IIS Handlers responseBufferLimit="0"

[Eric318650369eow]

I could not find discussion on this topic and I think it could be serious for some environments.

On IIS cfm handler why there is no use of buffer ? the handlers are configured with responseBufferLimit="0". Is there any reason. It seems it could be a serious issue.

[Charlie Arehart]

"Serious", why? Are you aware of a clear detriment, or merely a suspected or hypothetical one?

Are you afraid that "having an unlimited response buffer" could negatively affect processing? Or perhaps that it can be abused? With respect to cf request processing, are you aware that CF itself has a built-in response buffer size of 1mb (set on the first page of the cf admin)? As such cf would not somehow "overrun" iis when it's set without a buffer limit?

I've not heard of this responsebufferlimit being a concern with respect to cf.. There used to be more talk out it with respect to classic asp--around the turn of the century. Indeed, the only significant discussion of it I find in cf circles is a post by Ben N from 2013, when he was still using cf9, and the jrun wildcard handler mapping had that 0 limit--and it affected serving of non-cf files. Cf10 and above no longer uses such a wildcard mapping (it uses an isapi filter instead). 

So again, tell us more about the "serious" concerns you have. We're always open to learning and solving problems. 

[BKBK · Accepted solution]

On IIS cfm handler why there is no use of buffer ? the handlers are configured with responseBufferLimit="0". Is there any reason. 

By @Eric318650369eow

Yes, I think there is at least one good reason for the configuration responseBufferLimit="0". By disabling response-buffering in the web server (IIS in this case), the setting ensures that output tags such as cfcontent (for streaming of file downloads) and cfflush will work as intended.

The web server is not the only response-bufferer. ColdFusion usually buffers output itself before sending it to IIS. That is to say, response-buffering is a shared responsibility between ColdFusion and IIS. Disabling response-buffering in IIS ensures that, by default, ColdFusion alone will have the responsibility of determining whether or not responses are buffered, and the maximum size of the buffer.. 

My guess is that ColdFusion is designed on the premise that the web server doesn't buffer responses. In other words, on the premise that responseBufferLimit="0" for the web server. This ensures that IIS sends output to the client immediately as it is received from ColdFusion. Otherwise, output tags such as cfcontent and cfflush will not work as intended. 

However, depending on the requirements of your application, the setting responseBufferLimit="0" could be a serious issue. For example, if your application outputs responses in many small chunks. Especially, under high concurrency (multi-threading). Then, if ColdFusion's own response-buffering is insufficient, a lot of network overhead will be generated. That would usually result in high CPU usage.

But, even for such use cases, you can usually find a way out.  You could, for example, implement solutions such as:

• Avoid or minimize response flushing;
• Avoid or minimize explicit response streaming;
• Wherever possible, aggregate (that is, buffer) many small chunks of output into a large response payload.

[Eric318650369eow]

Thanks. I was facing IIS CPU spikes. I was exploring this buffer setting to know if it could be a source of my issue. The response was no in my case. I was excpecting more documentation on this setting that is not a small one and I was questionning it in regard of Coldfusion application that is running on our servers.

For my understanding
responseBufferLimit="0" disables IIS buffering — data is flushed immediately to Coldfusion.

Concerning <cfcontent>, I changed it and it did not affected files delivered with <cfcontent>. I will put it back to 0 because content could be stream to client.

If we take <cfcontent> as an example. When you serve a file with <cfcontent> or via Java I/O streaming:

1. The request is received by IIS.
2. The ISAPI connector (via isapi_redirect.dll) sends it to the ColdFusion servlet (Tomcat).
3. ColdFusion generates the response (possibly buffering it itself).
4. The response goes back through the ISAPI connector to IIS.
5. IIS can then buffer or stream the response to the client depending on responseBufferLimit.

So:

• ColdFusion’s buffering happens first (especially if you use <cfcontent file="...">).

• IIS buffering is after ColdFusion, which can help smooth delivery to clients, or reduce TCP fragmentation, but won’t reduce ColdFusion memory usage.

For cfcontent there are many discussions over the years. Asking an AI about cfcontent and buffering today I have this answer. 

By default, when you use <cfcontent> to serve a file:

• ColdFusion reads the entire file into memory.

• Then it writes it to the output buffer, which is eventually flushed to the client.

