Fly.io provides basic infrastructure metrics and logs, but can't explain why an app is slow or which endpoint is causing issues. AppSignal fills this gap by auto-instrumenting Node.js apps (Express, Fastify, Koa, Next.js) to capture per-route response times, grouped errors, database queries, host metrics, and logs in one dashboard. Key Fly.io-specific patterns to watch include cold starts, machine restarts indicating crash loops, and shared-CPU variance causing erratic p95 times. Adding deploy markers to the fly deploy step lets you instantly correlate performance changes with deployments. Recommended alerts include error rate spikes, slow route thresholds, and memory limits set below Fly.io's OOM ceiling.
Table of contents
What Fly.io Gives You (and What It Doesn’t)What AppSignal Captures in Your Node.js AppThe Signals AppSignal Surfaces on Fly.ioFly-Specific Things to WatchConnecting Deploys to ChangesAlerts Worth SettingWrapping it UpSort: