Documentation
Session
A session is one browser visit. EFU is the share of recent visits that did not hit an error.
What it is
Not your app's auth session — a Retrace Kit visit id for this browser until storage is cleared. Every
error carries sessionId. Lightweight session pings mark the visit as active.
Error-free users
EFU on the dashboard is the share of active sessions in the last hour that did not hit an error.
Active sessions
pinged in the last 60 min
Minus
sessions with an error
EFU %
shown on dashboard
EFU = (active sessions − sessions with errors) / active sessions × 100- 100 active sessions, 0 errors → 100%
- 3 active sessions, 1 with an error → 66.7%
- no pings in the last hour → 100% (nothing to measure)
Recalculates every hour. Health signal, not billing.
Errors and sessions
An error with a sessionId marks that session as affected for EFU. Multiple errors in one
visit still count as one affected session.
Separate from the Users column on issues, which needs current user. EFU works for anonymous traffic.
In the SDK
init creates and stores sessionId, sends pings, and attaches the id to errors.
Details: SDK → Session.