Auth System for a Large Number of Users

I'm being asked to look at technologies to use for a site that could potentially have millions of registered users. Current site has over 500,000.

The actual site itself will only get moderate traffic, perhaps 10-20 thousand visits per day. Database activity from those might only result in a few thousand records being added, plus an activity log, as most visits will be look ups.

Of the registered users, most will only ever visit the site once a year to update their details, and many not even that.

Comments from forum user Kaosweaver indicate that CI won't have any issues at this scale.


My question is about the authentication system. I've used Ion Auth and Community Auth with a few hundred users, but will they happily scale to millions?