Function hash_anonymous_identifier 

pub fn hash_anonymous_identifier(
    course_id: Uuid,
    hashing_key_for_the_day: &[u8],
    user_agent: &str,
    ip_address: &str,
) -> String

A visitor identifier that cannot be traced back to the visitor once the day’s key is rotated away.

hashing_key_for_the_day is what keeps the identifier from being reproducible from an IP address alone, so it must be the current day’s secret and never a constant. The same visitor gets a different identifier on the next day and on another course.

Touching how the parts are combined re-keys every identifier the moment it deploys, so COUNT(DISTINCT anonymous_identifier) counts every visitor already seen that day a second time. The key rotation caps that to the day of the deploy; nothing else does.