Working with Custom Buildpacks
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A buildpack repository contains three main scripts, situated in a folder named
detect script determines whether or not to apply the buildpack to an app. The script is called with one argument, the
build directory for the app. The
build directory contains the app files uploaded when a user performs a
detect script returns an exit code of
0 if the buildpack is compatible with the app. In the case of system buildpacks, the script also prints the buildpack name, version, and other helpful information to
The following is an example
detect script written in Ruby that checks for a Ruby app based on the existence of a
#!/usr/bin/env ruby gemfile_path = File.join ARGV, "Gemfile" if File.exist?(gemfile_path) puts "Ruby" exit 0 else exit 1 end
Optionally, the buildpack detect script can output additional details decided by the buildpack developer. These additional details include buildpack versioning information and a detailed list of configured frameworks and their associated versions.
The following is an example of the detailed information returned by the Java buildpack:
java-buildpack=v3.0-https://github.com/cloudfoundry/java-buildpack.git#3bd15e1 open-jdk-jre=1.8.0_45 spring-auto-reconfiguration=1.7.0_RELEASE tomcat-access-logging-support=2.4.0_RELEASE tomcat-instance=8.0.21 tomcat-lifecycle-support=2.4.0_RELEASE ...
compile script builds a droplet by packaging the app dependencies, assuring that the app has all the necessary components needed to run.
The script is run with two arguments: the
build directory for the app and the
cache directory, which is a location the buildpack can use to store assets during the build process. During the execution of the
compile script, all output sent to
STDOUT is relayed through the cf CLI to the user.
The following is an example of a simple
#!/usr/bin/env ruby #sync output $stdout.sync = true build_path = ARGV cache_path = ARGV install_ruby private def install_ruby puts "Installing Ruby" # !!! build tasks go here !!! # download ruby # install ruby end
release script provides feedback metadata to Cloud Foundry indicating how the app should be executed. The script is run with one argument, the
build directory. The script must generate a YAML file in the following format:
default_process_types: web: start_command.filetype
default_process_types indicates the type of app being run and the command used to start it.
This start command is used if a start command is not specified in the
cf push or in a Procfile.
At this time, only
web type of apps are supported.
Note: To define environment variables for your buildpack, add a bash script to the
.profile.d directory in the root folder of your app.
The following example shows what a
release script for a Rack app might return:
default_process_types: web: bundle exec rackup config.ru -p $PORT
web command runs as
bash -c COMMAND when Cloud Foundry starts your app. Refer to the command attribute section for more information about custom start commands.
The buildpack staging process extracts the droplet into the
/home/vcap directory inside the instance container, and creates the following filesystem tree:
app/ logs/ tmp/ staging_info.yml
app directory contains
BUILD_DIR contents, and
staging_info.yml contains the staging metadata saved in the droplet.
Cloud Foundry buildpacks work with limited or no Internet connectivity. A Cloud Foundry operator can use the buildpack packager RubyGem to give the same flexibility to custom buildpacks, enabling them to work in partially or completely disconnected environments.
Use the Buildpack Packager
- Ensure that you have installed the buildpack packager RubyGem.
- Create a manifest.yml in your buildpack.
- Run the packager in cached mode:
$ buildpack-packager --cached
The packager will add (almost) everything in your buildpack directory into a zip file. It will exclude anything marked for exclusion in your manifest.
In cached mode, the packager will download and add dependencies as described in the manifest.
The packager has the following option flags:
--force-download: By default, the packager stores the dependencies that it downloads while building a cached buildpack in a local cache at
~/.buildpack-packager. Storing dependencies enables the packager to avoid re-downloading them when repackaging similar buildpacks. Running
buildpack-packager --cachedwith the
--force-downloadoption forces the packager to download dependencies from the S3 host and ignore the local cache. When packaging an uncached buildpack,
--use-custom-manifest: To include a different manifest file in your packaged buildpack, you can call the packager with the
--use-custom-manifest PATH/TO/MANIFEST.YMLoption. The packager generates a buildpack with the specified manifest. If you are building a cached buildpack, the packager vendors dependencies from the specified manifest as well.
For more information, see the documentation at the buildpack-packager Github repo.
Use and Share the Packaged Buildpack
After you have packaged your buildpack using
buildpack-packager you can use the resulting
.zip file locally, or share it with others by uploading it to any network location that is accessible to the CLI. Users can then specify the buildpack with the
-b option when they push apps. See Deploying Apps with a Custom Buildpack for details.
buildpack-packager version 2.3.0, you can specify the default version for a dependency by adding a
default_versions object to the
default_versions object has two properties,
version. For example:
default_versions: - name: go version: 1.6.3 - name: other-dependency version: 1.1.1
To specify a default version:
default_version_forscript from the compile-extensions repository, passing the path of your
manifest.ymland the dependency name as arguments. The following command uses the example manifest from step 1:
$ ./compile-extensions/bin/default_version_for manifest.yml go 1.6.3
buildpack-packager script validates this object according to the following rules:
You can create at most one entry under
default_versionsfor a single dependency. The following example causes
buildpack-packagerto fail with an error because the manifest specifies two default versions for the same
# Incorrect; will fail to package default_versions: - name: go version: 1.6.3 - name: go version: 1.3.1
If you specify a
default_versionfor a dependency, you must also list that dependency and version under the
dependenciessection of the manifest. The following example causes
buildpack-packagerto fail with an error because the manifest specifies
version: 1.6.3for the
godependency, but lists
# Incorrect; will fail to package default_versions: - name: go version: 1.6.3 dependencies: - name: go version: 1.5.4 uri: https://storage.googleapis.com/golang/go1.5.4.linux-amd64.tar.gz md5: 27b1c469797292064c65c995ffe30386 cf_stacks: - cflinuxfs2
Once a custom buildpack has been created and pushed to a public git repository, the git URL can be passed via the cf CLI when pushing an app.
For example, for a buildpack that has been pushed to Github:
$ cf push my-new-app -b git://github.com/johndoe/my-buildpack.git
Alternatively, you can use a private git repository, with https and username/password authentication, as follows:
$ cf push my-new-app -b https://username:email@example.com/johndoe/my-buildpack.git
By default, Cloud Foundry uses the default branch of the buildpack’s git repository. You can specify a different branch using the git url as shown in the following example:
$ cf push my-new-app -b https://github.com/johndoe/my-buildpack.git#my-branch-name
Additionally, you can use tags or shas in a git repository, as follows:
$ cf push my-new-app -b https://github.com/johndoe/my-buildpack#v1.4.2
$ cf push my-new-app -b https://github.com/johndoe/my-buildpack#a2951e298bd22732fceb3968d541015120dbaf93
The app will then be deployed to Cloud Foundry, and the buildpack will be cloned from the repository and applied to the app.
Note: If a buildpack is specified using
cf push -b the
detect step will be skipped and as a result, no buildpack
detect scripts will be run.
Note: A common development practice for custom buildpacks is to fork existing buildpacks and sync subsequent patches from upstream. To merge upstream patches to your custom buildpack, use the approach that Github recommends for syncing a fork.