I am developing a backend system for my application on Google App Engine.
My application and backend server communicating with json. Like http://server.example.com/api/check_status/3838373.json or only http://server.example.com/api/check_status/3838373/
And I am planning to use CloudFlare for caching JSON pages.
Which one I should use on header? :
Content-type: application/json
Content-type: text/html
Is CloudFlare cache my server's responses to reduce my costs? Because I'll not use CSS, image, etc.
Answer
The standard Cloudflare cache level (under your domain's Performance Settings) is set to Standard/Aggressive, meaning it caches only certain types by default scripts, stylesheets, images. Aggressive caching won't cache normal web pages (ie at a directory location or *.html) and won't cache JSON. All of this is based on the URL pattern (e.g. does it end in .jpg?) and regardless of the Content-Type header.
The global setting can only be made less aggressive, not more, so you'll need to setup one or more Page Rules to match those URLs, using Cache Everything as the custom cache rule.
http://blog.cloudflare.com/introducing-pagerules-advanced-caching
BTW I wouldn't recommend using an HTML Content-Type for a JSON response.