Building blocks of a Cloud Native Buildpack

Now we will set up the buildpack scaffolding.

Let’s create the directory where your buildpack will live:

mkdir ruby-buildpack

buildpack.toml

You will now need a buildpack.toml to describe our buildpack.

Create the ruby-buildpack/buildpack.toml file and copy the following into it:

# Buildpack API version
api = "0.2"

# Buildpack ID and metadata
[buildpack]
id = "com.examples.buildpacks.ruby"
version = "0.0.1"
name = "Ruby Buildpack"

# Stacks that the buildpack will work with
[[stacks]]
id = "io.buildpacks.samples.stacks.bionic"

You will notice two specific fields in the file: buildpack.id and stack.id. The buildpack ID is the way you will reference the buildpack when you create buildpack groups, builders, etc. The stack ID is the root file system in which the buildpack will be run. This example can be run on one of two different stacks, both based upon Ubuntu Bionic.

detect and build

Next you will need to create the detect and build scripts. These files must exist in a bin directory in your buildpack directory.

Create your bin directory and then change to that directory.

mkdir ruby-buildpack/bin

Now create your ruby-buildpack/bin/detect file and copy in the following contents:

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

exit 1

Now create your ruby-buildpack/bin/build and copy in the following contents:

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

echo "---> Ruby Buildpack"
exit 1

You will need to make both of these files executable, so run the following command:

chmod +x ruby-buildpack/bin/detect ruby-buildpack/bin/build

These two files are now executable detect and build scripts. You are now able to use this buildpack.

Using your buildpack with pack

In order to test your buildpack, you will need to run the buildpack against your sample Ruby app using the pack CLI.

Set your default builder by running the following:

pack set-default-builder cnbs/sample-builder:bionic

Then run the following pack command:

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

The pack build command takes in your Ruby sample app as the --path argument and your buildpack as the --buildpack argument.

After running the command, you should see that it failed to detect, as the detect script is currently written to simply error out.

===> DETECTING
[detector] err:  [email protected] (1)
[detector] ERROR: No buildpack groups passed detection.
[detector] ERROR: failed to detect: buildpack(s) failed with err

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