ADR023: Product image selection

Deciders:
- Malte Sander
- Sebastian Bernauer
- Felix Henning
- Maximilian Wittich
- Lars Francke
Date: 2022-08-31

Note: Parts of this ADR were expanded by ADR028: Automatic stackable version selection

Context and Problem Statement

Currently users have to specify the full image tag in the Products CRD, as described in ADR018: Product Image Versioning.

spec:
  version: 1.4.1-stackable2.1.0

This must be a tag belonging to an image from the stackable repository. In order to give the user the opportunity to provide a own repository, mirror our repository or provide a complete own image, we’d like to propose a solution for it.

Decision Drivers

Flexible and extendable solution for possible use cases
Preserve the possibility of magic version completion of operators for images (e.g. superset:1.4.1 ⇒ superset:1.4.1-stackable2.1.0)
Allow the user to easily paste in a complete image name
Allow users to use some defaults, allow them to only override certain things

In detail we used the following use-cases to drive the decision:

use Stackable repo
- users specified version 1.4.1-stackable2.1.0 ⇒ operator should use image docker.stackable.tech/stackable/superset:1.4.1-stackable2.1.0
- users specified version 1.4.1 ⇒ operator should use image docker.stackable.tech/stackable/superset:1.4.1-stackable2.1.0 based on compatibility knowledge (which operator works with which image)
- users specifies no version ⇒ operator should use image docker.stackable.tech/stackable/superset:1.4.1-stackable2.1.0 based on recommended product version of the release/operator and knowledge (which operator works with which image)
use self-hosted (mirrored) repo
- users specified version 1.4.1-stackable2.1.0 and imageRepository my.corp/superset ⇒ operator should use image my.corp/superset:1.4.1-stackable2.1.0 based on imageRepository ⇒ Equivalent to user saying use the image "my.corp/superset:1.4.1-stackable2.1.0"
- users specified version 1.4.1 and imageRepository my.corp/superset ⇒ operator should use image my.corp/superset:1.4.1-stackable2.1.0 based on imageRepository and compatibility knowledge (which operator works with which image) ⇒ Equivalent to user saying use the image "my.corp/superset:1.4.1-stackable2.1.0" ⇒ NOT equivalent to user saying use the image "my.corp/superset:1.4.1" ⇒ When specifying the image directly users loose the ability to let the operator determine its compatible stackable image version
- user just specifies a repo: my.corp/superset with no versions → operator should use image my.corp/superset:1.4.1-stackable2.1.0 based on recommended product version of the release/operator and knowledge (which operator works with which image)

Decision Outcome

Chosen option: "Option 6" because it is the only option to cover all use cases.

Pros and Cons of the Options

Option 1

version: 1.4.1-stackable2.1.0 # optional (currently operator will error out but in the future according to ADR 18 operator should pick a good version automatically)
imageRepository: docker.stackable.tech/stackable/superset # Option<String>

Good, because it’s non-breaking
Good, because users can specify version: 1.4.1 and the operator will resolve it to 1.4.1-stackable2.1.0 (or whatever stackable versions works for him). This mechanism continues working. For this to work the customers needs to mirror the exact same tags as we at Stackable have (he can’t rename the tags).

Option 2

version: 1.4.1-stackable2.1.0 # optional (currently operator will error out but in the future according to ADR 18 operator should pick a good version automatically)
image: docker.stackable.tech/stackable/superset:1.4.1-stackable2.1.0 # Option<String>. Will overwrite version (if specified)

Good, because it’s non-breaking because mechanism from Option 1 is not implemented yet
Bad, because mechanism explain in Option 1 breaks.

Option 3

image:
  version: 1.4.1-stackable2.1.0 # optional (currently operator will error out but in the future according to ADR 18 operator should pick a good version automatically)
  imageRepository: docker.stackable.tech/stackable/superset # Option<String>

Bad, because it’s breaking
Bad, because versions can only contain the product version (e.g. 1.4.1). Having this "hidden" under image may seem like an implementation detail to the user. It can be argued that version is important enough to be a top-level field.

Option 4

version: 1.4.1-stackable2.1.0 # optional (currently operator will error out but in the future according to ADR 18 operator should pick a good version automatically)
imageOverwrite: # Option<struct>
  repository: docker.stackable.tech/stackable/superset # String
  tag: 1.4.1-stackable2.1.0 # Option<String>. Will overwrite tag (the specified version is ignored/overwritten)

Good, because it’s non-breaking
Good, because when only imageOverwrite.repository is specified, it’s the same as option 1 with all the benefits
Good, because when imageOverwrite.repository and imageOverwrite.tag is specified, it’s the same as option 2 with all the benefits

Option 5

image: # mandatory complex enum
  stackableImageTag: 1.4.1-stackable2.1.0 # String
  # OR
  custom: docker.stackable.tech/stackable/superset:1.4.1-stackable2.1.0 # String
  # OR (later on)
  stackableVersion: 1.4.1 # String
  # OR (later on)
  recommendedVersion: true # needs to be set to true. if set to false operator will error out

We want to start with the first two variants stackableImageTag and custom. The magicVersionResolving and recommendedVersion variants might be added later on.

Bad, because it’s breaking
Good, because it gives all flexibility of all previous options
Good, because we can non-breaking introduce new "magic" in the future by adding new image enum variants
Good, because we can implement it as enum called e.g. ImageSpec in operator-rs which will offer a function like resolve_image that will make it easy for operators to use

Option 6

This option is breaking. It uses a complex enum, similar to Option 5. Option 5 does not account for the need to specify the product version with a custom image. It is also not possible to just use a custom docker repository and still use the operator recommended version (i.e. just mirroring the stackable repository). This option makes that possible. We first start with implementing stackableVersion and custom. The stackable enum variant will be implemented as soon as we have the magic stackableVersion resolution.

image: # complex enum
  stackableVersion: # (1)
    repo: docker.stackable.tech # String. Defaults to docker.stackable.tech (kind of optional).
    productVersion: 1.4.1 # mandatory
    stackableVersion: stackable2.1.0 # mandatory
  # OR
  stackable: # (2)
    repo: docker.stackable.tech # String. Defaults to docker.stackable.tech (kind of optional).
    productVersion: 1.4.1 # Option<String>. If not specified use recommended product version ("magic").
  # OR
  custom: # (3)
    productVersion: 1.4.1
    custom: docker.stackable.tech/stackable/superset:1.4.1-stackable2.1

  pullPolicy: IfNotPresent
  pullSecrets: # Option<Vec<LocalObjectReference>>
    name: regcred # reference to secret in same namespace

Known issues: We will start to implement our own schema since kube-rs is not supporting flatten yet.

Use-case: I don’t want to specify anything, just give me defaults!

→ Don’t specify anything.

Use-case: I want a specific version of the product:

image:
  productVersion: 1.5.1

This resolves to the enum variant 2, with just the product version specified

Use-case: I’ve mirrored the stackable repo locally, but want to use automatic image resolution:

image:
  repo: my.repo.company.org/stackable

This resolves to variant 2.

Use-case: I have built my own custom image with i.e. additional dependencies for the product, which has a different tag than the original stackable image. I’ve uploaded it to my custom repo:

image:
  custom: my.repo.company.org/stackable/superset:my-custom-tag
  productVersion: 1.4.1

This resolves to enum variant 3. The product version is mandatory so the operator knows what to do.