Environment variables
Per-application variables, what build-time means, and why a value never comes back.
Variables are set per application, in that application's settings. Each key is either a runtime variable or a build-time variable.
Runtime and build-time
A runtime variable is present when your application runs. A build-time variable is present while the build runs too, which is what a framework needs when it inlines configuration at build time.
Values are write-only
Once a value is saved it never comes back through the console or the API — only the key does. To change a value, set that key again; the keys you leave alone keep exactly what is stored.
Prefer runtime variables for secrets. A build-time value is present during the build and can end up inside what the build produces or caches.
Removing a variable
Removing is by key, and it applies to the next deployment: a running application keeps the environment it started with until it is deployed again.
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