[RFC] Attributes in image automation

rfcs/0005-managed-attributes-image-automation/README.md

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+# RFC-0005 Extend supported list of image automation marker reference attributes
+
+**Status:** provisional
+
+**Creation date:** 2021-12-16
+
+**Last update:** 2022-01-25
+
+## Summary
+
+Flux should allow referencing more metadata in the image automation Setters strategy.
+
+## Motivation
+
+Some automation or observability tools can use label to identify better a 
+kubernetes object. It can be linked to a version, a date, a code 
+origin... For multiple reason, the image tag can reflect poorly this
+data. An example can be given by the image reflector controller which 
+can extract a part of the tag and use it to sort and select the correct one.
+
+### Goals
+
+This RFC aims to describe
+
+- A way to extract such additional attributes from the image tag.
+- Use those new attributes to update the kubernetes object.
+
+### Non-Goals
+
+This RFC will focus on Image Automation Controller and Image Reflector Controller.
+
+It is a non goal to keep in sync the attributes if the kubernetes object is 
+updated manually.
+
+## Proposal
+
+### User Stories
+
+As a user, I can update the filter pattern on the image policy object to 
+capture additional data. 
+Then, I can reference the name of the captured group in the comment of a 
+kubernetes object so that the attribute linked to this comment can be updated. 
+
+
+### Alternatives
+
+An alternative would be to build a mutation web hook which would be able to 
+filter all object and interact with them directly. 
+
+It would be more generic, more customizable and safer (fix the manual update use case)
+to create such mutation web hook, but will be more complex to build. 
+(new kubernetes object, new controller)  
+
+This raise the question on should this feature to be included in flux or not. 
+
+## Design Details
+
+Two options are possible here:
+
+- Only modify the Image Automation Controller to make it read ImagePolicies spec
+and compute attributes
+- Modify the Image Reflector Controller, to extract the attributes, stores them
+in the status and update the Image Automation Controller to use this new data storage. 
+
+The second option seems to be preferable to separate concerns.
+
+A simple option would be to allow multiple capture group in the filter in the ImagePolicy: 
+
+```yaml
+    extract: $ts
+    pattern: ^pr-(?P<pr>.*)-(?P<ts>\d*)-(?P<sha1>.*)$
+```
+
+And then to modify the Image Automation Controller to take comment like:
+
+```yaml
+  # {"$imagepolicy": "{namespace}:{imagepolicy}:{attributes}"
+```
+
+with `attributes` a name of a capture group on the pattern. 
+
+From previous pattern example, accepted attributes will be:
+
+- pr
+- ts
+- sha1
+
+If a user try to capture an attribute with a name like `tag` or `name` (already defined 
+by flux core), then the original value will be kept and a warning should show on the 
+Image Reflector Controller logs.
+
+As reminder, here is the definition for those default attributes:
+
+- tag: the full tag string
+- name: the image name
+
+## Implementation History
+
+_not implemented yet_