Why is the OPTIONS method sent in an HTTP request before executing GET/POST requests?

The OPTIONS method is used to determine the available methods and other parameterized options for a web resource or server without requiring any special action or access to the resource itself. It is part of the CORS (Cross-Origin Resource Sharing) mechanism.

When a web page tries to make a request to a resource on another domain (cross-origin request), the browser may send a preflight request with the OPTIONS method. This request includes headers like Access-Control-Request-Method and Access-Control-Request-Headers, indicating which HTTP method and headers will be used in the actual request.

The server receiving the OPTIONS request can respond with headers such as Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers, and Access-Control-Max-Age to inform the browser whether the actual request is permitted and which methods/headers are allowed.

If the server permits the request, the browser proceeds with the actual request (GET, POST, etc.). If not, the request is blocked by the browser, and the actual request is not sent, thus protecting the server from unauthorized cross-origin requests.

The OPTIONS request is not sent before every GET/POST request, but only before certain types of cross-origin requests considered "non-simple". Simple requests (e.g., GET or POST with certain content types and no custom headers) are sent without a preflight OPTIONS request.

# Example of OPTIONS request headers
# Access-Control-Request-Method: POST
# Access-Control-Request-Headers: Content-Type, Authorization
# Origin: http://client-domain.com

# Example of server response headers to OPTIONS
# Access-Control-Allow-Origin: http://client-domain.com
# Access-Control-Allow-Methods: POST, GET, OPTIONS
# Access-Control-Allow-Headers: Content-Type, Authorization
# Access-Control-Max-Age: 86400 # Cache the preflight result for 24 hours

In general, this is a security mechanism to control access to resources during cross-origin requests, allowing the server to determine whether the browser intends to perform permitted actions before sending the main request.