Before any tool gets installed, the vocabulary has to be right. This chapter separates monitoring from observability, names the three signals and the six-stage pipeline every product implements, and hands you the three costs that explain every design decision to come — then introduces Harborline, the ferry-booking system the whole book instruments, at its lowest point: flying blind.
5 topics
A customer says checkout "sometimes takes forever" on Saturday mornings, and nobody at Harborline can say whether it's true, how often it happens, or which of five services is responsible. The toolkit on hand — ssh, docker logs, htop — cannot answer a single question about last Saturday. That gap between a running system and an answerable one is what this book closes, and this chapter defines it precisely.
Five topics build the foundation: the monitoring-versus-observability distinction and the new-question test that makes it operational; the three signals — metrics, logs, traces — and what each structurally cannot do; the six-stage pipeline under every product logo; the three costs (cardinality, sampling, retention) that price every decision ahead; and the Harborline system itself, names fixed, wound open.