haproxy/haproxy-sample.cfg

global
  ...

frontend mysql_ro
  <% (1..@nbproc.to_i).each do |proc| %>
  bind  0.0.0.0:3306 process <%= proc %>
  <% end %>
  mode tcp
  acl mysql_not_enough_capacity nbsrv(mysql_ro_main) lt 2
  use_backend mysql_ro_backup if mysql_not_enough_capacity
  default_backend mysql_ro_main
  maxconn 20000
  timeout client 86400000

backend mysql_ro_main
  option httpchk GET /check-lag
  http-check disable-on-404
  balance roundrobin
  retries 1
  timeout connect 1000
  timeout check 300
  timeout server 86400000

  default-server port 9876 fall 2 inter 5000 rise 1 downinter 5000 on-marked-down shutdown-sessions weight 10
  server my-db-0001 my-db-0001.heliumcarbon.com:3306 check
  server my-db-0002 my-db-0002.heliumcarbon.com:3306 check
  server my-db-0003 my-db-0003.heliumcarbon.com:3306 check
  server my-db-0004 my-db-0004.heliumcarbon.com:3306 check
  server my-db-0005 my-db-0005.heliumcarbon.com:3306 check
  server my-db-0006 my-db-0006.heliumcarbon.com:3306 check

backend mysql_ro_backup
  option httpchk GET /ignore-lag
  http-check disable-on-404
  balance roundrobin
  retries 1
  timeout connect 1000
  timeout check 300
  timeout server 86400000

  default-server port 9876 fall 2 inter 10000 rise 1 downinter 10000 on-marked-down shutdown-sessions weight 10
  server my-db-0001 my-db-0001.heliumcarbon.com:3306 check
  server my-db-0002 my-db-0002.heliumcarbon.com:3306 check
  server my-db-0003 my-db-0003.heliumcarbon.com:3306 check
  server my-db-0004 my-db-0004.heliumcarbon.com:3306 check
  server my-db-0005 my-db-0005.heliumcarbon.com:3306 check
  server my-db-0006 my-db-0006.heliumcarbon.com:3306 check

listen monitoring 0.0.0.0:1234
  mode health
  option tcplog

listen statsctl
  ...