[Yes, I know I'm complaining about ancient technology]
After using Response.Flush, any errors encountered will not "trigger" IIS's configured "Custom Error" page. I can't find any mention of this on MSDN.
I find it frustrating that there are several articles that suggest using Response.Flush for all kinds of performance improvements on slow pages. The articles always fail to mention this crippling "side-effect" of using it.
Does anyone know why it fails? Where's the KB article?
1) Configure a "Custom Error" using IIS mgr for the 500;100 error. Choose a File or URL and point to the page you'd like to have as your error message.
2) Create a Test.asp page, and make the contents the following:
<%@ Language=VBScript %>
<%
' TURN ON BUFFERING
Response.Buffer = TRUE
' FLUSH THE CONTENT AND HEADERS TO THE CLIENT
' Response.Flush()
' CAUSE AN ERROR
This_Function_Does_Not_Exist()
%>
3) Browse to the page and observe that your selected error page appears before you. Finally, UNcomment the "Response.Flush()" line of code. Reload page in your browser. Your "error handling" page is not invoked.
From my understanding of ASP, when you execute a flush you're basically sending the content of the processed page to the Web browser. The HTTP headers have already been written and sent to the browser once you send the Response.Flush. Basically you shut off
of the Buffer and sent out the response. Since the headers have already been sent to the browser you can longer issue a 500 http response code, and call your custom page. This isn't an error, it is just how HTTP works.
You are correct, that after Response.Flush has been called the headers are sent to the client. So, it is therefore impossible to change that 200OK into a 500 response code. I have no issues with that.
However, there seems to be no reason that the "Custom Error" page could not be invoked at the time of the error. Often times, developers used code in the "Custom Error" page to alert them that an error had occurred on their web site. If they use Response.Flush
before the error occurs their alerting/logging mechanism will not be triggered.
The only alternative (when using Response.Flush on a page) is to use the horrible OnError ResumeNext style error trapping.
However, there seems to be no reason that the "Custom Error" page could not be invoked at the time of the error. Often times, developers used code in the "Custom Error" page to alert them that an error had occurred on their web site. If they use Response.Flush
before the error occurs their alerting/logging mechanism will not be triggered.
The custom-error feature was not designed to logging/alerting when an error occurs but for sending a customized error message to the client - as it is too late to send a customized error message to the client at this point, no custom-error page is invoked.
Anil Ruia
Software Design Engineer
IIS Core Server
Just so we're clear, here is a relevant quote from the article:
"The examples mentioned in this article are simplified to demonstrate the Custom Error feature for 404 errors. They can be easily extended to handle most other HTTP errors and modified to take immediate action, like sending administrative alerts."
Regardless of what that document says, that is not how custom-error behaved on any IIS version - it does not even get invoked in many cases when the error status is set before the response is flushed to the client - write an aspx page which generates a 404
response and no custom-error will be invoked for it.
On IIS7, you may be able to write a module (native or managed) which registers for Log/EndOfRequest) notification and does any logging/administrative alert on failed requests.
Anil Ruia
Software Design Engineer
IIS Core Server
2. In Global.asax, add the following to Application_Error:
if (Convert.ToBoolean(HttpContext.Current.Items["responseFlushed"]))
{
HttpContext.Current.Server.Transfer("CustomErrorPage.htm");
}
That should do it. You can add additional code to get your actual customErrors settings from web.config using System.Web.Configuration instead of hard coding the custom error page file path. You might also want to make sure Server isn't null.
Note that you should use HttpContext.Current.Items and NOT HttpContext.Current.Session, as the Items collection will reset itself on each request, resetting Items["responseFlushed"] to false in the process.
Just so we're clear, here is a relevant quote from the article:
"The examples mentioned in this article are simplified to demonstrate the Custom Error feature for 404 errors. They can be easily extended to handle most other HTTP errors and modified to take immediate action, like sending administrative alerts."
@renniqer, The question here is about Classic Asp. Your answer is for Asp.Net, it does not apply.
Otherwise, the best I have found for handling this case it to put all the rendering in a Render sub, and use
On Error Resume Next
Render
If Err.number <> 0 Then
' Handle the error Response.Write "Error" End If
When an error occur in the render, the render sub is immediately interrupted and you can handle the error with whatever custom code you wish, including coming from an include. So that is no more a catch all solution put only in one place, but for pages requiring
that flush trick, well, just use one more trick for handling their errors.
4 Posts
Response.Flush breaks IIS's "Custom Errors"
May 23, 2006 03:29 PM|goinoutwest|LINK
[Yes, I know I'm complaining about ancient technology]
After using Response.Flush, any errors encountered will not "trigger" IIS's configured "Custom Error" page. I can't find any mention of this on MSDN.
I find it frustrating that there are several articles that suggest using Response.Flush for all kinds of performance improvements on slow pages. The articles always fail to mention this crippling "side-effect" of using it.
Does anyone know why it fails? Where's the KB article?
415 Posts
Microsoft
Re: Response.Flush breaks IIS's "Custom Errors"
May 23, 2006 06:02 PM|bills|LINK
Hi - could you post some sample code that reproduces the issue?
thanks-
bill
Bill Staples
Product Unit Manager, IIS
blog: http://blogs.iis.net/bills
4 Posts
Re: Response.Flush breaks IIS's "Custom Errors"
May 23, 2006 06:44 PM|goinoutwest|LINK
Yeah, here's some repro instructions:
1) Configure a "Custom Error" using IIS mgr for the 500;100 error. Choose a File or URL and point to the page you'd like to have as your error message.
