BuildPack

About

A buildpack is a program that turns source code into a runnable container image.

These buildpacks are bundled into a builder stack and are called upon to build an app.

The buildpack idea has been standardized with Cloud Native Buildpacks. ¹⁾

Heroku is the originator of the buildpack idea.

One of the benefits of buildpacks is that they are multi-process - an image can have multiple entrypoints for each operational mode.

Buildpacks are grouped together into a builder.
Each buildpack will inspect the application source code and detect if it should participate in the build process. For example:
- a Go buildpack would look for files ending in *.go,
- a Ruby buildpack would look for *.rb files.
- an NPM buildpack might look for a package.json,
- a Go buildpack might look for Go source files.
Every buildpack that has matched contribute to the build step.

A builder is an image that contains three components:

a set of buildpacks, which provide your app’s dependencies
a stack, which provides the OS layer for your app image
the CNB lifecycle ²⁾, which puts everything together to produce your final app image

A stack consists of two images:

build image: the environment in which your app is built (when the buildpacks are analyzing the application source code and building the application’s container image)
run image: the OS layer for your app image (the base of the final application image).

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