

That is a good idea, Aaron, but this solution is "IE only"

What? You can't use a hidden frame like this, in other browsers:

<frameset rows="99%, 1%">
<frame ...
<frame ...
</frameset>

???

 

[Aaron Bertrand - MVP]

It is possible that the user will want to "bail out".

And such a user would use the STOP button, why are you trying to invent your
own methodology to replace it?

 

[Michael D. Kersey]

I'll look into the Google suggestion.

I want the user to terminate the page, but not the present session.

Clicking on the Stop button resets the HTTP connection; it does not
affect the current session.

In another response to my question I have explained my application. The delay
is a possible looooong Web search by the server.

Consider doing the XMLHTTP fetches in a background task or program and
not by an ASP page if they require more than a few seconds of
processing. Otherwise your own web server's memory and CPU will be tied
up waiting.

A periodic VBScript batch run or a VB program can do the XMLHTTP
searches and won't bog down your web server. Users' experiences will be
better since they are free to do other browsing while waiting for the
XMLHTTP searches to complete: users could check results hours or days
later. When your web server goes down, requests are not lost. When your
database server goes down, once it is restarted requests will be
requeued automatically (the user doesn't have to re-enter them). You
would get a more reliable, better-performing system.

You could also provide users with a mechanism for killing a search if
that search takes too long or if a user changes his/her mind and decides
the search results are not necessary. All that is possible with the
suggestion that I made on Google, but most is not available to you
otherwise.

Good Luck,
Michael D. Kersey

 

[Larry Woods]

Sorry, guys! Just going by the statement by the author at the beginning of
the article on aspfaq. Didn't check any further. Figured he knew. Oooops!

Larry

 

[Michael D. Kersey]

Michael,
Could you clarify one of your statements:
"Otherwise your own web server's memory and CPU will be tied up waiting."
Are you saying that an ASP site does not multi-thread?

No. What I meant was that:
- each thread that loads an XMLHTTP component waits for completion of
that component's request,
- that wait apparently can take many seconds, or can timeout, or can
fail,
- during that wait time, the resources (memory, CPU) used by that thread
are tied up and are unavailable for processing other HTTP requests.

ASP (indeed, HTTP, but that is a bigger story) is designed so that
requests are received, processed quickly and responses returned. Waiting
for *any* I/O process (file access, database access, an XMLHTTP request
to another website) ties up resources (CPU and memory). However, while
file access and database access are typically performed in micro- or
milli-seconds, an HTTP request is measured in seconds or tens of
seconds. And the likelihood of timeout or failure is much, much higher
for an HTTP request to a remote server than for a database or file
access on a local server.

Summary: long-running ASP scripts can bog down a web server if many
requests arrive in a short time.

Good Luck,
Michael D. Kersey

 

[Adrian Forbes - MVP]

The ASP page can see if the client is still active by
checking Response.IsClientConnected (I think it is a
memebr of Response, check the docs for more details) and
if the client is not connected then stop processing the
page.

Alternatively you could try something detaild here;

http://www.aspfree.com/articles/1333,1/articles.aspx

Which is bascially Micahel's suggesting with code.

ASP is mutli-threaded but you don't get too many threads,
and the reason is that pages are supposed to execute very
quickly. When you have long-running tasks then those
threads can quickly get used up.

 

[Larry Woods]

Great info, Adrian..and article, also.

What determines the number of threads that ASP gets?

Larry

 

[Adrian Forbes - MVP]

What determines the number of threads that ASP gets?

IIS5 defaults to 25/processor and IIS4 10/processor but
you can change the defaults if you want.

http://msdn.microsoft.com/library/default.asp?
url=/library/en-us/dnserv/html/server02282000.asp

http://www.microsoft.com/serviceproviders/whitepapers/tunin
giis.asp

More worrying that IIS's thread pool is if your long-
running code is inside MTS/COM+ as that has even less
threads given to it.

 

[Larry Woods]

Ignorance on my part...

I am instantiating a dll (visual basic-based) that is doing a bunch of
calculations. I have not defined it within a transaction, or anything like
that. It don't access any of the ASP objects, etc. The exposes some
properties and methods. The user "feeds" some data via properties; then
executes a method, which does the calculations, returning a value.

Question: Is this running under MTS/COM+ ??

TIA,

Larry Woods