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I'm testing a 32-bits ISAPI extension on a web-server with IIS 7.5 and get strange behaviour in some cases. At a certain point in the web application, a URL serves a large (a few MB) JPEG file directly to the client (Content-type: image/jpeg).

The image gets transmitted for a first part, but the response seems to halt for an unknown reason. Nothing happens until, from another browser, another request comes in and gets handled. Then another few KB of the image gets sent to the client. Only if I fire a couple of small requests from the other browser I get the entire image and the response completes.

The ISAPI extension is configured as a wildcard mapping, no caching is configured. The application pool has the 32-bits setting enabled, and runs with a domain account as identity (to access things on the back-end). I tried switching 'Enable cache' and 'Enable kernel caching' off. I tried on IIS 5.1 and 6.0 and there it works as expected.

Is there another configuration setting I am overlooking, or did I stumble upon a bug in IIS 7.5?

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Any further detail on how it's being sent, what the extension is, etc? – TristanK Mar 27 '12 at 11:32
It's an extension that provides access to a gallery of photo's stored on the internal storage solution. The data is sent over the ECB's WriteClient function. – Stijn Sanders Mar 27 '12 at 12:39
up vote 1 down vote accepted


ISAPI extensions that send long responses to the client may not transmit any output in Internet Information Services 7.0 until the session is closed

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Thank you, for a moment I thought it was (temporarily) fixed by disabling the HttpCacheModule, but this is the real fix indeed.… – Stijn Sanders Mar 29 '12 at 19:39

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