Config at boot

A release decides its configuration at two distinct moments. Some values are fixed when the image is built — they never vary per deploy, so they are baked into config/prod.exs at compile time. Others are known only when the release boots on its target machine — a secret, the public host, the port — so they are read from the environment in config/runtime.exs, which is evaluated once at startup. The rule that sorts them is simple to state: a value that is secret or varies per deploy belongs at runtime; a value fixed at build belongs at compile time. Select a configuration value to see which layer owns it and why.

Fly’s private network is IPv6-only. Before the BEAM can reach the database over it, three independent settings must agree — and one of them teaches the difference between setting an environment variable and consuming it. ERL_INETRC and ERL_AFLAGS take effect the moment they are set, because the BEAM runtime reads them with no application code involved. ECTO_IPV6 is different: setting it in fly.toml does nothing on its own — runtime.exs must read it into the repository’s socket_options. Omit that one line, and Postgrex issues an IPv4 (A) query for a host that has only an IPv6 (AAAA) record, and the boot crash-loops on :nxdomain — the same error an unreachable host gives. Select a variable to see where it is set, what consumes it, and whether setting it is enough.

In order: what runtime.exs resolves at boot against what prod.exs bakes at build; the three settings Fly’s IPv6-only network needs and the one that must be consumed by hand; and the two private-name schemes with the single command that isolates an unreachable host from a wrong address family.