• This can be inefficient or problematic for large files (e.g., videos, high-res images, large PDFs).

it proposes this solution that I will certainly implement soon.

<cfscript>
    filePath = expandPath("./largefile.mp4");
    response = getPageContext().getResponse();
    response.setContentType("video/mp4");

    file = createObject("java", "java.io.File").init(filePath);
    inputStream = createObject("java", "java.io.FileInputStream").init(file);
    outputStream = response.getOutputStream();

    buffer = createObject("java", "java.lang.reflect.Array").newInstance("byte", 8192);

    while ((len = inputStream.read(buffer)) != -1) {
        outputStream.write(buffer, 0, len);
    }

    inputStream.close();
    outputStream.flush();
    outputStream.close();
    abort;
</cfscript>

[Charlie Arehart]

Eric, thanks for the additional context. That said, this is your first mention of cfcontent. Are you saying you know that these cpu spikes relate to that?

I'd be surprised. Indeed, rather than focusing on such OUTPUT processing (cf sending results through IIS), I'd wonder instead about what was coming INTO IIS at that time. Indeed, it might be an impact of a change in the rate or nature of MANY incoming requests. There are various diagnostics you could assess such things, from iis logs to Windows Perfmon stats, and more.

But if you feel the code you've found is somehow the solution, do let us know how it goes. 

[Dave Watts]

Please don't implement that solution until you can accurately describe the problem you're having. If you're not having a problem, don't do anything!

First, this response is marked as the correct answer. I don't know who did that, but what question does it answer? The CFCONTENT tag works well enough for most uses. Have you encountered a problem with CFCONTENT?

Second, there's at least one error in your message. The IIS response buffer just buffers the response - the communication between IIS and the browser. If you're using IIS exclusively to serve a CF application, you almost certainly don't need to worry about the default IIS setting.

Third, none of your proposed solution has anything to do with IIS response buffering. In fact, I think you'd need to keep IIS response buffering disabled to get any value from your solution.

Fourth - and don't take this the wrong way - a lot of your messages in this thread have a bad AI smell. There's nothing wrong with using AI to help answer questions or learn something, but you don't want to use an AI solution without understanding the code it gives you, and without understanding your problem.

[Eric318650369eow]

Yes you are right, as I wrote before my IIS spikes was not related to this setting.

In my answer I was just trying to point that cfcontent is not directly affected by responseBufferLimit="0" as wrote before by BKBK "output tags such as cfcontent and cfflush will not work as intended." I had the same guess but I tested it, I put a buffer limit of 4mg and the cfcontent still worked as excpected. That's why I put the description of the request pipe. For cfflush I did not test it.

And yes I will not implement but testing it. I could not edit my post, I was thinking more about testing.
Consuming less buffer memory to prevent issues could be a good thing. More bots are coming on our websites and are consuming the web differently with more concurrent requests that could cause new "issues".

The "default" IIS setting is not with no buffering for most of the handlers. Somebody decided to use this setting for CF handler and I was wondering why. 

[BKBK]

@Eric318650369eow , thanks for sharing that. Your code is an implementation of the suggestion in the last sentence of my post: "Wherever possible, aggregate (that is, buffer) many small chunks of output into a large response payload."

[BKBK]

Concerning <cfcontent>, I changed it and it did not affected files delivered with <cfcontent>. I will put it back to 0 because content could be stream to client.

By @Eric318650369eow

Yes, setting responseBufferLimit back to 0 was the correct thing to do. As I said last week, the value responseBufferLimit="0" could only be a problem for <cfcontent>:
 
-  "if your application outputs responses in many small chunks. Especially, under high concurrency (multi-threading). "

and

-  "if ColdFusion's own response-buffering is insufficient" 

[Dave Watts]

Why does it seem it could be a serious issue, or any issue at all? It's sort of common* to set it to zero if you're using IIS with PHP via FastCGI, and IIS buffered output could cause problems rendering dynamic content if you have, say, short amounts of text to output.

* "sort of" in the sense that it's pretty rare to run PHP apps on IIS

[Charlie Arehart]

Eric, care to reply to the considered thoughts offered here? Or are you waiting to hear from someone else? Or do you feel the responses don;t meet your expectations?

[Charlie Arehart]

Eric offered a reply a couple days later, appearing within a thread above, here.