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+# Meta
+[meta]: #meta
+- Name: Defining Mixins for io.buildpacks.stacks.bionic
+- Start Date: 2019-12-05
+- CNB Pull Request: (leave blank)
+- CNB Issue: (leave blank)
+- Supersedes: (put "N/A" unless this replaces an existing RFC, then link to that RFC)
+
+# Summary
+[summary]: #summary
+
+The Platform API specification allows stack maintainers to define stack mixins for a given stack.
+The Cloud Native Buildpacks project informally defines stack build and run images with ID `io.buildpacks.stacks.bionic` as containing exactly the set of Ubuntu packages in the `ubuntu:bionic` image on Docker Hub.
+This RFC proposes that we formalize the existing definition of `io.buildpacks.stacks.bionic` and define mixins on that stack to be Ubuntu 18.04 LTS packages. 
+
+In general, a stack is a contract that should be more like "Ubuntu Bionic" and less like "Platform XYZ's Re-distribution of Ubuntu Bionic."
+Stack images can still be provided by platforms/vendors, but they should use the standard CNB stack contracts where applicable.
+
+# Proposal
+[proposal]: #proposal
+
+This RFC proposes that apply the following set of restrictions to base images marked with stack ID `io.buildpacks.stacks.bionic`:
+1. The base set of packages is the set of packages distributed in `ubuntu:bionic` with `ca-certificates`, `libssl1.1`, and `openssl` added. The base image does not have to be derived from the `ubuntu:bionic` image, but must contain packages that provide exactly the same functionality to processes running as the CNB-specified, non-root user.
+2. When invoked as root, `apt-get` must be configured to use at minimum the set of package repositories that `ubuntu:bionic` is configured to use. `apt-get update` may be needed to download the package repository indices before `apt-get install` is functional.
+3. Mixins without an `=` are Ubuntu 18.04 LTS package names without version or architecture info from official package mirrors.
+4. Mixins that begin with `set=` are defined by the project as such:
+  a. `build:set=shell-utils` represents the change set: `build:curl`, `build:jq`, and yj CLI at build-time.
+
+
+# Motivation
+[motivation]: #motivation
+
+This will allow:
+1. Buildpack authors to publish buildpacks that are compatible with more stack images without requiring those authors to explicitly specify vendor-specific stack IDs.
+2. Vendor-specific buildpacks to work together in the same build, as long as those buildpacks support `io.buildpacks.stacks.bionic`.
+3. App developers to create custom stack images that are compatible with any set of buildpacks that support `io.buildpacks.stacks.bionic`.
+
+# Example
+[example]: #example
+
+Say `org.cloudfoundry.stacks.cfbionicstack` is `ubuntu:bionic`, but with `build-essentials` in the build image and `git` in both build and run images.
+
+We would change the image metadata from:
+```
+run stackID: "org.cloudfoundry.stacks.cfbionicstack"
+run mixins: []
+build stackID: "org.cloudfoundry.stacks.cfbionicstack"
+build mixins: []
+```
+To:
+```
+run stackID: "io.buildpacks.stacks.bionic"
+run mixins: ["git"]
+build stackID: "io.buildpacks.stacks.bionic"
+build mixins: ["build:build-essential", "git"]
+```
+
+And buildpack authors would update their `buildpack.toml` files from:
+```toml
+[[stacks]]
+id = "org.cloudfoundry.stacks.cfbionicstack"
+```
+To:
+```toml
+[[stacks]]
+id = "io.buildpacks.stacks.bionic"
+mixins = ["build:build-essential", "git"] # NOTE: only need to list mixins they actually require!
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- This could encourage buildpack authors and app developers to create a large number of unmaintained stack images that have different sets of Ubuntu packages.
+- Breaking change for buildpack authors once old stack images are removed.
+
+# Unresolved Questions
+[unresolved-questions]: #unresolved-questions
+
+- Neither the spec nor this RFC suggest whether or not stack authors or app developers are allowed to make additional changes to stack images without using mixin metadata.
+- When package groups (like `build-essential`) or dependent packages are installed, must stack images always specify all the packages added? Or is it only necessary when stack image authors want buildpacks to be able to depend on the dependent packages?
+- For usability, should we create a single-mixin "shortcut" notation for the stack ID (e.g., `io.buildpacks.stacks.bionic#shell-utils`)? This would require a spec change and updates to pack, the lifecycle, and other CNB tools, so I'd like to leave it for a different RFC. 
+- Where should we list all of the package sets for `io.buildpacks.stacks.bionic`? On buildpacks.io? In a new section of the spec?
+
+# Alternatives
+[alternatives]: #alternatives
+
+- Continue to maintain separate vendor-specific stack IDs.