IIS: “Compress Application Files” breaks DSOs and TDCs

HTTP Compression is a wonderful idea.  Unfortunately, its implementation in IIS 5 was deeply flawed

Besides the usual issues, I’ve also found that HTTP Compression breaks Data Source Objects (DSOs) and Tabular Data Controls (TDCs) in IE (wonderful features for client-side data-binding in web apps).  IE loads the data (showing “XX items remaining” counting down in the status bar, and eventually “Done”), but it never data-binds to the table template or gives any error messages, leaving an empty table.  My guess is that IE uses a different mechanism for requesting DSO data resources from a webserver than it does for normal resources (pages and their referenced files like scripts, stylesheets and images).  I’ve never seen this issue documented by anyone else, which I attribute to the relative obscurity of DSOs and TDCs (they were never marketed to devs enough).

So you can imagine it was a big pain figuring this out the first time…

When IIS 6 came out, I heard a lot of excitement that it fixed HTTP Compression, so I was excited too.  Well I’ve recently/finally moved into IIS 6, enabled HTML compression, and then just got confused all over again today because my TDCs were broken.  Grrr…

Sure enough, turning off “Compress Application Files” (and restarting IIS) fixed it.  At least IIS 6 gives separate compression settings for dynamic and static files, so now I’m just compressing static files (e.g. HTML, CSS, JS, VBS, HTC), which should still help a bit (of course those files are cached by the browser anyway…).

Apparently Jeff Atwood has also found other big issues with IIS 6’s HTTP compression.   He’s got a lot of good info, plus a handy metabase snippet to help fix those problems.  Check it out.

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