Beware: BLOB cache may miss modifications via SPD

There are 2 alternative approaches to manually (not via a SharePoint solution) update content resources (.css, .js, page layouts, masterpage) that are provisioned in a SharePoint library. Via the browser: you can download the content resource/file, make the changes in the downloaded file, and upload, checkin and publish the modified file into the SharePoint library. Alternative approach is to open the SharePoint site in SharePoint Designer, open + edit the content resource direct from within SPD, and afterwards checkin + publish the modified file from SPD.

Although both approaches work to modify the content file administrated in a SharePoint library, the SPD alternative has a caveat. This week we noticed that the SharePoint BLOB cache may miss the modification trigger when the modification is done through SPD. One of our developers changed a CSS file via SPD, but when browsing the site did not see the effect of his modifications. I immediately had my suspicion towards the BLOB cache. To verify, I inspected the BLOB cache folder on the WFEs, and noticed that on all 3 of the WFEs the date of the file in cache was earlier as the published date of the file with modifications. All other files in the cache appeared up-to-date, so it was not a situation of complete BLOB cache corruption. Merely the file changed via SPD was outdated in the cache. Pragmatic resolution here is to delete the outdated file from the BLOB cache folder on all WFEs.