Live tail logs
You can live tail logs from the dashboard or with thefly logs command. For more information, see Live tail logs.
Monitoring
Logging overview
See how Fly.io app logs flow from stdout through Vector and NATS, then live tail with fly logs, use the Logs API, search logs in Grafana, or export them.
Fly.io app logs are the stdout from the processes run in your apps: whatever an app outputs becomes a log.
Your apps run in Machines. Inside each Machine, we inject a process (init) that runs and monitors your app. Since we build Machines from Docker images, init is taking ENTRYPOINT + CMD and running that. The init program is, among other things, gathering process output from stdout and redirects it to a socket.
Outside of the Machine, on the host, we take that output and send it to Vector via yet another socket. Vector ships logs (your appās output) to an internal NATS cluster. Clients can subscribe to specific topics, and NATS sends the requested data to those subscribers.
A proxy sits in front of NATS, to ensure you only see your own logs, and you can hook into NATS via this proxy to get your logs.
Vector can act as a NATS client, read the logs, and ship them somewhere. An example of this is our Fly Log Shipper app.