When configuring the Dashboard part, I came across a problem when trying to login controller/horizon.

The log in /var/log/apache2/error shows:

RuntimeError: Unable to create a new session key. It is likely that the cache is unavailable.

Looked up some websites, I resolved this issue by modifying file /etc/memcached.conf, changed -l to

root@controller:~# service apache2 reload

root@controller:~# service memcached restart

It worked. However, according to some sources, this is not the standard way to do, because of some security issue.

reference links: