scope utm parameters and referrer to a session

[pjhul]

Context

Recently, I made some changes to how we track events on posthog.com, and afterwards we noticed a fairly significant drop when looking at insights filtering based on UTM source and Referrer. After some digging, we found that this had to do with the fact that we persist UTM and Referrer information using a super property.

Before, we were seeing an inflated number of users with a specific UTM source, which was caused by users returning back to the website during a later session, but still having a UTM source sent on their events.

Solution

Ideally, we do still want these properties to be persisted, but only within a session rather than indefinitely. It looks like we do have the sessionStore object already. Perhaps creating a register_session function that registers a super property using the sessionStore?

Overall, I think this should be given quite high priority, as it affects some pretty critical properties, especially for marketing teams.

Further context

• https://github.com/PostHog/company-internal/issues/908#issuecomment-1331495075
• Utm properties get set on later visits but likely shouldn't posthog#7710