3.2. Monitoring & Debugging Applications

💡 First Principle: If you can't observe it, you can't fix it. Every application needs metrics, logs, and traces — the three pillars of observability.

What happens when a Lambda function starts timing out in production but worked fine in testing? Without CloudWatch metrics showing duration trends, CloudWatch Logs capturing the error, and X-Ray traces revealing which downstream call is slow, you're debugging blind. The DVA-C02 frequently presents troubleshooting scenarios where the correct answer depends on knowing which observability tool to use.

Think of monitoring like a car's dashboard: metrics are the gauges (speed, fuel, temperature), logs are the engine diagnostic codes, and traces are GPS breadcrumbs showing exactly which route each request took through your distributed system.