Graceful VM exits, some dials

Summary

Fly.io introduces configurable VM shutdown behavior for applications whose clients, connections, or transactions need more than the default five-second termination window. After sending SIGINT, Fly normally waits five seconds before forcefully terminating a VM; the new kill_timeout setting extends that wait to as much as five minutes on Shared CPU VMs or 24 hours on dedicated CPU VMs, while kill_signal selects among several signals. The post demonstrates HAProxy’s soft-stop mode with SIGUSR1 to drain existing TCP connections and Postgres’s smart shutdown with SIGTERM to let open transactions commit before termination. It also describes infrastructure considerations: deploys and other releases can disrupt in-flight connections, so Fly drains connections where possible and offers ways to reduce reliance on shared HTTP and TLS handlers, plus stateless UDP and experimental stateless TCP load balancing.

Takeaways

1. 1

   The default VM shutdown sequence sends SIGINT, waits five seconds, and then forcefully terminates a still-running VM.

2. 2

   HAProxy can use SIGUSR1 for soft-stop behavior, stopping new connections while existing TCP connections drain.

3. 3

   Postgres can use SIGTERM with a five-minute timeout to commit open transactions through its smart shutdown mode.