JSON content compression stops working after some time with Azure WebRole

Question

I have JSON compression configured for my Web API in Azure, following this MSDN article Use AppCmd.exe to Configure IIS at Startup.

I publish my roles and start testing and all is well according to Fiddler.

Here is an example request header:

GET http://x.cloudapp.net:8080/api/xyz HTTP/1.1
Accept: application/json
Host: x.cloudapp.net:8080
Accept-Encoding: gzip

Here is an example response header:

HTTP/1.1 200 OK
Cache-Control: no-cache
Pragma: no-cache
Content-Type: application/json; charset=utf-8
Content-Encoding: gzip
Expires: -1
Vary: Accept-Encoding
Server: Microsoft-IIS/8.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Thu, 18 Jul 2013 22:27:38 GMT
Content-Length: 2472

Just a few Web API calls later (like 6 seconds later) all responses are no longer compressed.

Request header:

GET http://xyz HTTP/1.1
Accept: application/json
Host: sp-test-server2012.cloudapp.net:8080
Accept-Encoding: gzip

Response header:

HTTP/1.1 200 OK
Cache-Control: no-cache
Pragma: no-cache
Content-Type: application/json; charset=utf-8
Expires: -1
Server: Microsoft-IIS/8.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Thu, 18 Jul 2013 22:27:44 GMT
Content-Length: 16255

Note the missing Content-Encoding in the second response.

So I get a few hundred calls that are compressed, and then most of the rest are uncompressed. Every now and again I can see that another response is compressed. Or if I stop testing for a while and then resume it seems that compression starts again.

Is compression in IIS 8 'throttled' or something? Say, if the CPU is nearly maxed out, does IIS stop compressing?

In monitoring my WebRole in Azure, my CPU usage can go above 90% during my heavy load testing. It is hard to tell if this is correlated with the lack of compression on the results. Memory usage does not appear to be an issue at all.

I would like this to be more reliable and predictable!

Answer 1

Well, apparently yesterday my Google Fu failed me. I found the answer today and it is true that IIS will or will not dynamically compress content based on CPU usage. HTTP Compression

There are two settings that control dynamic compression. One specifies when it is disabled: dynamicCompressionDisableCpuUsage, default 90%. Another specifies when it is re-enabled dynamicCompressionEnableCpuUsage, default 50%.

The things you learn.

Answer 2

This article might be helpful to force compression:

ASP.NET Web API GZip compression ActionFilter with 8 lines of code

Of course they'll charge for CPU time in heavy load situations.