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Making your buildpack configurable

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

Select NodeJS version

We’ll allow buildpack users to define the desired NodeJS version via a .node-js-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 (NodeJS), 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 node-js-buildpack/bin/detect to look like this:

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

if [[ ! -f package.json ]]; then
   exit 100
fi

# ======= ADDED =======
version=3.1.3

if [[ -f .node-js-version ]]; then
    version=$(< .node-js-version tr -d '[:space:]')
fi

cat > "${CNB_BUILD_PLAN_PATH}" << EOL
provides = [{ name = "node-js" }]
requires = [{ name = "node-js", metadata = { version = "$version" } }]
EOL
# ======= /ADDED =======

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

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

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

echo "---> NodeJS Buildpack"

# ======= MODIFIED =======
# 1. GET ARGS
plan=${CNB_BP_PLAN_PATH}

# 2. CREATE THE LAYER DIRECTORY
node_js_layer="${CNB_LAYERS_DIR}"/node-js
mkdir -p "${node_js_layer}"

# ======= MODIFIED =======
# 3. DOWNLOAD node-js
default_node_js_version="18.18.1"
node_js_version=$(cat "$plan" | yj -t | jq -r '.entries[] | select(.name == "node-js") | .metadata.version' || echo ${default_node_js_version})
node_js_url=https://nodejs.org/dist/v${node_js_version}/node-v${node_js_version}-linux-x64.tar.xz
remote_nodejs_version=$(cat "${CNB_LAYERS_DIR}/node-js.toml" 2>/dev/null | yj -t | jq -r .metadata.nodejs_version 2>/dev/null || echo 'NOT FOUND')
if [[ "${node_js_url}" != *"${remote_nodejs_version}"* ]]; then
    echo "-----> Downloading and extracting NodeJS" ${node_js_version}
    wget -q -O - "${node_js_url}" | tar -xJf - --strip-components 1 -C "${node_js_layer}"
else
    echo "-----> Reusing NodeJS"
fi

# 4. MAKE node-js AVAILABLE DURING LAUNCH and CACHE the LAYER
cat > "${CNB_LAYERS_DIR}/node-js.toml" << EOL
[types]
cache = true
launch = true
[metadata]
nodejs_version = "${node_js_version}"
EOL

# ========== ADDED ===========
# 5. SET DEFAULT START COMMAND
cat > "${CNB_LAYERS_DIR}/launch.toml" << EOL
[[processes]]
type = "web"
command = ["node", "app.js"]
default = true
EOL

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

18.18.1

In the following pack invocation we choose to --clear-cache so that we explicitly do not re-use cached layers. This helps us demonstrate that the NodeJS runtime layer does not get restored from a cache.

pack build test-node-js-app --clear-cache --path ./node-js-sample-app --buildpack ./node-js-buildpack

You will notice that version of NodeJS specified in the app’s .node-js-version file is downloaded.

===> BUILDING
...
[builder] ---> NodeJS Buildpack
[builder] -----> Downloading and extracting NodeJS 18.18.1

Going further

Now that you’ve finished your buildpack, how about extending it? Try:

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