hdfs、yarn、hbase這些組件的master支持多個，實現自動主備切換，其中hdfs、hbase無論訪問主master或者備master都可以正常訪問頁面，但是yarn比較特別，只有主master的頁面可以訪問，備master會返回Refresh，3s后重定向；
一種方式是提供兩個域名，分別對應兩個yarn的master，一旦有master切換，需要手工切換到另外一個，有沒有更好的方式？
訪問備master過程如下：
curl http://standby_ip:8088/cluster -v
* About to connect() to standby_ip port 8088 (#0)
* Trying standby_ip...
* Connected to standby_ip (standby_ip) port 8088 (#0)
> GET /cluster HTTP/1.1
> User-Agent: curl/7.29.0
> Host: standby_ip:8088
> Accept: */*
>
< HTTP/1.1 200 OK
< Cache-Control: no-cache
< Expires: Tue, 25 Sep 2018 03:59:22 GMT
< Date: Tue, 25 Sep 2018 03:59:22 GMT
< Pragma: no-cache
< Expires: Tue, 25 Sep 2018 03:59:22 GMT
< Date: Tue, 25 Sep 2018 03:59:22 GMT
< Pragma: no-cache
< Content-Type: text/plain; charset=UTF-8
< Refresh: 3; url=http://active_ip:8088/cluster
< Content-Length: 103
< Server: Jetty(6.1.26)
<
This is standby RM. Redirecting to the current active RM: http://active_ip:8088/cluster
* Connection #0 to host standby_ip left intact
可見備master響應http status為200，包含Refresh頭，同時body為This is standby RM. Redirecting to the current active RM:***
有沒有可能在load balancer（比如nginx）上配置實現自動切換？這里非法響應需要判斷header包含‘Refresh’或者判斷body包含‘This is standby RM’
1）被動切換，支持http status以及timeout等判斷，不滿足
http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_next_upstream
proxy_next_upstream error | timeout | invalid_header | http_500 | http_502 | http_503 | http_504 | http_403 | http_404 | http_429 | non_idempotent | off ...;
2）主動健康檢查，最常用的開源module，只支持http status級別的健康檢查，不滿足
https://github.com/yaoweibin/nginx_upstream_check_module
check_http_expect_alive
syntax: *check_http_expect_alive [ http_2xx | http_3xx | http_4xx |
http_5xx ]*
default: *http_2xx | http_3xx*
context: *upstream*
description: These status codes indicate the upstream server's http
response is ok, the backend is alive.
3）nginx收費版本的ngx_http_upstream_hc_module，支持header和body的判斷，可以滿足
Dynamically configurable group with periodic health checks is available as part of our commercial subscription:
http://nginx.org/en/docs/http/ngx_http_upstream_hc_module.html
match active {
body !~ "This is standby RM";
}