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Comment by owickstrom | Hacker News Reader

Hey, yeah the default specification includes a set of action generators that are picked from randomly. If you write a custom spec you can define your own action generators and their weights.

Rerunning things: nothing built for that yet, but I do have some design ideas. Repros are notoriously shaky in testing like this (unless run against a deterministic app, or inside Antithesis), but I think Bombadil should offer best-effort repros if it can at least detect and warn when things diverge.

Shrinking: also nothing there yet. I'm experimenting with a state machine inference model as an aid to shrinking. It connects to the prior point about shaky repros, but I'm cautiously optimistic. Because the speed of browser testing isn't great, shrinking is also hard to do within reasonable time bounds.

Thanks for the questions and feedback!

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For re-running, I assume you want to do this all on a review app with a snapshot of the DB, so you start with a clean app state.

Should be pretty easy to make it deterministic if you follow that precondition.

(How I had my review apps wired up was I dumped the staging DB nightly and containerized it, I believe Neon etc make it easy to do this kind of thing.)

Ages ago I wired up something much more basic than this for a Python API using hypothesis, and made the state machine explicit as part of the action generator (with the transitions library), what do you think about modeling state machines in your tests? (I suppose one risk is you don’t want to copy the state machine implementation from inside the app, but a nice fluent builder for simple state machines in tests could be a win.)

That's true, clean app state gets you far. And that's something I'm going to add to Bombadil once it gets an ability to run many tests (broad exploration, reruns, shrinking), i.e. something in the spec where you can supply reset hooks, maybe just bash commands.

Regarding state machines: yeah, it can often become an as-complex mirror of the system your testing, if the system has a large complicated surface. If on the other hand the API is simple and encapsulates a lot of complexity (like Ousterhout's "Deep Modules") state machine specs and model-based testing make more sense. Testing a key-value store is a great example of this.

If you're curious about it, here's a very detailed spec for TodoMVC in Bombadil: https://github.com/owickstrom/bombadil-playground/blob/maste... It's still work-in-progress but pretty close to the original Quickstrom-flavored spec.

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