0026-environment-variable-configuration-of-buildpacks.md

Environment Variable Configuration Of Buildpack

Supersedes RFC #0003

Summary

Environment variables passed in at build time via the command line or project.toml should be the main means for user configuration of a buildpack's detect/build processes.

Motivation

There are often situations in which users need to configure or customize aspects about their applications build process (i.e. build flags in Go, project path in .Net, etc). There are also some buildpacks specific settings (dependency versions) that the user might also want/need to control.

Detailed Explanation

In RFC #0003 it was proposed to use the Build Plan buildpack to pass all configuration as part of the build plan. This proved to have unforeseen negative consequences, chief among them there was no way to pass configuration data to the detect process of a buildpack because the information embedded in the build plan would only become available to the buildpack in the buildpack plan during build. It has since been decided in a number of the buildpack families to use environment variables instead of the Build Plan buildpack.

This RFC is meant to catalog this distributed design decision. This RFC is also meant to be a formal superseding RFC #0003 to ensure the chain of decision making remains somewhat intact.