3 posts tagged with "design"

Billing at Fibe runs on a unit we call the service-day. A Marquee is either funded for today or it isn't. Every day, for every funded environment, a small charge is taken from the player's Wallet and the Marquee's paid-through time is pushed to the end of the current day. There is no monthly subscription to reason about, no proration math, no surprise cliff at the end of a cycle — just one question, asked once a day, forever: did you pay for today?

This post is about why we chose that shape, how the daily charge keeps an environment running, and the single design rule underneath it that keeps every part of the billing lifecycle agreeing on what "today" actually means.

Most of our nastiest bugs were never typos. They were a state machine that looked right in the PR, passed its tests, and then met a customer doing something perfectly reasonable that nobody had drawn on the whiteboard. A Playground stuck in an error state forever. A chat that vanished because a healthcheck flapped. A runtime launched on a Marquee that hadn't been funded.

So we started writing down the hard lifecycles in Alloy, a relational modeling language, and letting a solver do the part humans are worst at: trying every small, weird combination of states and asking "does your invariant actually hold here?" Often it doesn't — and the solver tells us exactly which world breaks it.

Most of the time, the dangerous question in a distributed system isn't "what does this delete cascade to?" It's "what does it not cascade to — and are you sure?"

We spend a lot of energy reasoning about the edges that do fire: delete a Marquee, and a chain of consequences unfolds. But the bugs that wake you at 3am are usually the opposite shape. Someone toggles an innocuous setting — disables a Prop, edits a Secret, demotes a teammate — and a long-running environment they didn't mean to touch quietly falls over. The edge that surprised them is one nobody designed; it emerged because some well-meaning "keep things in sync on every save" hook reached one relationship too far.