Session Replay: What It Is, How It Works, and When You Need It

Session replay records what users actually do on your site: mouse movements, clicks, scrolls, keypresses, and every DOM mutation. Play it back and you're watching a user's experience as they lived it. It sounds invasive. It kind of is. That tension is worth understanding before you pick a tool. How session replay actually works Almost every session replay tool in existence — FullStory, Hotjar, LogRocket, PostHog Replay, Microsoft Clarity, and most self-hosted options — is built on the same foundation: rrweb. rrweb (record and replay the web) is an open-source library that works in two steps. First, it takes a full snapshot of the DOM at the start of a session. Then it records every subsequent mutation as a compact, serialized diff. Scroll positions, CSS changes, added or removed elements — every change gets timestamped and appended to the recording stream. On replay, rrweb reconstructs the DOM from the initial snapshot and replays the mutations in order, synced to a virtual timeline. W