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Making your buildpack configurable

It’s likely that not all Ruby apps will want to use the same version of Ruby. Let’s make the Ruby version configurable.

Select Ruby version

We’ll allow buildpack users to define the desired Ruby version via a .ruby-version file in their app. We’ll first update the detect script to check for this file. We will then record the dependency we can provide (Ruby), as well as the specific dependency the application will require, in the Build Plan, a document the lifecycle uses to determine if the buildpack will provide everything the application needs.

Update ruby-buildpack/bin/detect to look like this:

#!/usr/bin/env bash
set -eo pipefail

if [[ ! -f Gemfile ]]; then
   exit 100
fi

# ======= ADDED =======
plan=$2
version=3.1.3

if [[ -f .ruby-version ]]; then
    version=$(cat .ruby-version | tr -d '[:space:]')
fi

echo "provides = [{ name = \"ruby\" }]" > "$plan"
echo "requires = [{ name = \"ruby\", metadata = { version = \"$version\" } }]" >> "$plan"
# ======= /ADDED =======

Then you will need to update your build script to look for the recorded Ruby version in the build plan:

Your ruby-buildpack/bin/build script should look like the following:

#!/usr/bin/env bash
set -eo pipefail

echo "---> Ruby Buildpack"

# ======= MODIFIED =======
# 1. GET ARGS
layersdir=$1
plan=$3

# 2. CREATE THE LAYER DIRECTORY
rubylayer="$layersdir"/ruby
mkdir -p "$rubylayer"

# ======= MODIFIED =======
# 3. DOWNLOAD RUBY
ruby_version=$(cat "$plan" | yj -t | jq -r '.entries[] | select(.name == "ruby") | .metadata.version')
echo "---> Downloading and extracting Ruby $ruby_version"
ruby_url=https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/heroku-22/ruby-$ruby_version.tgz
wget -q -O - "$ruby_url" | tar -xzf - -C "$rubylayer"

# 4. MAKE RUBY AVAILABLE DURING LAUNCH
echo -e '[types]\nlaunch = true' > "$layersdir/ruby.toml"

# 5. MAKE RUBY AVAILABLE TO THIS SCRIPT
export PATH="$rubylayer"/bin:$PATH
export LD_LIBRARY_PATH=${LD_LIBRARY_PATH:+${LD_LIBRARY_PATH}:}"$rubylayer/lib"

# 6. INSTALL GEMS
# Compares previous Gemfile.lock checksum to the current Gemfile.lock
bundlerlayer="$layersdir/bundler"
local_bundler_checksum=$((sha256sum Gemfile.lock || echo 'DOES_NOT_EXIST') | cut -d ' ' -f 1)
remote_bundler_checksum=$(cat "$layersdir/bundler.toml" | yj -t | jq -r .metadata.checksum 2>/dev/null || echo 'DOES_NOT_EXIST')
# Always set the types table so that we re-use the appropriate layers
echo -e '[types]\ncache = true\nlaunch = true' >> "$layersdir/bundler.toml"

if [[ -f Gemfile.lock && $local_bundler_checksum == $remote_bundler_checksum ]] ; then
    # Determine if no gem dependencies have changed, so it can reuse existing gems without running bundle install
    echo "---> Reusing gems"
    bundle config --local path "$bundlerlayer" >/dev/null
    bundle config --local bin "$bundlerlayer/bin" >/dev/null
else
    # Determine if there has been a gem dependency change and install new gems to the bundler layer; re-using existing and un-changed gems
    echo "---> Installing gems"
    mkdir -p "$bundlerlayer"
    cat >> "$layersdir/bundler.toml" << EOL
[metadata]
checksum = "$local_bundler_checksum"
EOL
    bundle config set --local path "$bundlerlayer" && bundle install && bundle binstubs --all --path "$bundlerlayer/bin"

fi

# 7. SET DEFAULT START COMMAND
cat > "$layersdir/launch.toml" << EOL
# our web process
[[processes]]
type = "web"
command = "bundle exec ruby app.rb"
default = true

# our worker process
[[processes]]
type = "worker"
command = "bundle exec ruby worker.rb"
EOL

Finally, create a file ruby-sample-app/.ruby-version with the following contents:

3.1.0

Now when you run:

pack build test-ruby-app --path ./ruby-sample-app --buildpack ./ruby-buildpack

You will notice that version of Ruby specified in the app’s .ruby-version file is downloaded.

===> BUILDING
---> Ruby Buildpack
---> Downloading and extracting Ruby 3.1.0

Next, let’s see how buildpacks can store information about the dependencies provided in the output app image for introspection.

Next Step

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