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Summary

This RFC proposes changes to the structure of a process type in order to make the interface more similar to k8s, docker and other familiar tools and simultaneously simplify the implementation.

Definitions

process type: A named process definition, contributed by a buildpack at build-time and executed by the launcher at run-time.

direct process: A process that is executed directly.

shell process: A process that is executed by a shell.

script process: A special type of shell process where there are zero args and command is a shell script.

build-provided profiles scripts: Profile scripts that are provided by buildpacks at build-time. When the launcher executes a shell process these scripts will be sourced in the shell, prior to process execution.

user-provided profile script: A profile scripts that is provided by the user in the app root of the app dir. When the launcher executes a shell process this script will be sourced in the shell, after buildpack-provided profile scripts and prior to process execution.

Motivation

This RFC aims to:

  1. Reduce complexity in the CNB Specification
  2. Enable new use cases by supporting overridable arguments
  3. Improve interoperability between buildpacks and minimal or distroless stacks

We will achieve these goals be addressing the following problems with our current process type model:

Argument Handling

In RFC 0045 we introduced a change to the launcher interface that allows users to append additional arguments to a process type at runtime. This was a major improvement to the runtime interface for app images, as users no longer needed to respecify the entire command in order to append a single argument. However, in the Launcher Arguments proposal we made two mistakes:

  1. complexity - We tried to be clever and support appending argument appending to shell processes. This "works" but can result in surprising behavior such as the need to escape literal " characters to prevent Bash from removing them during evaluation. Users must know whether the process is a direct process or a shell process in order to provide arguments correctly, making process types a leaky abstraction.
  2. use cases - Arguments can serve two different purposes. Some are required and should always be included in the command. Some are merely defaults that may be overridden by the user. The current spec does not support the latter case.

Shell-Specific Logic

interoperability - Currently, unless a buildpack author specifically indicates that the given process type is a direct process, a shell process with a dependency on Bash (linux) or Cmd (windows) is created. This is not ideal, as many in the industry are moving towards minimal images that do not include a shell, in order to reduce surface attack area. Buildpack authors may inadvertantly create process types that depend on a shell even when no such dependency is necessary.

complexity - Having a direct dependency between the launcher and specific shells is inelegant. It makes the spec more complex and end users and buildpack author are forced to understand that complexity in order to understand the behavior of the resulting usage or debug issues. For example end users and buildpacks authors must understand:

  • The difference between a direct and shell process
  • The nuances of argument handling in direct vs. Bash vs. Cmd cases
  • That buildpack-provided profile scripts will not apply to direct processes
  • That a user-provided .profile script will not apply to a direct processes

Removing this special behavior and requiring buildpacks to explicitly include any required shell in the command itself creates a simpler, more comprehensible model for buildpack authors and end users alike.

What it is

Process Schema

The existing schema for the processes table in launch.toml is

# Old Schema

[[processes]]
type = "required"
command = "required"
args = ["optional"]
direct = false
default = false

The new proposed schema is:

# New Schema

[[processes]]
type = "required"
command = ["required"]
args = ["optional"]
default = false

The following changes have been made:

  • direct has been removed - all processes are executed directly. If bash or cmd.exe is required it should be included in command. No surprises.
  • command is now an array - Arguments in command will not be overwritten if a user provides additional arguments at runtime. args are default arguments that will be overwritten if a user provides additional arguments at runtime. The makes command analogous to Entrypoint in the OCI spec and command in a Kubernetes PodSpec. args is analogous to cmd and args in docker and Kubernetes respectively.

How it Works

Buildpack-provided process types

Example 1 - A Shell Process

The Paketo .Net Execute Buildpack may generates shell processes similar to the following:

[[processes]]
type = "web"
command = "dotnet my-app.dll --urls http://0.0.0.0:${PORT:-8080}"
direct = false

NOTE: the buildpack API used by this buildpack (0.5) predates the introduction of default.

Using the new API this process could look like:

[[processes]]
type = "bash"
command = ["bash", "-c", "dotnet", "my-app.dll", "--urls", "http://0.0.0.0:${PORT:-8080}"]
default = true

Things to note:

  • If the buildpack authors believed that --urls should be overridable they could set move the last two arguments from command to args.

Example 2 - A Script Process

When a buildpack upgrade to the new buildpack API, it must convert any existing shell processes into direct processes.

The Paketo Yarn Start Buildpack currently may generate a script processes similar to the following:

[[processes]]
type = "web"
command = "pre-start.sh && nodejs server.js && post-start.sh"
direct = false
default = false

Using the new API this process look like:

[[processes]]
type = "web"
command = ["bash", "-c", "pre-start.sh && nodejs server.js && post-start.sh"]
default = false

User-provided process types

Example 1 - A Script Process

The follow custom script command:

docker run --entrypoint launcher <image> 'for opt in $JAVA_OPTS; do echo $opt; done'

will become the following, using the new platform API

docker run --entrypoint launcher <image> bash -c 'for opt in $JAVA_OPTS; do echo $opt; done'

What About Profile Scripts?

