understand-buildpacks.html.md.erb

---
title: How Buildpacks Work
owner: Buildpacks
---

This topic describes how buildpacks are structured and detected in Cloud Foundry.


## <a id='buildpack-scripts'></a> Buildpack Scripts

A buildpack repository may contain the following five scripts in the `bin` directory:

* `bin/detect` determines whether or not to apply the buildpack to an app.
* `bin/supply` provides dependencies for an app.
* `bin/finalize` prepares the app for launch.
* `bin/release` provides feedback metadata to Cloud Foundry indicating how the app should be executed.
* `bin/compile` is a deprecated alternative to `bin/supply` and `bin/finalize`.

The `bin/supply` and `bin/finalize` scripts replace the deprecated `bin/compile` script. Older buildpacks may still use `bin/compile` with the latest version of Cloud Foundry. In this case, applying multiple buildpacks to a single app is not supported. Similarly, newer buildpacks may still provide `bin/compile` for compatibility with Heroku and older versions of Cloud Foundry.

The `bin/supply` script is required for non-final buildpacks. The `bin/finalize` (or `bin/compile`) script is required for final buildpacks.

<p class="note"><strong>Note:</strong> In this topic, the terms <em>non-final buildpack</em> and <em>final buildpack</em>, or <em>last buildpack</em>, are used to describe the process of applying multiple buildpacks to an app. For example: <code>cf push APP-NAME -b FIRST-BUILDPACK -b SECOND-BUILDPACK -b FINAL-BUILDPACK</code>.</p>

<p class="note"><strong>Note:</strong> If you use only one buildpack for your app, this buildpack behaves as a final, or last, buildpack.</p>

<p class="note"><strong>Note:</strong> When using multi-buildpack support, the last buildpack in order is the final buildpack, and is able to make changes to the app and determine a start command. All other specified buildpacks are non-final and only supply dependencies.</p>

### <a id='detect-script'></a> bin/detect

The `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 you run `cf push`. 

The `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 information to `STDOUT`.

The following is an example `detect` script that checks for a Ruby app based on the existence of a `Gemfile`:

~~~
#!/usr/bin/env ruby

gemfile_path = File.join ARGV[0], "Gemfile"

if File.exist?(gemfile_path)
  puts "Ruby"
  exit 0
else
  exit 1
end
~~~

Optionally, the buildpack `detect` script can output additional details provided by the buildpack developer. This includes buildpack versioning information and a 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 ...
```

<p class="note"><strong>Note:</strong> Cloud Foundry detects only one buildpack by default. When multiple buildpacks are desired, you must explicitly specify them.</p>

For more information, see [Buildpack Detection](#buildpack-detection).

### <a id='supply-script'></a> bin/supply

The `supply` script provides dependencies for the app and runs for all buildpacks. All output sent to `STDOUT` is relayed to the user through the Cloud Foundry Command Line Interface (cf CLI).

The script is run with four arguments:

* The `build` directory for the app
* The `cache` directory, which is a location the buildpack can use to store assets during the build process
* The `deps` directory, which is where dependencies provided by all buildpacks are installed
* The `index`, which is a number that represents the ordinal position of the buildpack

The `supply` script stores dependencies in `deps`/`index`. It may also look in other directories within `deps` to find dependencies supplied by other buildpacks.

The `supply` script must not modify anything outside of the `deps`/`index` directory. Staging may fail if such modification is detected.

The `cache` directory provided to the `supply` script of the final buildpack is preserved even when the buildpack is upgraded or otherwise changes. The `finalize` script also has access to this cache directory.

The `cache` directories provided to the `supply` scripts of non-final buildpacks are cleared if those buildpacks are upgraded or otherwise change.

The following is an example of a simple `supply` script:

~~~
#!/usr/bin/env ruby

#sync output

$stdout.sync = true

build_path = ARGV[0]
cache_path = ARGV[1]
deps_path = ARGV[2]
index = ARGV[3]

install_ruby

private

def install_ruby
  puts "Installing Ruby"

  # !!! build tasks go here !!!
  # download ruby 
  # install ruby
end
~~~

### <a id='finalize-script'></a> bin/finalize

The `finalize` script prepares the app for launch and runs only for the last buildpack. All output sent to `STDOUT` is relayed to the user through the cf CLI.

The script is run with four arguments:

* The `build` directory for the app
* The `cache` directory, which is a location the buildpack can use to store assets during the build process
* The `deps` directory, which is where dependencies provided by all buildpacks are installed
* The `index`, which is a number that represents the ordinal position of the buildpack

The `finalize` script may find dependencies installed by the `supply` script of the same buildpack in `deps`/`index`. It may also look in other directories within `deps` to find dependencies supplied by other buildpacks.

The `cache` directory provided to the `finalize` script is preserved even when the buildpack is upgraded or otherwise changes. The `supply` script of the same buildpack also has access to this cache directory.

The following is an example of a simple `finalize` script:

~~~
#!/usr/bin/env ruby

#sync output

$stdout.sync = true

build_path = ARGV[0]
cache_path = ARGV[1]
deps_path = ARGV[2]
index = ARGV[3]

setup_ruby

private

def setup_ruby
  puts "Configuring your app to use Ruby"

