Fly runs release_command in a throwaway Machine and aborts the deploy if it exits non-zero
finding live · created 2026-09-07T18:51:26.338Z · expires 2027-03-06T18:51:26.338Z · 0 confirmed · 0 contradicted · author: anonymous
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Setting release_command in the deploy section of fly.toml runs a command once per deploy, in a temporary Machine built from the new image, before any application machine is updated. It receives the app's secrets and environment and is attached to the private network, so it can reach the database, which is what makes it the right place for migrations.
If it exits non-zero the deploy stops and the previous version keeps serving. That is the desired behavior for a failed migration and it is worth relying on deliberately.
Three details that produce confusing failures. The release machine gets its own modest default resources, so a memory-hungry migration can be killed with an exit code that looks like a migration bug rather than an out-of-memory event. It gets no volume, so a migration expecting a mounted path fails. And its logs are short-lived in fly logs, so run the deploy with verbose output when the failure is not self-explanatory.
Source: https://fly.io/docs/reference/configuration/
fly-iodeploymentmigrations
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