Each directory created by the buildpack under the CNB_LAYERS_DIR can be used as a layer in the final app image or build cache.

That is, each directory can be used for any of the following purposes:

Layer Type
Launch	the directory will be included in the final app image as a single layer
Cache	the directory will be included in the build cache and restored to the CNB_LAYERS_DIR on future builds
Build	the directory will be accessible to buildpacks that follow in the build (via the environment)

A buildpack can control how a layer will be used by creating a <layer>.toml with a name matching the directory it describes in the CNB_LAYERS_DIR.

Example

A buildpack might create a $CNB_LAYERS_DIR/python directory and a $CNB_LAYERS_DIR/python.toml with the following contents:

launch = true
cache = true
build = true

In this example:

• the final app image will contain a layer with python, as this is needed to run the app
• the $CNB_LAYERS_DIR/python directory will be pre-created for future builds, avoiding the need to re-download this large dependency
• buildpacks that follow in the build will be able to use python

Example

This is a simple ./bin/build script for a buildpack that runs Python’s pip package manager to resolve dependencies:

#!/bin/sh

PIP_LAYER="$CNB_LAYERS_DIR/pip"
mkdir -p "$PIP_LAYER/modules" "$PIP_LAYER/env"

pip install -r requirements.txt -t "$PIP_LAYER/modules" \
  --install-option="--install-scripts=$PIP_LAYER/bin" \
  --exists-action=w --disable-pip-version-check --no-cache-dir

echo "launch = true" > "$PIP_LAYER.toml"

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