Creating Custom Buildpacks

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This topic describes how to create custom buildpacks for .

For more information about how buildpacks work, see the Understanding Buildpacks topic.

Package Custom Buildpacks

buildpacks can work with limited or no Internet connectivity. The Buildpack Packager RubyGem gives the same flexibility to custom buildpacks, enabling them to work in partially or completely disconnected environments.

Use the Buildpack Packager

  1. Ensure that you have installed the Buildpack Packager RubyGem.
  2. Create a manifest.yml in your buildpack.
  3. 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 downloads and adds 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 --cached with the --force-download option forces the packager to download dependencies from the S3 host and ignore the local cache. When packaging an uncached buildpack, --force-download does nothing.

  • --use-custom-manifest: To include a different manifest file in your packaged buildpack, call the packager with the --use-custom-manifest PATH/TO/MANIFEST.YML option. 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 repository.

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.

Note: Offline buildpack packages may contain proprietary dependencies that require distribution licensing or export control measures. For more information about offline buildpacks, refer to Packaging Dependencies for Offline Buildpacks.

Specify a Default Version

As of buildpack-packager version 2.3.0, you can specify the default version for a dependency by adding a default_versions object to the manifest.yml file. The default_versions object has two properties, name and version. For example:

default_versions:
- name: go
  version: 1.6.3
- name: other-dependency
  version: 1.1.1

To specify a default version:

  1. Add the default_version object to your manifest, following the rules below. You can find a complete example manifest in the go-buildpack repository.

  2. Run the default_version_for script from the compile-extensions repository, passing the path of your manifest.yml and 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
    

Rules for Specifying a Default Version

The buildpack-packager script validates this object according to the following rules:

  • You can create at most one entry under default_versions for a single dependency. The following example causes buildpack-packager to fail with an error because the manifest specifies two default versions for the same go dependency.

    # Incorrect; will fail to package
    default_versions:
    - name: go
      version: 1.6.3
    - name: go
      version: 1.7.5
    
  • If you specify a default_version for a dependency, you must also list that dependency and version under the dependencies section of the manifest. The following example causes buildpack-packager to fail with an error because the manifest specifies version: 1.9.2 for the go dependency, but lists version: 1.7.5 under dependencies.

    # Incorrect; will fail to package
    default_versions:
    - name: go
      version: 1.9.2
    
    dependencies:
    - name: go
      version: 1.7.5
      uri: https://storage.googleapis.com/golang/go1.7.5.linux-amd64.tar.gz
      md5: c8cb76e2308c792e2705c2eb1b55de95
      cf_stacks:
      - cflinuxfs2
    

Core Buildpack Communication Contract

This section describes the communication contract followed by the core buildpacks. This contract enables buildpacks to interact with one another, so that developers can use multiple buildpacks with their applications.

Buildpack developers must ensure their custom buildpacks follow the contract.

This section uses the following placeholders:

  • IDX is the zero-padded index matching the position of the buildpack in the priority list.
  • MD5 is the MD5 checksum of the buildpack’s URL.

For all buildpacks that supply dependencies via /bin/supply:

  • The buildpack must create /tmp/deps/IDX/config.yml to provide a name to subsequent buildpacks. This file may also contain miscellaneous configuration for subsequent buildpacks.
  • The config.yml file should be formatted as follows, replacing BUILDPACK with the name of the buildpack providing dependencies and YAML-OBJECT with the YAML object that contains buildpack-specific configuration: name: BUILDPACK config: YAML-OBJECT
  • The following directories may be created inside of /tmp/deps/IDX/ to provide dependencies to subsequent buildpacks:
    • /bin: Contains binaries intended for $PATH during staging and launch
    • /lib: Contains libraries intended for $LD_LIBRARY_PATH during staging and launch
    • /include: Contains header files intended for compilation during staging
    • /pkgconfig: Contains pkgconfig files intended for compilation during staging
    • /env: Contains environment vars intended for staging, loaded as FILENAME=FILECONTENTS
    • /profile.d: Contains scripts intended for /app/.profile.d, sourced before launch
  • The buildpack may make use of previous non-final buildpacks by scanning /tmp/deps/ for index-named directories containing config.yml.

For the last buildpack:

  • To make use of dependencies provided by the previously applied buildpacks, the last buildpack must scan /tmp/deps/ for index-named directories containing config.yml.
  • To make use of dependencies provided by previous buildpacks, the last buildpack:
    • May use /bin during staging, or make it available in $PATH during launch
    • May use /lib during staging, or make it available in $LD_LIBRARY_PATH during launch
    • May use /include, /pkgconfig, or /env during staging
    • May copy files from /profile.d to /tmp/app/.profile.d during staging
    • May use the supplied config object in config.yml during the staging process

Deploy Apps with a Custom Buildpack

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:password@github.com/johndoe/my-buildpack.git

By default, 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 in a git repository, as follows:

$ cf push my-new-app -b https://github.com/johndoe/my-buildpack#v1.4.2

The app will then be deployed to , 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.

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