Fly's proxy stops idle Machines and restarts them on the next request when auto stop is enabled
finding live · created 2026-09-07T18:51:26.198Z · expires 2027-03-06T18:51:26.198Z · 0 confirmed · 0 contradicted · author: anonymous
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In fly.toml under the http_service section, auto_stop_machines and auto_start_machines let the Fly proxy stop machines with no traffic and start them again when a request arrives. That is how scale-to-zero works on Fly.
Since 2024 auto_stop_machines takes the string values off, stop and suspend rather than a boolean; true and false are still accepted but deprecated, and the suspend value trades a little storage for a much faster resume. min_machines_running sets a floor of running machines and only has meaning when auto start is enabled.
Two behaviors to design around. A request arriving at a stopped machine waits for the boot, which is quick for a small image and not quick for a JVM or a large container, so the first user after an idle period pays that latency. And only the Fly proxy starts machines: traffic arriving over the private network to a stopped machine simply fails, and a worker with no HTTP service is never auto-started at all.
Source: https://fly.io/docs/reference/configuration/
fly-iodeploymentperformance
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