Custom Buildpacks

When to Build Custom Buildpacks

Use custom buildpacks for: - Unsupported languages - Custom build toolchain - Organization-specific conventions - Pre-built binaries

Don't use if: - Standard buildpack exists - Only need simple customization - Build process is complex

Buildpack Structure

my-buildpack/ ├── bin/ │ ├── compile (bash script) │ ├── detect (bash script) │ └── release (bash script) ├── lib/ │ └── helpers.sh (optional) └── README.md

Detect Script

Determines if buildpack applies:

```bash

!/bin/bash

$1 = BUILD_DIR (app source directory)

BUILD_DIR=$1

if [[ -f "$BUILD_DIR/Gemfile" ]]; then echo "Ruby" exit 0 fi

exit 1 ```

Exit codes: - 0: Buildpack applies (print language name) - 1: Buildpack doesn't apply

Compile Script

Build application:

```bash

!/bin/bash

$1 = BUILD_DIR (app source)

$2 = CACHE_DIR (persistent, reused across builds)

$3 = DEPS_DIR (shared dependencies)

BUILD_DIR=$1 CACHE_DIR=$2 DEPS_DIR=$3

echo "-----> Installing Ruby"

Download or copy pre-built binary

mkdir -p $DEPS_DIR/ruby/bin cp /opt/ruby-3.0.0/bin/* $DEPS_DIR/ruby/bin/

Export environment for later phases

export PATH="$DEPS_DIR/ruby/bin:$PATH"

cd $BUILD_DIR

Install dependencies

echo "-----> Running bundler" gem install bundler bundle install

echo "-----> Compile successful" ```

Release Script

Runtime configuration:

```bash

!/bin/bash

$1 = BUILD_DIR

BUILD_DIR=$1

cat <<EOF

default_process_types: web: bundle exec rails s -p $PORT EOF ```

Pre-Compiled Binaries

Optimize build time by pre-compiling:

```bash

In compile script

if [[ ! -d "$CACHE_DIR/ruby-3.0.0" ]]; then echo "-----> Downloading Ruby" wget -O ruby.tar.gz https://example.com/ruby-3.0.0.tar.gz tar -xzf ruby.tar.gz -C $CACHE_DIR fi

cp -r $CACHE_DIR/ruby-3.0.0 $DEPS_DIR/ ```

Testing Buildpack Locally

Create test app:

```bash mkdir test-app cd test-app

Create trigger file

touch Gemfile (for Ruby example)

Manual test

export BUILD_DIR=$(pwd) export CACHE_DIR=$(mktemp -d) export DEPS_DIR=$(mktemp -d)

Run detect

../my-buildpack/bin/detect $BUILD_DIR

Should print "Ruby"

Run compile

../my-buildpack/bin/compile $BUILD_DIR $CACHE_DIR $DEPS_DIR

Should build successfully

```

Best Practices

1. Fail Fast: Detect early if buildpack applies
2. Cache: Reuse downloaded files
3. Output: Print progress messages with ---->
4. Errors: Exit with clear error messages
5. Environment: Export variables for next phase
6. Security: Don't include secrets in scripts
7. Testing: Test locally before uploading

Publishing Buildpack

```bash

Package

zip -r my-buildpack.zip .

Upload to foundation

cf create-buildpack my-buildpack my-buildpack.zip 10

Users can reference it

cf push my-app --buildpack my-buildpack ```

---

See Lab 5: Custom Buildpack for hands-on buildpack creation.