Minimal Rack::Cors + Roda app

This minimal roda app demonstrates a cors preflight and subsequent POST request using curl.

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1) A cors preflight request inquires about access:

• The -H Origin: header identifies the caller making the preflight request. (i.e. The app which wishes to make cross origin requests to this endpoint.)
• The -X OPTIONS method specifier defines this as a preflight request.

curl --head \
  -H 'Origin: https://pizza-app.example.com' \
  -H 'Access-Control-Request-Method: POST' \
  -H 'Access-Control-Request-Headers: X-Requested-With' \
  -X OPTIONS \
  https://rack-cors-roda.herokuapp.com/pizza/toppings

2) Preflight response headers indicate what's possible:

• The origin which may consume data from this endpoint.
• Methods the origin can use with this endpoint.
• Headers the endpoint will allow the origin to read. (A default set is allowed.)
• Seconds before another preflight request is required. (Technically, the number of seconds a preflight response can be cached by the browser.)
• Which headers are allowed during the subsequent request.

Access-Control-Allow-Origin: https://rack-cors-roda.herokuapp.com
Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS, HEAD
Access-Control-Expose-Headers:
Access-Control-Max-Age: 7200
Access-Control-Allow-Headers: X-Requested-With

3) A subsequent POST request to that endpoint:

curl \
  --request POST 'https://rack-cors-roda.herokuapp.com/pizza/toppings' \
  --header 'Content-Type: text/plain' \
  --data-raw '{ "topping": "cheese" }'

4) Returns a JSON response:

{"your_topping":"cheese"}

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Notes:

• Testing your cors configuration using curl or postman does not guarantee browser requests will work. (YMMV)
• The cors standard is defined within the JavaScript Fetch API.
• Configuring cors on your API necessitates coordination with your front-end app.
• A cors preflight request lets the browser inquire if cors is supported and whether cors headers are understood. (Browsers make preflight requests as needed so front-end developers don't usually need to code them.)
• This curl testing methodology comes from this SO answer.
• Using the rack-cors gem:
  • Multiple origins are allowed
  • Multiple allow blocks are allowed
  • Origins must include scheme and no trailing slash
  • Using wildcard origin (Origins '*') enables requests from anywhere (i.e. calls to a public API)
  • Cookies sent from a cors endpoint with wildcard origin are not accepted by the calling browser. (This prevents abuse from public APIs with wildcard origins.)