HTTP status codes
Also written: 200, 201, 400, 404, 409, 429, 500, 502, 4xx, 5xx, 4xx and 5xx status codes, 409 Conflict, 502 Bad Gateway, Too Many Requests
Three-digit codes in an HTTP reply: 200 OK, 201 Created (new order), 400 Bad Request (bad input), 404 Not Found, 409 Conflict (too little stock), 429 Too Many Requests (slow down), 500 server error, and 502 Bad Gateway (a service we depend on failed). 4xx means the request itself is wrong, so retrying won't help; 5xx and 429 may pass and are worth a retry.