Launching Livebook using LiveView

Summary

Fly.io’s Livebook launcher uses a single LiveView process to deploy a Livebook instance, but temporarily held deployment secrets could be lost when a user refreshed the browser or the server restarted. The secrets, including liveview_password, were sent to deployment and stored in Vault, so the values could not be retrieved; keeping them in a database or Redis also conflicted with avoiding persistent server-side storage. The implementation encrypts the private state with Phoenix.Token.encrypt/4, stores the resulting blob in browser SessionStorage, and uses LiveView hooks and websocket events to restore it through Phoenix.Token.decrypt/4. After recovery, the UI resumes tracking deployment progress, then clears the cached data when deployment succeeds or fails, while closing the tab also clears SessionStorage.

Takeaways

1. 1

   Phoenix.Token.encrypt/4 both signs the deployment data to prevent tampering and encrypts its contents to keep them private.

2. 2

   LiveView hooks use websocket events to store encrypted state in SessionStorage and push it back to the server for restoration.

3. 3

   Cached state is removed from SessionStorage after deployment success or failure, and closing the browser tab clears the session storage as well.