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When doing upgrades of websites it’s often useful to be able to signal to users that your service is offline for maintenance either in part or in entirety which is quite straightforward to implement unless you’ve got something like an AngularJS or React app, that could well be cached in the browser, and that actually wants to respond to 503 status calls returned from a web based API. Then CORS has a habit of getting in the way.
To help with that I’ve just pushed this super simple and lightweight ASP.Net website to GitHub that will respond with a 503 status code to any request made of it while ensuring that the CORS protocol will succeed meaning that the 503 status code will make its way through to your own error handling.
It’s ideal for hosting in an Azure deployment slot during upgrades that swap slots.
Note: an alternative approach would be to use the URL rewriter in web.config. It’s not particularly intuitive or, to my taste, readable but I believe can be configured to perform the same task.