Orchestrating Millions Across the Globe: Reactive Payments at Scale

“Just integrate Stripe” works, until it doesn’t. As soon as a user-facing product accepts payments, whether one-time purchases, subscriptions, upgrades, or add-ons, payment flows begin to behave like distributed systems. Gateways, regions, retries, experiments, fallbacks, and partial failures all interact in ways that linear checkout flows were never designed to handle. This talk distills how to think about payments through the lens of reactivity. Instead of modeling payments as a sequence of steps, we’ll explore how treating them as reactive orchestration systems, driven by state changes and events, leads to architectures that are easier to extend, reason about, and recover under real-world conditions. Using concrete production examples, we'll walk through the technical lessons learned from operating high-volume, global payment flows and show how frontend payment orchestration becomes a first-class concern once products move beyond trivial checkout logic. While the examples use React, the underlying ideas apply to any frontend framework or library. The best part? I’ll end with a live demo of this running in production on Smallpdf, where we built and shipped our in-house reactive frontend orchestration system in React, now serving tens of millions of users worldwide.