feat: Add Transformer type

pkg/api/config/config.go

@@ -22,9 +22,10 @@ import (
 
 func New(ctx context.Context, cfg *config.Config) (*api.Configuration, error) {
 	c := &api.Configuration{
-		Definitions: containers.MapStore[string, *core.ResourceDefinition]{},
-		Controllers: containers.MapStore[string, api.Controller]{},
-		Bindings:    containers.MapStore[string, *core.Binding]{},
+		Definitions:  containers.MapStore[string, *core.ResourceDefinition]{},
+		Controllers:  containers.MapStore[string, api.Controller]{},
+		Bindings:     containers.MapStore[string, *core.Binding]{},
+		Transformers: containers.MapStore[string, *core.Transformer]{},
 	}
 
 	dir := os.DirFS(cfg.API.Resources)
@@ -109,6 +110,14 @@ func New(ctx context.Context, cfg *config.Config) (*api.Configuration, error) {
 			}
 
 			c.Bindings[binding.Metadata.Name] = &binding
+		case core.TransformerKind:
+			var transformer core.Transformer
+
+			if err := json.NewDecoder(buf).Decode(&transformer); err != nil {
+				return fmt.Errorf("parsing transformer: %w", err)
+			}
+
+			c.Transformers[transformer.Spec.Kind] = &transformer
 		}
 
 		return nil

pkg/api/core/resource.go

@@ -7,6 +7,7 @@ import (
 const (
 	ResourceDefinitionKind = "ResourceDefinition"
 	BindingKind            = "Binding"
+	TransformerKind        = "Transformer"
 )
 
 // Resource is the core API resource definition used to communicate
@@ -39,3 +40,11 @@ type BindingSpec struct {
 	Resources  []string
 	Controller string
 }
+
+type Transformer Object[TransformerSpec]
+
+type TransformerSpec struct {
+	Kind       string
+	Controller string
+	Template   string // template to transform a ResourceDefinition into
+}

pkg/api/server.go

@@ -1,9 +1,11 @@
 package api
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
+	"html/template"
 	"io"
 	"io/fs"
 	"log/slog"
@@ -67,6 +69,7 @@ type Configuration struct {
 	Definitions     containers.MapStore[string, *core.ResourceDefinition]
 	Controllers     containers.MapStore[string, Controller]
 	Bindings        containers.MapStore[string, *core.Binding]
+	Transformers    containers.MapStore[string, *core.Transformer]
 	TailscaleClient tailscale.Client
 }
 
@@ -233,6 +236,45 @@ func (s *Server) register(cntl Controller, version string, def *core.ResourceDef
 			resource.Metadata.Namespace, resource.Metadata.Name,
 		)
 
+		transformer, err := s.cfg.Transformers.Get(resource.Kind)
+		if err == nil && transformer != nil {
+			m := map[string]interface{}{}
+
+			err := json.Unmarshal(resource.Spec, &m)
+			if err != nil {
+				http.Error(w, err.Error(), http.StatusInternalServerError)
+				return
+			}
+
+			t := template.Must(template.New("").Parse(transformer.Spec.Template))
+			bb := &bytes.Buffer{}
+			err = t.Execute(bb, m)
+			if err != nil {
+				http.Error(w, err.Error(), http.StatusInternalServerError)
+				return
+			}
+
+			// For transformed values store the transformed payload within the spec, along with the
+			// definition, that the user has specified.
+			// Also add a metadata label to mark this resource as transformed. So the clients that
+			// retrieve the definition are informed.
+			resource.Metadata.Labels = map[string]string{
+				"transformed": "yes",
+			}
+			a := map[string]json.RawMessage{}
+
+			a["definition"] = resource.Spec
+			a["transformed"] = bb.Bytes()
+
+			spec, err := json.Marshal(a)
+			if err != nil {
+				http.Error(w, err.Error(), http.StatusInternalServerError)
+				return
+			}
+
+			resource.Spec = spec
+		}
+
 		result, err := s.fs.Update(r.Context(), s.rev, message, func(f controllers.FSConfig) error {
 			return cntl.Put(r.Context(), &controllers.PutRequest{
 				Request: controllers.Request{