CI/CD

We package an app with its dependencies into an image, which runs as a container — identical on your laptop and in production. "Works on my machine" stops being an excuse.

We run two identical environments: blue is live, green is the new version. Once green is verified we flip traffic to it; on trouble we flip back instantly. Zero-downtime releases with instant rollback.

We add more machines/instances (scale out) instead of one bigger box (vertical, scale up). For stateless services this wins: cheaper, more elastic, no ceiling. State goes to a separate store.

Distributes incoming traffic across multiple instances — the front door that gives you redundancy and smooth scaling. Health checks remove dead instances, so one failure stays invisible to users.

We follow one request across every service using a trace ID (e.g. OpenTelemetry). In a microservices system this is how we pinpoint which service slowed down or failed — no guessing.

Atomicity, Consistency, Isolation, Durability: the guarantees that keep transactions correct, so money never half-moves. Why we default to a relational DB for anything financial.