Calculate session replay percentage based on stable identifier

[nightpool]

Hi all! Right now we're using record_sessions_percent to roll out session replay to a small percentage of our users. However, due to the way that Mixpanel uses Math.random to calculate a page view-level eligibility for session replay, it's hard to compare replays in a funnel or between different sessions by the same user.

Instead, Mixpanel should hash the user's distinct_id to calculate a stable identifier that can be used for determining whether the user is in the "record sessions" cohort, so that replays from the same user can be analyzed together.

[tdumitrescu]

This is largely a product decision (whether you want user-based sampling or not), but we are indeed planning to offer some more complete controls over sampling in future releases. In the meantime, if you want user-based sampling, the recommended workaround is to write your own sampling logic and then initialize with record_sessions_percent set to either 0 or 100 based on your determination of whether the current user falls in sample or not.