DIY Session Replay - How tools like Sentry and Posthog Work

This month Sean Beard will be showing us how "session replay" in services like Sentry, PostHog, and Pendo work.

We are always looking for speakers. Submit your talk here: https://forms.gle/5A26sY5475X8J14S8

Food and drinks will be provided.

This meetup will be recorded and uploaded (not live-streamed) to our YouTube channel - https://www.youtube.com/@DenverScriptMeetup

Agenda
6:00 PM - Networking
6:15 PM - JS News
6:30 PM - Short Talk TBD
7:00 PM - DIY Session Replay: Record and Replay the Web

Talk Descriptions

DIY Session Replay: Record and Replay the Web by Sean Beard - Session replay tools like Sentry, PostHog, and Pendo let you "watch" user sessions without recording video. How? They capture DOM state and mutations, not pixels. This talk breaks down the engineering behind this technology. You'll see how rrweb - the open source standard - works under the hood. Three core mechanisms: serializing the DOM, tracking changes with MutationObserver, and replaying everything in an isolated iframe. The bandwidth savings are dramatic compared to actual video. There will be a demo of a working proof-of-concept, walking through how to capture browser state, record incremental changes, and reconstruct sessions. Then we'll tackle production: client/server architecture patterns, handling incomplete sessions, and storage strategies for managing replay data at scale. Useful if you work with browser APIs, build debugging tools, or want to understand how modern observability products actually work.

Speaker Bios

Sean Beard is a software engineer with over 15 years of experience building and scaling production-grade web applications across startups, consultancies, and large enterprises. He enjoys turning hard-won lessons into practical insights that help other developers level up their skills and confidence.