OPTIONS method in REST API

23 February 2023

The OPTIONS method in REST API is used to retrieve information about the communication options available for a resource.

It allows a client to discover the available methods and the communication options that the server supports, such as the supported HTTP methods, headers, and the format of the request and response.

Components of OPTIONS request

A typical OPTIONS request will include the following:

  • The HTTP verb “OPTIONS”.
  • The endpoint or resource location, such as “/products”.
  • The headers, which may include additional information such as the accept type or the authentication information.
  • The request body is empty as it is not needed for an OPTIONS request.

Components of OPTIONS response

A typical OPTIONS response will include the following:

  • The HTTP status code, such as 200 OK if the request was successful or 403 Forbidden if the client is not authorized to access the requested resource.
  • The headers, which may include additional information such as the Allow header which indicates which HTTP methods are supported by the resource, and the content type or caching directives.
  • The response body is empty as it is not needed for an OPTIONS request.

It’s worth noting that the OPTIONS method is not widely used in REST APIs, but it’s considered a best practice to implement it to allow clients to discover the communication options available.

Preflight call in REST API

A Preflight call, also known as an OPTIONS request, is an HTTP request sent by a client to a server as part of the Cross-Origin Resource Sharing (CORS) mechanism. CORS is a security feature implemented in web browsers to prevent web pages from making requests to servers outside their origin domain, unless explicitly allowed by the server.

When a web page tries to make a cross-origin request (i.e., a request to a different domain, protocol, or port), the browser automatically sends an OPTIONS request to the server to check if the actual request (e.g., GET, POST, PUT) is allowed from the origin domain. This OPTIONS request includes specific headers such as Access-Control-Request-Method and Access-Control-Request-Headers to inquire about the permissions.

The server then responds to this Preflight call with appropriate CORS headers, indicating whether the requested operation is allowed or not. If the server allows the operation, the browser proceeds with the actual request; otherwise, it blocks the request, preventing potential security vulnerabilities.

In summary, a Preflight call in REST API refers to the OPTIONS request sent by a browser to a server to determine whether cross-origin requests are permitted, as part of the CORS protocol.

Pros and Cons of OPTIONS method in REST API

The OPTIONS method in REST API offers several pros and cons:

Pros OPTIONS method in REST API

  1. Cross-Origin Resource Sharing (CORS) Compliance: The OPTIONS method plays a crucial role in CORS compliance by allowing servers to specify which origins, methods, and headers are permitted for cross-origin requests. This helps enhance the security of REST APIs by preventing unauthorized access from web applications hosted on different domains.
  2. Pre-flight Requests Optimization: By using the OPTIONS method, the client can determine the server’s CORS policy before making an actual request. This pre-flight mechanism saves bandwidth and server resources by avoiding unnecessary requests and responses, especially for requests that may be blocked due to CORS restrictions.
  3. Improved API Documentation and Discovery: When implemented effectively, the OPTIONS method can provide valuable metadata about the supported HTTP methods, headers, and other capabilities of the REST API. This information can be utilized for API documentation, auto-discovery, and client-side validation, enhancing the overall developer experience.
  4. Enhanced Security: By explicitly defining CORS policies with the OPTIONS method, REST APIs can mitigate certain security risks, such as cross-site request forgery (CSRF) attacks and unauthorized data access from malicious websites. CORS policies help enforce access controls and protect sensitive resources from unauthorized access.

Cons OPTIONS method in REST API

  1. Complexity of CORS Configuration: Setting up and configuring CORS policies, including handling OPTIONS requests, can be complex, especially for large and distributed systems. Inconsistencies or misconfigurations in CORS policies may lead to unexpected behavior, such as blocked requests or security vulnerabilities.
  2. Potential Overhead: While the pre-flight mechanism provided by the OPTIONS method helps optimize cross-origin requests, it can introduce additional overhead, especially for high-frequency API interactions. The need to perform an extra OPTIONS request before each actual request may impact the overall performance and latency of the API, particularly in scenarios with tight response time requirements.
  3. Browser Support and Compatibility: Although modern web browsers support CORS and the OPTIONS method, compatibility issues may arise with older browsers or custom user agents. Ensuring consistent behavior across different client environments and handling edge cases effectively can be challenging, particularly in mixed-browser or legacy systems.
  4. Exposure of API Metadata: While the OPTIONS method can expose valuable metadata about the API’s capabilities, it may also inadvertently reveal sensitive information to potential attackers. Careful consideration should be given to the information exposed in the response to OPTIONS requests to avoid leaking sensitive details about the API implementation or server configuration.

Conclusion

In conclusion, while the OPTIONS method in REST API provides essential capabilities for implementing CORS and enhancing security, it also introduces complexities and potential overhead that must be carefully managed and balanced against the benefits it offers.