Decision Record: keep capi images recommended (#582)

Signed-off-by: Matthias Büchse <[email protected]>
Co-authored-by: Malte Münch <[email protected]>
Co-authored-by: Kurt Garloff <[email protected]>

Standards/scs-0120-v1-capi-images.md

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+---
+title: Cluster-API images
+type: Decision Record
+status: Draft
+track: IaaS
+---
+
+## Abstract
+
+The SCS reference implementation for the Kubernetes-as-a-service layer is built on top of Cluster API (CAPI), and therefore it depends on the corresponding VM images, which may or may not be present on the underlying infrastructure-as-a-service layer. Current tooling will make sure to upload the required image in case it's not present or outdated. However, these ad-hoc uploads will not be shared across environments, which may lead to waste of bandwidth (for transferring the image), storage (if images are not stored in a deduplicated manner), and not least time (because the upload does take multiple minutes). Needless to say, it may also lead to excessive greenhouse-gas emissions.
+
+This decision record investigates the pros and cons of making the CAPI images mandatory. Ultimately, the decision is made to keep them recommended; we stress, however, that providers who offer the images by default should advertise this fact.
+
+## Terminology
+
+- _Kubernetes as a service (KaaS)_: A service that offers provisioning Kubernetes clusters.
+- _Cluster API (CAPI)_: "Cluster API is a Kubernetes sub-project focused on providing declarative APIs and tooling to simplify provisioning, upgrading, and operating multiple Kubernetes clusters." ([source](https://cluster-api.sigs.k8s.io/)) This API can thus be used to implement KaaS.
+- _CAPI image_: Virtual machine image that contains a standardized Kubernetes setup to be used for CAPI. The SCS reference implementation for KaaS depends on these images.
+- _CSP_: Cloud-service provider
+
+## Design considerations
+
+We consider the following two options:
+
+1. Make CAPI image mandatory.
+2. Keep CAPI image recommended.
+
+For reasons of symmetry, it suffices to consider the pros and cons of the first option.
+
+Pros:
+
+- Save time, money, physical resources and power for both CSP and customer.
+- Regardless of CSP taste, this KaaS tech is part of SCS.
+
+Neutral:
+
+- The CAPI image can be provided in an automated fashion that means virtually no burden to the CSP.
+- The KaaS implementation will work either way.
+- Willing CSPs may offer the image by default and advertise as much.
+
+Cons:
+
+- Additional regulations would be necessary to guarantee quality and timeliness of image.
+- Some CSPs may be opposed to being forced to offer a certain service, which may hurt the overall acceptance
+  of the SCS standardization efforts.
+
+## Decision
+
+Ultimately, we value the freedom of the CSPs (and the acceptance of the standardization efforts) highest;
+willing CSPs are welcome to opt in, i.e., to provide up-to-date images and advertise as much.
+
+Therefore we decide to _keep the CAPI images recommended_.
+
+## Consequences
+
+None, as the status quo is being kept.
+
+## Open questions
+
+Some interesting potential future work does remain, however: to find a way to certify that a willing provider
+does indeed provide up-to-date images. It would be possible with today's methods to certify that a CAPI
+image is present (the image_spec yaml file would have to be split up to obtain a separate test case), but
+we there is no way to make sure that the image is up to date.