在HTTP协议中，响应状态码 429 Too Many Requests 表示在一定的时间内用户发送了太多的请求，即超出了“频次限制”。
在响应中，可以提供一个 Retry-After 首部来提示用户需要等待多长时间之后再发送新的请求。
429 Too Many Requests
HTTP/1.1 429 Too Many Requests Content-Type: text/html Retry-After: 3600
| Specification | Title |
|---|---|
| RFC 6585, section 4: 429 Too Many Requests | Additional HTTP Status Codes |
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