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RFC: Dynamic Runtime Base Image Selection #174

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RFC: Dynamic Runtime Base Image Selection #174

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c58c859
Add RFC: Dynamic Runtime Base Image Selection
sclevine Jun 30, 2021
8d2be16
Add PURL link
sclevine Jul 1, 2021
2e79865
Fix PURL link
sclevine Jul 1, 2021
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sclevine Jul 7, 2021
61b1dc0
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sclevine Jul 8, 2021
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54 changes: 54 additions & 0 deletions text/0000-run-image-selection.md
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# Meta
[meta]: #meta
- Name: Dynamic Runtime Base Image Selection
- Start Date: 2021-06-30
- Author(s): sclevine
- RFC Pull Request: (leave blank)
- CNB Pull Request: (leave blank)
- CNB Issue: (leave blank)
- Supersedes: [RFC0069](https://github.com/buildpacks/rfcs/blob/main/text/0069-stack-buildpacks.md), [RFC#167](https://github.com/buildpacks/rfcs/pull/167), many others
- Depends on: [RFC#172](https://github.com/buildpacks/rfcs/pull/172)

# Summary
[summary]: #summary

*NOTE: This proposal is part of a larger initiative to reduce complexity originally outlined in https://github.com/buildpacks/rfcs/pull/167*

This RFC introduces functionality for dynamically selecting runtime base images, as a partial alternative to stackpacks (along with [RFC#173](https://github.com/buildpacks/rfcs/pull/173)).

# Motivation
[motivation]: #motivation

- Buildpacks on the same builder may have different OS package requirements.
- OS package requirements may vary based on the type of application.

# What it is
[what-it-is]: #what-it-is

This RFC proposes that we allow buildpacks to select a minimal runtime base image during detection.

# How it Works
[how-it-works]: #how-it-works

Builders may specify an ordered list of runtime base images, where each entry may contain a list of runtime base image mirrors.
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Can we include an example of what this would look like?

[[stacks]]
id = "centos"
build-image = "example/centos7-build:123"
run-image = "example/centos7-run:123"

[[stacks]]
id = "bionic"
build-image = "example/ubuntu-build:123"
run-image = "example/ubuntu-run:123"

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Will do


Buildpacks may specify a list of package names (as [PURL URLs](https://github.com/package-url/purl-spec) without versions or qualifiers) in a `packages` table in the build plan during detection.
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Does this mean that the lifecycle needs to be able to inspect a base image and determine what packages it includes?

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Yep, just like it works with mixins and buildpack.toml today.

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But does this mean it needs to be aware of how to do that on a particular stack (apt, yum, etc)? With mixins it can do this generically.

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The PURL URLs in the CycloneDX-formatted SBoM provide a generic abstraction for this.


The first runtime base image that contains all required packages is selected. When mirrors are present, the runtime base image mirror matching the app image is always used, including for package queries.

# Drawbacks
[drawbacks]: #drawbacks

- Involves breaking changes.
- Buildpacks cannot use this mechanism to install OS packages directly, only select runtime base images.

# Alternatives
[alternatives]: #alternatives

- Stackpacks
- Only allow app developers to install OS packages (using Dockerfiles via [RFC#172](https://github.com/buildpacks/rfcs/pull/172))

# Unresolved Questions
[unresolved-questions]: #unresolved-questions

- Should packages be determined during the detect or build phase? Opinion: detect phase, so that a runtime base image's app-specified Dockerfiles may by applied in parallel to the buildpack build process.