我找到解决办法了。
我在nginx（open resty）配置文件中做了以下更改

upstream algoapp {
   server 127.0.0.0.1:543;
}
upstream main {
   server 127.0.0.1:443;
}

location /rest/ws/login {
   proxy_set_header X-Forwarded-Host $host;
   proxy_set_header X-Forwarded-Server $host;
   proxy_set_header X-Real-IP $remote_addr;
   proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
   proxy_set_header X-Url-Scheme $scheme;
   proxy_set_header X-Forwarded-Proto $scheme;
   proxy_set_header Host $http_host;
   proxy_redirect off;
   if ($request_method = OPTIONS ) {
   proxy_pass https://127.0.0.1:543;
   }
   if ($request_method = POST ) {
   set $upstream '';
   access_by_lua '
   ngx.req.read_body()
   local data = ngx.req.get_body_data()
   local  match = ngx.re.match(ngx.var.request_body, "appId")
   if match then
      ngx.var.upstream = "algoapp"
   else
   ngx.var.upstream = "main"
   end
   ';
   proxy_pass https://$upstream;
   }
}

我能说的最好的问题是，在执行if语句时，变量$request_body可能没有被读入内存。
建议的替代方案是使用lua支持或compile nginx with the echo modules and run echo_request_body。

一个自动化测试的工作示例，以防对其他人有用：https://github.com/ericminio/learning-nginx/tree/master/payload_filter
配置

map $request_body $matching {
    default       'not_matching';
    include       /etc/nginx/conf.d/matching;
}

server {                                                                              
    default_type  text/plain;

    location /ping {
        return 200 'pong';
    }

    location / {  
        proxy_set_header 'X-Matching' $matching;
        proxy_pass http://localhost/routing;           
    } 

    location /routing {
        try_files /unknown-file-to-trigger-redirect @_$http_x_matching;
    }
    location @_matching {
        proxy_pass http://host.docker.internal:5015;
    }
    location @_not_matching {
        proxy_pass http://host.docker.internal:5066;
    } 

}

当使用集成时，这并不容易，比如在某些情况下，你的nginx并不允许你做额外的代理来改变整个设计，那么你可以尝试nginx-if-request-body来实现结果。

http {
 ....
     
 map "$uri" $forward_status {
    default 100; # 100 means nothing return, continue to proxy phase
    "~*.+?\.(css|js|bmp|gif|ico|jpeg|jpg|pict|png|svg|swf|tif)$" 418;
 }
 map "$request_body" $forward_status_by_body {
    default 100;
    "abc123xxx" 418;
    "~*.+?\.(css|js|bmp|gif|ico|jpeg|jpg|pict|png|svg|swf|tif)$" 418;
  }

server {
 ...
    error_page 418 =200 @welcome_if_request_body;

    location @welcome_if_request_body {
        add_header Content-Type text/plain;
        return 200 "welcome_if_request_body, you hit it";
    }

    location = / {
        if_request_body on;
        return_status_if_body_eq "ASD" 418 on;
        return_status_if_body_eq "foo" 418;
        return_status_if_body_eq "john" 418;
        return_status_if_body_startswith "report" 418;
        return_status_if_body_contains "report" 418;
        return_status_if_body_regex "^[\d]+?abc" 418;
        return_status_if_variable_map_to $forward_status;
        return_status_if_variable_map_to $forward_status_by_body;
        proxy_pass http://localhost:7777;

    }
...
  }
}