Event-Driven Notification Platform · View 03 of 26 · 2 · Structure

Layered Architecture

What depends on what, and the single dependency that points the wrong way.

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The layering rule

A layer may call the layer below it and may publish upward only through the messaging layer
The decision layer never calls a delivery worker directly; it publishes an intent
Producers depend on the API layer alone and know nothing about channels, templates or providers

The exception, drawn in red

The in-app worker pushes into the experience layer over WebSocket, which is an upward call
The alternative is client polling at 750 M notifications a day, which costs more than the violation
Contained by making Centrifugo the only upward target and treating it as infrastructure, not application code

Why open source throughout

Every layer has a mature Apache-2 or MIT option with an operator, so there is no managed-service lock-in
The two places this costs effort are Kafka and Flink operations, budgeted as two platform engineers
Channel providers are the one unavoidable exception — FCM and APNs have no self-hosted equivalent

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