Nginx 在 1.13.4 版本 release 的 mirror 模組 ngx_http_mirror_module，用途是能夠 mirror request
The ngx_http_mirror_module module (1.13.4) implements mirroring of an original request by creating background mirror subrequests. Responses to mirror subrequests are ignored.
可以針對 uri 來指定 mirror
Sets the URI to which an original request will be mirrored. Several mirrors can be specified on the same level.
看起來能應用在 Proxy or Load balancer 上來做分析 or debug