balancer:

[1] mod_proxy - Apache HTTP サーバ (2007-09-02 22:53:05 +09:00 版) http://httpd.apache.org/docs/2.2/ja/mod/mod_proxy.html#ProxyPass

Proxy ディレクティブのスキームが balancer:// になっている場合は、バックエンドサーバと実際には通信しない仮想ワーカーが生成されます。このワーカーは幾つかの "本物の" ワーカーの管理をつかさどります。この場合パラメータは、この仮想ワーカーに対して設定されます。

[2] mod_proxy - Apache HTTP Server Version 2.4 (2015-02-15 17:51:02 +09:00 版) http://httpd.apache.org/docs/current/en/mod/mod_proxy.html

A balancer worker is created if its worker URL uses balancer as the protocol scheme. The balancer URL uniquely identifies the balancer worker. Members are added to a balancer using BalancerMember.

[3] mod_proxy - Apache HTTP Server Version 2.4 (2015-02-15 17:51:02 +09:00 版) http://httpd.apache.org/docs/current/en/mod/mod_proxy.html#ProxyPass

If the Proxy directive scheme starts with the balancer:// (eg: balancer://cluster, any path information is ignored) then a virtual worker that does not really communicate with the backend server will be created.

[4] mod_proxy_ajp - Apache HTTP Server Version 2.4 (2015-01-02 20:49:24 +09:00 版) http://httpd.apache.org/docs/current/en/mod/mod_proxy_ajp.html

<Proxy balancer://cluster>
BalancerMember ajp://app1.example.com:8009 loadfactor=1
BalancerMember ajp://app2.example.com:8009 loadfactor=2
ProxySet lbmethod=bytraffic
</Proxy>
ProxyPass /app balancer://cluster/app

[5] mod_proxy_balancer - Apache HTTP Server Version 2.4, 2020-07-28T12:39:54.000Z, 2020-10-01T02:03:35.602Z https://httpd.apache.org/docs/current/en/mod/mod_proxy_balancer.html