Week 3: Thinking Tools

We talk about our beliefs and assertions that led to the creation of Replay.
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Jason Laster
I’d like to step back this week and discuss some of the beliefs that underpin Replay.
  1. People understand what they can see. If we can help you visualize what happened in your application, you can better understand why it happened.
  1. Anyone can think programatically. We believe that debugging is fundamentally about testing hypotheses, and that you do not need to be a developer to explore how your software works.
These ideas were articulated really well by Bret Victor in Learnable Programming.

A new way of thinking

When you make a recording with Replay, you’re recording your software at the lowest level (system calls) and replaying it exactly as it ran before.
Let’s say there is an issue with your application’s checkout flow. With Replay you can record the problem and
  1. Play to the point where your checkout button turned red. Or the button was clicked 3 times and nothing happened!
  1. Visualize all of the times onCheckout function was called and see if the checkout workflow broke before or after onCheckout was called.
  1. Visualize all of the checkout API calls and see if the cart was not updated because of a client-side or server-side problem.
And just as exciting, you can share the recording with others on the team or compare multiple recordings to see why something happened in one case, but did not happen in another.


  • This week we slowed down and focused on the fundamentals (user accounts and monitoring). We're in a much better spot to onboard users reliably and get back to shipping!
  • We're actively hiring for a founding designer and systems engineers. Reach out if you're interested!
  • We have 5 good first issues and had 2 new contributors this week! Say hi in Slack if you'd like to get involved. 👋
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