Events & Configuration

Event-driven decoupling

A synchronous direct call couples the caller to the callee's lifetime, latency, and failure. An event bus breaks those: publishers emit facts ("OrderPlaced{...}"), subscribers react. The cost is dispatch ordering — a serious bus documents and guarantees its order (registration order for sync buses is the common deterministic choice), because "usually fine" ordering is a production incident generator.

The configuration ladder

Real systems resolve config in layers with defined precedence — the common ladder:

defaults  <  file  <  environment  <  explicit flag

Each layer overrides the previous only where it speaks. Debugging misconfiguration means asking, per key: "which layer set this?" A config system that cannot answer that question is a rumour mill.

Secrets never ride the ladder

Values from secret stores must never be logged, never land in error messages, and never become part of a string you format. Redaction is a sink responsibility (the logging layer), but the config layer's job is to mark secrets so the sink can.

What the sandbox grades

The exercises grade the mechanics: a DI seam that makes time-based logic deterministic, an event bus with registration-order dispatch and unsubscribe, and a precedence ladder you can interrogate per key.