How to apply CORS preflight cache to an entire domain

Question

I am building a REST application that makes use of CORS. Every REST call is different and I am finding that there is significant overhead in getting the preflight OPTIONS call. Is there a way to cache and apply a preflight OPTIONS result so that any subsequent calls to the same domain use the cached response?

Answer 1

Preflight can only be applied to the request, not to the entire domain. I brought the same question up on the mailing list, and there were security concerns. Here's the entire thread: http://lists.w3.org/Archives/Public/public-webapps/2012AprJun/0228.html

There are a few things to consider if you'd like to limit the number of preflight requests. First note that WebKit/Blink based browsers set a max preflight cache of 10 minutes:

https://github.com/WebKit/webkit/blob/master/Source/WebCore/loader/CrossOriginPreflightResultCache.cpp https://chromium.googlesource.com/chromium/blink/+/master/Source/core/loader/CrossOriginPreflightResultCache.cpp

(I'm not sure if this is true for other browsers). So while you should always set the Access-Control-Max-Age header, the max value is 10 minutes.

Next note that it is impossible to avoid a preflight on PUT/DELETE requests. So updates/deletes to your API will require at least one preflight every 10 minutes.

On GET/POST, avoid custom headers if at all possible, since these still trigger preflights. If your API returns JSON, note that a Content-Type of 'application/json' also triggers a preflight.

If you are willing to bend just how "RESTful" your API is, there are a few more things you can try. One is to use a Content-Type that doesn't need a preflight, like 'text/plain'. Custom headers always trigger preflights, so if you have any custom headers, you could move them into query parameters. At the extreme end, you could use a protocol like JSON-RPC, where all requests are made to a single endpoint.

In all honesty, because of the browser's preflight cache limit of 10 minutes, and REST's resource urls, the preflight cache is fairly useless. There's very little you can do to limit preflights over the course of a long running app. I'm hopeful the authors of the CORS spec will try to address this in the future.