How can I enable IIS7 to gzip static files like js and css and how can I test if IIS7 is really gziping them before sending to the client?
Why use deflate instead of gzip for text files served by Apache?
The simple answer is don't.
RFC 2616 defines deflate as:
deflate The "zlib" format defined in RFC 1950 in combination with the "deflate" compression mechanism described in RFC 1951
The zlib format is defined in RFC 1950 as :
0 1 +---+---+ |CMF|FLG| (more-->) +---+---+ 0 1 2 3 +---+---+---+---+ | DICTID | (more-->) +---+---+---+---+ +=====================+---+---+---+---+ |...compressed data...| ADLER32 | +=====================+---+---+---+---+
So, a few headers and an ADLER32 checksum
RFC 2616 defines gzip as:
gzip An encoding format produced by the file compression program "gzip" (GNU zip) as described in RFC 1952 . This format is a Lempel-Ziv coding (LZ77) with a 32 bit CRC.
RFC 1952 defines the compressed data as:
The format presently uses the DEFLATE method of compression but can be easily extended to use other compression methods.
CRC-32 is slower than ADLER32
Compared to a cyclic redundancy check of the same length, it trades reliability for speed (preferring the latter).
So ... we have 2 compression mechanisms that use the same algorithm for compression, but a different algorithm for headers and checksum.
Turns out many browsers over the years implemented an incorrect deflate algorithm. Instead of expecting the zlib header in RFC 1950 they simply expected the compressed payload. Similarly various web servers made the same mistake.
So, over the years browsers started implementing a fuzzy logic deflate implementation, they try for zlib header and adler checksum, if that fails they try for payload.
The result of having complex logic like that is that it is often broken. Verve Studio have a user contributed test section that show how bad the situation is.
For example: deflate works in Safari 4.0 but is broken in Safari 5.1, it also always has issues on IE.
So, best thing to do is avoid deflate altogether, the minor speed boost (due to adler 32) is not worth the risk of broken payloads.
There was a thread on forums.iis.net about this during the iis 7 beta. Turned out the guy didn't have the modules installed, but it sounds like you've ruled that out from your opening sentence.
Microsofts key advice for him was to enable failed request tracing to find out what was going wrong. This is possibly one of the most under-appreciated features of IIS7, but certainly one of the most powerful.
- Open IIS Manager.
- Go to your site, and on the actions pane (the very far right), click 'Failed Request Tracing...' under the 'Configure' section.
- Click 'enable'.
- Then, in the features view, click 'Failed request tracing rules'. Click add, next, enter 200 for the status code, next, click finish.
If you don't see "Failed Request Tracing" in the actions pane, you'll need to add the feature to the server - either using the "Add Role Services" wizard (Health and Diagnostics\Tracing) or through the Web Platform Installer (Products\Server\IIS: Tracing), and then close and re-open IIS Manager.
Next, rerun your test. This will generate some log info for us to examine.
Look in c:\inetpub\logs\FailedReqLogFiles\w3svcx. You will see a bunch of files named fr000xx.xml. Open up any one of them in your browser. (By the way, if you copy these files anywhere, make sure freb.xsl is there. Also, don't delete freb.xsl - if you do, just delete the whole directory or copy it from another location, as IIS only creates it once per folder.)
Click the 'request details' tab and select 'complete request trace'. Search the page for 'compress' - you should find it in several areas; once for static content, and once for dynamic content.
If you don't find either of them, IIS isn't configured correctly. If you do find them, you should see them followed by a compression_success and a compression_do. Success is self explanatory; the 'do' indicates what it did - in my case, it showed "OriginalSize 1462784 CompressedSize 179482"
Since yours isn't working, hopefully you will see something different that helps you solve the problem.
Make sure you turn this off when you're done by disabling failed request tracing in the actions pane for your website.