RFC: Add matchParams selector

rfc/rfc-0000-match-params.md

@@ -0,0 +1,138 @@
+# Meta
+[meta]: #meta
+- Name: Match params in field selector
+- Start Date: 2022-02-14
+- Author(s): @jwntrs
+- Status: Draft
+- RFC Pull Request:
+- Supersedes: N/A
+
+# Summary
+[summary]: #summary
+
+The [template switching RFC](https://github.com/vmware-tanzu/cartographer/pull/75) introduced the ability to match templates based on fields in the workload. However matching on params is not well defined. It is currently not possible to match on a field in the supply chain, and matching on fields in the workload is complex. You would need to define something as follows:
+
+```yaml
+  selector:
+    matchFields:
+      - key: workload.spec.params[?(@.name=="promotion")].value    #< ===== not so nice
+        operator: In
+        values: ["gitops"]
+```
+
+Lets introduce a top level params key:
+
+```yaml
+  selector:
+    matchFields:
+      - key: params.promotion    #< ===== use something like this instead
+        operator: In
+        values: ["gitops"]
+```
+
+
+# Motivation
+[motivation]: #motivation
+
+We should be able to match against supply chain params in our field selectors.
+
+# What it is
+[what-it-is]: #what-it-is
+
+```yaml
+apiVersion: kontinue.io/v1alpha1
+kind: ClusterSupplyChain
+metadata:
+  name: responsible-ops
+spec:
+  selector:
+    app: web
+  params:
+  - name: promotion       #< ===== params can be set here
+    value: (gitops|regops)
+  resources:
+   ...
+    - name: promote
+      templateRef:
+        kind: ClusterTemplate
+        options:
+        - name: git-promotion
+          selector:
+            matchFields:
+              - key: params.promotion           #< ===== introduce top level `params` that relies on our param hierarchy
+                operator: In
+                values: ["gitops"]
+        - name: registry-promotion
+          selector:
+            matchFields:
+              - key: params.promotion
+                operator: In
+                values: ["regops"]
+
+---
+apiVersion: kontinue.io/v1alpha1
+kind: Workload
+metadata:
+  name: petclinic
+  labels:
+    app: web
+spec:
+  params:
+  - name: promotion           #< ===== params can also be set here
+    value: (gitops|regops)
+  source:
+    git:
+      url: https://github.com/spring-projects/spring-petclinic.git
+      ref:
+        branch: main
+```
+
+# How it Works
+[how-it-works]: #how-it-works
+
+Compute the available params from the workload and supply chain based on the [param hierarchy](https://cartographer.sh/docs/v0.2.0/architecture/#parameter-hierarchy) and pass it to the field matcher as `params`.
+
+# Migration
+[migration]: #migration
+
+N/A
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Ultimately we want our template `selector` and top-level supply chain `selector` to work the same. However, if we introduce params into the matching context, then it would cause some divergence in these two cases. 
+
+The top level selector we only have access to params defined in the workload.
+
+The template selectors would have access to params from the workload as well as the supply chain.
+
+Neither selector would have access to params from the templates.
+
+
+# Alternatives
+[alternatives]: #alternatives
+
+???
+
+# Prior Art
+[prior-art]: #prior-art
+
+???
+
+# Unresolved Questions
+[unresolved-questions]: #unresolved-questions
+
+???
+
+# Spec. Changes (OPTIONAL)
+[spec-changes]: #spec-changes
+
+```yaml
+  selector:
+    matchFields:
+      - key: params.promotion           #< =================== introduce top level `params`
+        operator: In
+        values: ["gitops"]
+```
+
+