Add juju action to 'drain', so allow existing connections to finish but not accept new

[Haw Loeung]

Hi,

Mostly used for if/when we have some sort of frontend in front of content-cache units (HAProxy in front, Azure LB, etc.). We should have an action to trigger on a unit to drain the queue. So allow existing connections to finish and not accept new.

We'll need to see if HAProxy has a flag to do this and implement it as a juju action.

[Haw Loeung (hloeung)]

It's possible with 'set server backend/serv state drain' (with 'show servers state' to show the current status). Then 'set server backend/serv state ready' to restore.

e.g.

Current:

| ubuntu@juju-87625f-hloeung-13:~$ echo "show servers state" | sudo -u haproxy socat stdio $(awk '/stats socket/ { print $3 }' /etc/haproxy/haproxy.cfg)
| 1
| # be_id be_name srv_id srv_name srv_addr srv_op_state srv_admin_state srv_uweight srv_iweight srv_time_since_last_change srv_check_status srv_check_result srv_check_health srv_check_state srv_agent_state bk_f_forced_id srv_f_forced_id srv_fqdn srv_port srvrecord
| 7 backend-cached-archive-ubuntu-com 1 server_1 127.0.0.1 2 0 1 1 99487 15 3 6 6 0 0 0 - 6080 -

Drain:

| ubuntu@juju-87625f-hloeung-13:~$ echo "set server backend-cached-archive-ubuntu-com/server_1 state drain" | sudo -u haproxy socat stdio $(awk '/stats socket/ { print $3 }' /etc/haproxy/haproxy.cfg)
| ubuntu@juju-87625f-hloeung-13:~$ echo "show servers state" | sudo -u haproxy socat stdio $(awk '/stats socket/ { print $3 }' /etc/haproxy/haproxy.cfg)
| 1
| # be_id be_name srv_id srv_name srv_addr srv_op_state srv_admin_state srv_uweight srv_iweight srv_time_since_last_change srv_check_status srv_check_result srv_check_health srv_check_state srv_agent_state bk_f_forced_id srv_f_forced_id srv_fqdn srv_port srvrecord
| 7 backend-cached-archive-ubuntu-com 1 server_1 127.0.0.1 2 8 1 1 5 15 3 6 6 0 0 0 - 6080 -
| ...

Restore / undrain:

| ubuntu@juju-87625f-hloeung-13:~$ echo "set server backend-cached-archive-ubuntu-com/server_1 state ready" | sudo -u haproxy socat stdio $(awk '/stats socket/ { print $3 }' /etc/haproxy/haproxy.cfg)
| ubuntu@juju-87625f-hloeung-13:~$ echo "show servers state" | sudo -u haproxy socat stdio $(awk '/stats socket/ { print $3 }' /etc/haproxy/haproxy.cfg)
| 1
| # be_id be_name srv_id srv_name srv_addr srv_op_state srv_admin_state srv_uweight srv_iweight srv_time_since_last_change srv_check_status srv_check_result srv_check_health srv_check_state srv_agent_state bk_f_forced_id srv_f_forced_id srv_fqdn srv_port srvrecord
| 7 backend-cached-archive-ubuntu-com 1 server_1 127.0.0.1 2 0 1 1 71 15 3 6 6 0 0 0 - 6080 -