chore: add Expression-Based Assertions KEP (#568)

Signed-off-by: Kumar Mallikarjuna <kumarmallikarjuna.work@gmail.com>

keps/0009-expression-based-assertions.md

@@ -0,0 +1,117 @@
+---
+kep-number: 9
+short-desc: This KEP discusses adding expression-based assertions to Kuttl
+title: Expression-Based Assertions
+authors:
+    - "@kumar-mallikarjuna"
+owners:
+    - "@kumar-mallikarjuna"
+    - "@porridge"
+creation-date: 2024-09-23
+last-updated: 2024-09-23
+status: provisional
+---
+
+# Expression-Based Assertions
+
+## Summary
+
+Add expression-based assertions to Kuttl. This can be achieved by integrating support for languages such as Common Expression Language (CEL). This would extend Kuttl's ability to perform assertions based on complex expressions that are currently not possible with the existing syntax.
+
+## Motivation
+
+Currently, Kuttl lacks support for complex data manipulation in assertions.
+Here are a few examples where this limitation is apparent:
+
+-   Testing whether a Deployment has created _more than `n` Pods_.
+    -   While Kuttl supports equality testing, conditional matching poses a challenge.
+-   Slice assertion.
+    -   Kuttl's current method for handling slice comparisons is somewhat unclear. This is especially important when
+        the order of elements in the slice is not fixed, which is common in Kubernetes (see
+        https://github.com/kudobuilder/kuttl/issues/76#issuecomment-660944596).
+    -   Additionally, partial assertions on slices are not possible (see https://github.com/kudobuilder/kuttl/issues/76).
+
+These issues could be resolved by incorporating a CEL engine into Kuttl, allowing for expression
+evaluation and more flexible assertions.
+
+### Goals
+
+1. Add a CEL engine using [github.com/google/cel-go/cel](https://github.com/google/cel-go/cel).
+2. Update `TestAssert` CRD to specify CEL identifiers and evaluate expressions.
+3. Support any/all assertion of CEL expressions.
+
+### Non-Goals
+
+1. Reading non-Kubernetes resources for assertions.
+2. Reading resources from multiple clusters for assertions.
+
+> **Note:** These goals are not in the scope of the first development of this feature but should be targetted in its later phases.
+
+## Proposal
+
+### CRD Changes
+
+```yaml
+apiVersion: kuttl.dev/v1beta1
+kind: TestAssert
+resourceRefs:
+    - apiVersion: apps/v1
+      kind: Deployment
+      name: coredns
+      namespace: kube-system
+      ref: coredns_deployment
+    - apiVersion: apps/v1
+      kind: Deployment
+      name: metrics-server
+      namespace: kube-system
+      ref: metrics_server
+# Success if any expression evaluates to true
+assertAny:
+    - celExpr: "coredns_deployment.status.readyReplicas > 1"
+    - celExpr: ...
+    - ...
+# Success only if all expressions evaluate to true
+assertAll:
+    - celExpr: "metrics_server.status.readyReplicas > 0"
+    - celExpr: ...
+    - ...
+```
+
+### Implementation
+
+At a high level, we would use the [github.com/google/cel-go/cel](https://github.com/google/cel-go/cel) library to evaluate expressions as follows:
+
+```go
+func EvaluateExpression(resources...*unstructured.Unstructured)(ref.Val, error) {
+    env, err := cel.NewEnv(
+        cel.Variable("<id1>", cel.MapType(cel.StringType, cel.DynType)),
+        cel.Variable("<id2>", cel.MapType(cel.StringType, cel.DynType)),
+        ...
+    )
+    if err != nil {
+        return fmt.Errorf("failed to create environment: %w", err)
+    }
+
+    ast, issues := env.Compile(`<expression>`)
+    if issues != nil && issues.Err() != nil {
+        return fmt.Errorf("type-check error: %s", issues.Err())
+    }
+
+    prg, err := env.Program(ast)
+    if err != nil {
+        return fmt.Errorf("program construction error: %w", err)
+    }
+
+    out, _, err := prg.Eval(map[string] interface {} {
+        "<id1>": resources[0].Object,
+        "<id2>": resources[1].Object,
+        ...
+    })
+
+    if err != nil {
+        return fmt.Errorf("failed to evaluate program: %w", err)
+    }
+
+    return out, nil
+}
+```