Engagement
Session replay
Also called session recording.
Session replay is a reconstructed playback of a session's DOM mutations and input. It shows what one visitor saw and did.
How it is measured
The recorder serialises the page and each change, then rebuilds it on playback. Masking hides sensitive fields, and sampling limits volume.
Tarsier does not record sessions, by design. Cookieless counting and replay pull in opposite directions.
Worked example
A clinic's booking form loses 30 percent of starts. Five recordings show the date picker covering the Next button on small phones.
A CSS fix lifts completion from 52 to 71 percent.
How it differs
Session replay plays one visit. A heatmap aggregates many. A rage click can lead you to a replay worth watching. Replays raise personal data concerns that counts do not.
Common errors
Recording without masking inputs. Watching random sessions. Keeping footage forever. Skipping consent. Treating one recording as a trend.
In practice
Watch only sessions around a known problem. Mask all inputs and set a short retention.