<layer>/profile.d/*

When a buildpack upgrades to the new buildpack API, buildpack-provided profile scripts will no longer be supported. Instead buildpacks can use exec.d. Most existing profiles scripts can be easily converted.

**Example 1: jkutner/sshd **

The following profile script example was taken from https://github.com/jkutner/sshd-buildpack/blob/master/sbin/sshd.sh

#!/usr/bin/env bash

if [[ "${SSH_DISABLED:-}" != "true" ]]; then
  ssh_layer=$(realpath $(dirname ${BASH_SOURCE[0]})/..)
  ssh_dir=$(realpath $HOME/.ssh)

  mkdir -p $ssh_dir
  cat $ssh_layer/id_rsa.pub >> $ssh_dir/authorized_keys

  cat << EOF >> $ssh_dir/sshd_config
HostKey $ssh_layer/id_rsa
AuthorizedKeysFile $ssh_dir/authorized_keys
EOF

  chmod 600 /workspace/.ssh/*

  ssh_port=${SSH_PORT:-"2222"}
  echo "at=sshd state=starting user=$(whoami) port=${ssh_port}"
  /usr/sbin/sshd -f $ssh_dir/sshd_config -o "Port ${ssh_port}"
fi

This script does not require any changes and would work as an exec.d helper. To implement the new buildpack API the buildpack author would have to:

  • Move the above script from <layer>/profile.d/sshd to <layer>/exec.d/sshd.
  • Ensure script is executable by run-image user (if it isn't already).

**Example 2: paketo-buildpacks/node-engine **

The following profile script example was taken from https://github.com/paketo-buildpacks/node-engine/blob/main/environment.go

if [[ -z "$MEMORY_AVAILABLE" ]]; then
		memory_in_bytes="$(cat /sys/fs/cgroup/memory/memory.limit_in_bytes)"
		MEMORY_AVAILABLE="$(( $memory_in_bytes / ( 1024 * 1024 ) ))"
fi
export MEMORY_AVAILABLE

The following revised version of this script would implement the exec.d interface

#!/usr/bin/env bash

if [[ -z "$MEMORY_AVAILABLE" ]]; then
		memory_in_bytes="$(cat /sys/fs/cgroup/memory/memory.limit_in_bytes)"
		MEMORY_AVAILABLE="$(( $memory_in_bytes / ( 1024 * 1024 ) ))"
fi

cat << EOF >&3
MEMORY_AVAILABLE = "${MEMORY_AVAILABLE}" 
EOF

As in the example above. The buildpack author would need to move the script to the new directory and ensure the file is created with the correct permissions.

<app>/.profile

When a platform upgrades to the new platform API, user-provided profile scripts will no longer be supported. A buildpack could provide an identical or similar interface by detecting a .profile file in the app dir, wrapping it so that it implements the exec.d interface and add creating an executable with that layer. A platform that wishes to always provide that functionality could add a buildpack like this to every build. By making dynamic runtime environment modification a buildpack concern and not duplicating similar behavior in the platform API, we reduce the surface area and complexity of the platform API and thus improve normalization of the spec.

In order to prevent regressions for users, this RFC should not be implemented until a utility buildpack providing support for <app>/.profile is released, allowing platforms to provide uninterrupted support for the .profile interface.

API compat

Overriding Args for Shell Processes

We should convert args in launch.toml to command in metadata.toml to expose a consistent interface to users. For example, if a buildpack the following process type in launch.toml

[[process]]
type = "hi"
command = "echo"
args = ["hello", "world"]
direct = true

we would convert it to the following in metadata.toml

[[process]]
type = "hi"
command = ["echo", "hello", "world"]
args = []
direct = true

This spares users from having to learn about the differences between buildpack APIs in order to predict how additional arguments will behave.

Evaluating Args

For shell processes provided by older buildpacks, we must continue to Bash evaluate buildpack-provided args to avoid breaking older buildpacks

For example, if a buildpack using API 0.5 creates the following entry in launch.toml

[[process]]
type = "hi"
command = "echo"
args = ["hello", "${WORLD:-world}"]

It should be converted to the following in metadata.toml

[[process]]
type = "hi"
command = ["echo", "hello", "${WORLD:-world}"]
args = []
direct = false

and the launcher should Bash evaluate each entry in command to avoid breaking changes.

However, any additional user-provided args should NOT be Bash evaluated to reduce the amount of complexity exposed to end users.

profiles

Profiles contributed by older buildpacks will still be evaluated when executing shell process types. But user provided profiles will not be evaluated when using the new platform API, even when a shell process is executed. Again, this is done to prevent differences between the buildpack APIs from leaking into the user interface. Users will only need think about differences between platform APIs.