2) Create a Test.asp page, and make the contents the following:
<%@ Language=VBScript %>
<%
' TURN ON BUFFERING
Response.Buffer = TRUE
' FLUSH THE CONTENT AND HEADERS TO THE CLIENT
' Response.Flush()
' CAUSE AN ERROR
This_Function_Does_Not_Exist()
%>
3) Browse to the page and observe that your selected error page appears before you. Finally, UNcomment the "Response.Flush()" line of code. Reload page in your browser. Your "error handling" page is not invoked.
Thanks for taking a look!
19 Posts
Re: Response.Flush breaks IIS's "Custom Errors"
Jun 01, 2006 09:56 AM|wwwcoder|LINK
4 Posts
Re: Response.Flush breaks IIS's "Custom Errors"
Jun 05, 2006 10:01 PM|goinoutwest|LINK
However, there seems to be no reason that the "Custom Error" page could not be invoked at the time of the error. Often times, developers used code in the "Custom Error" page to alert them that an error had occurred on their web site. If they use Response.Flush before the error occurs their alerting/logging mechanism will not be triggered.
The only alternative (when using Response.Flush on a page) is to use the horrible OnError ResumeNext style error trapping.
2343 Posts
Microsoft
Re: Response.Flush breaks IIS's "Custom Errors"
Jun 13, 2006 09:02 PM|anilr|LINK
The custom-error feature was not designed to logging/alerting when an error occurs but for sending a customized error message to the client - as it is too late to send a customized error message to the client at this point, no custom-error page is invoked.
Software Design Engineer
IIS Core Server
4 Posts
Re: Response.Flush breaks IIS's "Custom Errors"
Jun 13, 2006 09:44 PM|goinoutwest|LINK
I believe you are incorrect. The custom-error feature was designed for both. See this Microsoft article:
http://www.microsoft.com/technet/prodtechnol/windows2000serv/technologies/iis/tips/custerr.mspx
Just so we're clear, here is a relevant quote from the article:
"The examples mentioned in this article are simplified to demonstrate the Custom Error feature for 404 errors. They can be easily extended to handle most other HTTP errors and modified to take immediate action, like sending administrative alerts."
2343 Posts
Microsoft
Re: Response.Flush breaks IIS's "Custom Errors"
Jun 26, 2006 06:36 PM|anilr|LINK
Regardless of what that document says, that is not how custom-error behaved on any IIS version - it does not even get invoked in many cases when the error status is set before the response is flushed to the client - write an aspx page which generates a 404 response and no custom-error will be invoked for it.
On IIS7, you may be able to write a module (native or managed) which registers for Log/EndOfRequest) notification and does any logging/administrative alert on failed requests.
Software Design Engineer
IIS Core Server
1 Post
Re: Response.Flush breaks IIS's "Custom Errors"
Sep 14, 2006 03:37 PM|renniquer|LINK
I've experienced the same error, and here's the workaround I came up with:
1. Right after you call Response.Flush(), add this line of code:
HttpContext.Current.Items["responseFlushed"] = true;
2. In Global.asax, add the following to Application_Error:
if (Convert.ToBoolean(HttpContext.Current.Items["responseFlushed"]))
{
HttpContext.Current.Server.Transfer("CustomErrorPage.htm");
}
That should do it. You can add additional code to get your actual customErrors settings from web.config using System.Web.Configuration instead of hard coding the custom error page file path. You might also want to make sure Server isn't null.
Note that you should use HttpContext.Current.Items and NOT HttpContext.Current.Session, as the Items collection will reset itself on each request, resetting Items["responseFlushed"] to false in the process.
1 Post
Re: Response.Flush breaks IIS's "Custom Errors"
Oct 10, 2006 05:42 PM|legacycoder|LINK
Goinoutwest, have you been able to resolve this issue? We're having the exact same problem...
[quote user="goinoutwest"][quote user="anilr"]
The custom-error feature was not designed to logging/alerting when an error occurs but for sending a customized error message to the client
I believe you are incorrect. The custom-error feature was designed for both. See this Microsoft article:
http://www.microsoft.com/technet/prodtechnol/windows2000serv/technologies/iis/tips/custerr.mspx
Just so we're clear, here is a relevant quote from the article:
"The examples mentioned in this article are simplified to demonstrate the Custom Error feature for 404 errors. They can be easily extended to handle most other HTTP errors and modified to take immediate action, like sending administrative alerts."
1 Post
Re: Response.Flush breaks IIS's "Custom Errors"
Apr 18, 2019 08:15 PM|fredericDelaporte|LINK
@renniqer, The question here is about Classic Asp. Your answer is for Asp.Net, it does not apply.
Otherwise, the best I have found for handling this case it to put all the rendering in a Render sub, and use
On Error Resume NextRenderIf Err.number <> 0 Then' Handle the errorResponse.Write "Error"End IfWhen an error occur in the render, the render sub is immediately interrupted and you can handle the error with whatever custom code you wish, including coming from an include. So that is no more a catch all solution put only in one place, but for pages requiring that flush trick, well, just use one more trick for handling their errors.