Observability: logs, metrics, traces

Metrics tell you something is wrong, traces tell you where, logs tell you why. You need all three, and they need to share request ids.

What this lesson covers

• Instrument the three signals and correlate them
• Alert on symptoms users feel, not on causes
• Trace a request across service boundaries

This lesson sits in Reliability and Operations, part of System Design Foundations. It assumes what came before it and leads directly into the next lesson in the module.