Migration Path

Buildpack Authors

When migrating to the new API, buildpack authors should take the following steps:

  1. Does the buildpack contribute any profile.d helpers? If so, replace these with equivalent exec.d helpers.
  2. Does the buildpack contribute a direct=true process? If so, remove direct=true from the process definition, this is now the default.
  3. Does the buildpack contribute a direct=false process? If so, there are two options:
    1. Explicitly add bash or cmd to the process definition, this is the safest path forward.
    2. Remove an unnecessary dependency on bash or cmd by:
      • Ensure that your process functions properly as PID1.

Platform Maintainers

Before upgrading to the new API platform, platforms that wish to support the <app>/.profile or <app>/.profile.bat should ensure that all builds contain a buildpack that provides support for this feature, whenever the stack provides the requisite shell.

Drawbacks

In exchange for a reduction in complexity and cognitive overhead buildpack-authors and end users lose certain conveniences like the more intuitive profile interface. This drawback could be remediated by tooling for buildpack authors (to make creation of exec.d helpers easy) or by buildpacks (to support .profile). However, in the .profile case, consistency across ecosystems will be a matter of convention (like Procfile) rather than a guarantee.

Alternatives

Lifecycle support for <app>/.exec

When we remove support for <app>/.profile we could add support for <app>/.exec or similar where <app>/.exec must implement the exec.d interface.

pros:

  • Users may still dynamically modify the runtime environment without requiring a specific buildpack

cons:

  • The exec.d interface must be duplicated in both the buildpack and platform API
  • If we modify the interface the same app might behave differently or fail to run on specific platforms depending on which version of the platform API they are using
  • Users must directly implement our less-than-perfectly-intuitive exec.d interfaces instead of whatever better UX buildpack authors invent.

Lifecycle support for <app>.profile

We could consider having the lifecycle convert <app>/.profile files into exec.d
If we do not remove shell logic from the spec, users will continue to find the launcher behavior vexingly complex. Also, there will be no sane path forward for supporting overridable arguments.

pros:

  • Fewer breaking changes
  • More consistency behavior across platforms/builders

cons:

  • complexity in the launcher
  • launcher behavior that fails on certain stacks
  • undesirable coupling between the lifecycle and specific shells

API compat strategy - Explicit Bash Evaluation

When adding legacy shell processes to metadata.toml we could replace the "direct=false" command with the literal command that will be evaluated. For example the following script process in launch.toml

[[process]]
type = "hi"
command = "echo hello "${WORLD:-world}"
direct = false

Could become the following in metadata.toml

[[process]]
type = "hi"
command = ["bash", "-c", 'echo hello "${WORLD:-world}"']
direct = true # we could even potentially remove this from the metadata.toml schema entirely

However, things get very complicated if we explicitly source profiles or shell evaluated args in command. The complexity of the resulting commands probably makes this strategy untenable.

Prior Art

The Paketo Java buildpacks have already converted an extensive collection of profile script to exec.d binaries and converted all processes to direct processes in order to support minimal stacks like io.paketo.stacks.tiny and to provide users with more intuitive argument handling.

The Procfile interface is supported by Paketo, Heroku, and Google buildpacks, demonstrating that it is possible to have a consistent interface across buildpack ecosystems without building direct support for that interface in the lifecycle.

Spec. Changes

Platform API

The launcher usage will change to the following.

/cnb/process/<process-type> [<arg>...]
# OR
/cnb/lifecycle/launcher [<cmd>...]

All references to <direct> will be removed from the usage.

Execution rules will become simpler. This following is a draft to convey the idea, not the final wording (which wil need some wordsmithing):

The launcher:

  • MUST derive the command to execute values of <cmd> and <args> as follows:
    • If the final path element in $0, matches the type of any buildpack-provided <process-type>
      • <cmd> SHALL be the <command> defined for <process-type> in <layers>/config/metadata.toml
      • If the user has provided <args> to the launcher
        • <args> SHALL be the user-provided <args>
      • If the user has not provided <args> to the launcher
        • <args> SHALL be the <args> defined for <process-type> in <layers>/config/metadata.toml
    • Else <cmd> shall be the user provided <cmd>

Buildpack API

All references to Bash, Command Prompt, profile.d, and <app>/.profile will be removed the launch section of the buildpack spec, significantly reducing the complexity of the specification.

The launch.toml data format will change to include the following:

[[processes]]
type = "<process type>"
command = ["<command>"]
args = ["<arguments>"]
default = false

History

Amended

Meta

  • Name: Removed references to custom env templating
  • Start Date: 2022-12-02
  • Author(s): natalieparellano
  • Amendment Pull Request: (leave blank)

Summary

As this is a breaking change, we decided to do this in a separate (yet to be created) RFC.

Created issue: #258

In addition to the changes described originally in 0093 we'd like some way of versioning the launcher interface, to avoid surprising end-users.

Motivation

Why was this amendment necessary?

The RFC text should reflect what was actually implemented / agreed upon to avoid confusion.