Buildpacks allow you to convert your source code into a secure, efficient, production ready container image.

What are buildpacks?

Buildpacks provide framework and runtime support for applications. Buildpacks examine your apps to determine all the dependencies it needs and configure them appropriately to run on any cloud.

How do they work?

Each buildpack comprises of two phases -

Detect phase

The detect phase runs against your source code to determine if the buildpack is applicable or not. Once a buildpack is detected to be applicable, it proceeds to the build stage. If the project fails detection the build stage for a specific buildpack is skipped.

For example:

• A Python buildpack may look for a requirements.txt or a setup.py file pass
• A Node buildpack may look for a package-lock.json file to pass

Build phase

The build phase runs against your source code to -

• Set up the build-time and run-time environment
• Download dependencies and compile your source code (if needed)
• Set appropriate entry point and startup scripts

For example:

• A Python buildpack may run pip install -r requirements.txt if it detected a requirements.txt file
• A Node buildpack may run npm install if it detected a package-lock.json file

What is a builder?

Builders are an ordered combination of buildpacks with a base build image, a lifecycle, and reference to a run image. They take in your app source code and build the output app image. The build image provides the base environment for the builder (for eg. an Ubuntu Bionic OS image with build tooling) and a run image provides the base environment for the app image during runtime. A combination of a build image and a run image is called a stack.

Under the hood a builder uses the lifecycle to run the detect phase for all the buildpacks it contains in order and then proceeds to run the build phase of all the buildpacks that passed detection.

This allows us to have a single builder that can detect and build various kinds of applications automatically.

For example, let’s say demo-builder contains the Python and Node buildpack. Then -

• If your project just has a requirements.txt, demo-builder will only run the Python build steps.
• If your project just has a package-lock.json, demo-builder will only run the Node build steps.
• If your project has both package-lock.json and requirements.txt, demo-builder will run both the Python and Node build steps.
• If your project has no related files, demo-builder will fail to detect and exit.

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