Create a custom Salesforce Apex REST endpoint using @RestResource to handle GET and POST requests from external systems

In-depth guide

Steps

1. Annotate a global class with @RestResource(urlMapping='/your-resource/*') to register it as a REST endpoint under /services/apexrest/your-resource/
2. Annotate methods with @HttpGet, @HttpPost, @HttpPut, @HttpPatch, or @HttpDelete; methods must be global static and accept no parameters (use RestContext.request to read the body and URL parameters)
3. Parse the request body from RestContext.request.requestBody.toString() and deserialize it using JSON.deserialize() into a typed Apex class
4. Build the response by setting RestContext.response.responseBody, statusCode, and headers, or simply return a typed object from the method for automatic JSON serialization
5. Authenticate external callers using Connected App OAuth (JWT or auth code flow) and ensure the running user's profile has API access enabled
6. Test using RestRequest and RestResponse instances set on RestContext.request and RestContext.response before calling the method in a test class