Notifications start small. "Send the user an email when their order ships." A function. A library. Done.
A year later, you have email, SMS, push, in-app, Slack, and Microsoft Teams. You have user preferences per channel. You have quiet hours, batching, throttling, and a "do not disturb" mode. You have unsubscribe links and bounce handling. You have analytics on open rates and template-level metrics. You have multi-language templates and timezone-aware scheduling.
What started as a function is now a system. If you keep it as a sprawling collection of sendEmail and sendSlack helpers across your codebase, that system will eat your engineering team alive.
This is the shape of the notification microservice I built (and have rebuilt twice). The pattern isn't novel — it's domain-driven design applied to notifications — but the specifics matter.
The core insight