  # !!! build tasks go here !!!
  # setup ruby 
end
~~~

### <a id='compile-script'></a> bin/compile (Deprecated)

The `compile` script is deprecated. It encompasses the behavior of the `supply` and `finalize` scripts for single buildpack apps by using the `build` directory to store dependencies.

The script is run with two arguments:

* The `build` directory for the app
* 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 to the user through the cf CLI.

### <a id='release-script'></a> bin/release

The `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 the `web` type of apps is supported.

<p class="note"><strong>Note:</strong> To define environment variables for your buildpack, add a Bash script to the <code>.profile.d</code> directory in the root folder of your app.</p>

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
~~~

<p class="note"><strong>Note:</strong> The <code>web</code> command runs as <code>bash -c COMMAND</code> when Cloud Foundry starts your app.</p>


## <a id='droplet-filesystem'></a> Droplet Filesystem

The buildpack staging process extracts the droplet into the `/home/vcap` directory inside the instance container and creates the following filesystem tree:

```
app/
deps/
logs/
tmp/
staging_info.yml
```

The `app` directory includes the contents of the `build` directory, and `staging_info.yml` contains the staging metadata saved in the droplet.


## <a id='buildpack-detection'></a> Buildpack Detection

When you push an app, Cloud Foundry uses a detection process to determine a single buildpack to use. For general information about this process, see the [How Diego Stages Buildpack Apps](../concepts/how-applications-are-staged.html#stage-buildpack) section of the _How Apps Are Staged_ topic.

During staging, each buildpack has a position in a priority list. You can retrieve this position by running `cf buildpacks`.

Cloud Foundry checks if the buildpack in position 1 is a compatible buildpack. If the position 1 buildpack is not compatible, Cloud Foundry moves on to the buildpack in position 2. Cloud Foundry continues this process until the correct buildpack is found. 

If no buildpack is compatible, the `cf push` command fails with the following error:

<pre class="terminal">
None of the buildpacks detected a compatible application
Exit status 222
Staging failed: Exited with status 222

FAILED
NoAppDetectedError
</pre>

For a more detailed account of how Cloud Foundry interacts with the buildpack, see [Sequence of Interactions](#interactions).


## <a id='interactions'></a> Sequence of Interactions 

This section describes the sequence of interactions between the Cloud Foundry platform and the buildpack. The sequence of interactions differs depending on whether the platform skips or performs buildpack detection.

### <a id='no-detection'></a> No Buildpack Detection

Cloud Foundry skips buildpack detection if the developer specifies one or more buildpacks in the app manifest or in the `cf push APP-NAME -b BUILDPACK-NAME` cf CLI command.

If you explicitly specify buildpacks, Cloud Foundry performs the following interactions:

1. For each buildpack except the last buildpack, the platform:
	1. Creates the `deps`/`index` directory.
	1. Runs `/bin/supply` with the `build`, `cache`, and `deps` directories and the buildpack `index`.
	1. Accepts any modification of the `deps`/`index` directory.
	1. Accepts any modification of the `cache` directory.
	1. May disallow modification of any other directories.

1. For the last buildpack, the platform:
	1. If `/bin/finalize` is present:
		1. Creates the `deps`/`index` directory if it does not exist.
		1. If `/bin/supply` is present, runs `/bin/supply` with the `build`, `cache`, and `deps` directories and the buildpack `index`.
		1. Accepts any modification of the `deps`/`index` directory.
		1. May disallow modification of the `build` directory.
		1. Runs `/bin/finalize` with the `build`, `cache`, and `deps` directories and the buildpack `index`.
		1. Accepts any modification of the `build` directory.
	1. If `/bin/finalize` is not present:
		1. Runs `/bin/compile` with the `build` and `cache` directories.
		1. Accepts any modification of the `build` directory.
	1. Runs `/bin/release` to determine staging information.

At the end of this process, the `deps` directory is included at the root of the droplet, adjacent to the `app` directory.

### <a id='detection'></a> Buildpack Detection

Cloud Foundry performs buildpack detection if the developer does not specify one or more buildpacks in the app manifest or in the `cf push APP-NAME -b BUILDPACK-NAME` cf CLI command.

<p class="note"><strong>Note:</strong> Cloud Foundry detects only one buildpack to use with the app.</p>

If the platform performs detection, it:

1. Runs `/bin/detect` for each buildpack.

1. Selects the first buildpack with a `/bin/detect` script that returns a zero exit status.

1. If `/bin/finalize` is present:
	1. Creates the `deps`/`index` directory if it does not exist.
	1. If `/bin/supply` is present, runs `/bin/supply` with the `build`, `cache`, and `deps` directories and the buildpack `index`.
	1. Accepts any modification of the `deps`/`index` directory.
	1. May disallow modification of the `build` directory.
	1. Runs `/bin/finalize` on the `build`, `cache`, and `deps` directories.
	1. Accepts any modification of the `build` directory.

1. If `/bin/finalize` is not present:
	1. Runs `/bin/compile` on the `build` and `cache` directories.
	1. Accepts any modification of the `build` directory.

1. Runs `/bin/release` to determine staging information.

At the end of this process, the `deps` directory is included at the root of the droplet, adjacent to the `app` directory.