Observability

Observability describes how well you can understand what a system is doing from the outside, using what it emits — logs, metrics and traces. The usual contrast is with monitoring: monitoring watches for conditions you thought of in advance and tells you when one occurs, while observability is about being able to answer questions you did not anticipate, without shipping new code to answer them.

The distinction is practical rather than philosophical. Monitoring answers "is it working" and is a small, closed set of known questions. Observability answers "why is it doing that", where the question is invented during the incident and might be "why are only Android users on this API version seeing timeouts". A system with excellent monitoring and poor observability tells you promptly that something is wrong and then leaves you guessing; the reverse tells you nothing until a customer complains, after which you can find out exactly what happened.

Both are needed, and they are not substitutes. External uptime checks are the outside view — they detect what customers experience, including the failures that never reach your code at all, like DNS and certificates and routing. Telemetry from inside is the inside view, and it is where diagnosis happens. Teams that have only the second usually discover their outages from customers.