Handle API lifecycle

cmd/api-syncagent/main.go

@@ -27,6 +27,7 @@ import (
 	"github.com/kcp-dev/logicalcluster/v3"
 	"github.com/spf13/pflag"
 	"go.uber.org/zap"
+	reconcilerlog "k8c.io/reconciler/pkg/log"
 
 	"github.com/kcp-dev/api-syncagent/internal/controller/apiexport"
 	"github.com/kcp-dev/api-syncagent/internal/controller/apiresourceschema"
@@ -80,6 +81,7 @@ func main() {
 
 	// set the logger used by sigs.k8s.io/controller-runtime
 	ctrlruntimelog.SetLogger(zapr.NewLogger(log.WithOptions(zap.AddCallerSkip(1))))
+	reconcilerlog.SetLogger(sugar)
 
 	if err := run(ctx, sugar, opts); err != nil {
 		sugar.Fatalw("Sync Agent has encountered an error", zap.Error(err))

cmd/crd-puller/.gitignore

@@ -0,0 +1,2 @@
+/crd-puller
+*.yaml

cmd/crd-puller/README.md

@@ -1,17 +1,18 @@
 # CRD Puller
 
 The `crd-puller` can be used for testing and development in order to export a
-CustomResourceDefinition for any Group/Version/Kind (GVK) in a Kubernetes cluster.
+CustomResourceDefinition for any Group/Kind (GK) in a Kubernetes cluster.
 
 The main difference between this and kcp's own `crd-puller` is that this one
-works based on GVKs and not resources (i.e. on `apps/v1 Deployment` instead of
+works based on GKs and not resources (i.e. on `apps/Deployment` instead of
 `apps.deployments`). This is more useful since a PublishedResource publishes a
-specific Kind and version.
+specific Kind and version. Also, this puller pulls all available versions, not
+just the preferred version.
 
 ## Usage
 
 ```shell
 export KUBECONFIG=/path/to/kubeconfig
 
-./crd-puller Deployment.v1.apps.k8s.io
+./crd-puller Deployment.apps.k8s.io
 ```

cmd/crd-puller/main.go

@@ -41,13 +41,10 @@ func main() {
 	pflag.Parse()
 
 	if pflag.NArg() == 0 {
-		log.Fatal("No argument given. Please specify a GVK in the form 'Kind.version.apigroup.com' to pull.")
+		log.Fatal("No argument given. Please specify a GroupKind in the form 'Kind.apigroup.com' (case-sensitive) to pull.")
 	}
 
-	gvk, _ := schema.ParseKindArg(pflag.Arg(0))
-	if gvk == nil {
-		log.Fatal("Invalid GVK, please use the format 'Kind.version.apigroup.com'.")
-	}
+	gk := schema.ParseGroupKind(pflag.Arg(0))
 
 	loadingRules := clientcmd.NewDefaultClientConfigLoadingRules()
 	loadingRules.ExplicitPath = kubeconfigPath
@@ -67,7 +64,7 @@ func main() {
 		log.Fatalf("Failed to create discovery client: %v.", err)
 	}
 
-	crd, err := discoveryClient.RetrieveCRD(ctx, *gvk)
+	crd, err := discoveryClient.RetrieveCRD(ctx, gk)
 	if err != nil {
 		log.Fatalf("Failed to pull CRD: %v.", err)
 	}

deploy/crd/kcp.io/syncagent.kcp.io_publishedresources.yaml

@@ -286,7 +286,7 @@ spec:
                         type: string
                       type: array
                     group:
-                      description: The API group, for example "myservice.example.com".
+                      description: The API group, for example "myservice.example.com". Leave empty to not modify the API group.
                       type: string
                     kind:
                       description: |-
@@ -316,8 +316,20 @@ spec:
                         type: string
                       type: array
                     version:
-                      description: The API version, for example "v1beta1".
+                      description: |-
+                        The API version, for example "v1beta1". Leave empty to not modify the version.
+
+                        This field must not be set when multiple versions have been selected.
+
+                        Deprecated: Use .versions instead.
                       type: string
+                    versions:
+                      additionalProperties:
+                        type: string
+                      description: |-
+                        Versions allows to map API versions onto new values in kcp. Leave empty to not modify the
+                        versions.
+                      type: object
                   type: object
                 related:
                   items:
@@ -674,12 +686,23 @@ spec:
                       description: The resource Kind, for example "Database".
                       type: string
                     version:
-                      description: The API version, for example "v1beta1".
+                      description: |-
+                        The API version, for example "v1beta1". Setting this field will only publish
+                        the given version, otherwise all versions for the group/kind will be
+                        published.
+
+                        Deprecated: Use .versions instead.
                       type: string
+                    versions:
+                      description: |-
+                        Versions allows to select a subset of versions to publish. Leave empty
+                        to publish all available versions.
+                      items:
+                        type: string
+                      type: array
                   required:
                     - apiGroup
                     - kind
-                    - version
                   type: object
               required:
                 - resource

docs/content/api-lifecycle.md

@@ -0,0 +1,64 @@
+# API Lifecycle
+
+In only the rarest of cases will the first version of a CRD be also its final version. Instead usually
+CRDs evolve over time and Kubernetes has strong, though sometimes hard to use, support for managing
+different versions of CRDs and their resources.
+
+To understand how CRDs work in the context of the Sync Agent, it's important to first get familiar
+with the [regular Kubernetes behaviour](https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definition-versioning/)
+regarding CRD versioning.
+
+## Basics
+
+The Sync Agent will, whenever a published CRD changes (this can also happen when the projection rules
+inside a `PublishedResource` are updated), create a new `APIResourceSchema` (ARS) in kcp. The name and
+version of this ARS are based on a hash of the projected CRD. Undoing a change would make the agent
+re-use the previously created ARS (ARS are immutable).
+
+After every reconciliation, the list of latest resource schemas in the configured `APIExport` is
+updated. For this the agent will find all ARS that belong to it (based on an ownership label) and
+then merge them into the `APIExport`. Resource schemas for unknown group/resource combinations are
+left untouched, so admins are free to add additional resource schemas to an `APIExport`.
+
+This means that every change to a CRD on the service cluster is applied practically immediately in
+each workspace that consumes the `APIExport`. Administrators are wise to act carefully when working
+with their CRDs on their service cluster. Sometimes it can make sense to turn-off the agent before
+testing new CRDs, even though this will temporarily suspend the synchronization.
+
+## Single-Version CRDs
+
+A very common scenario is to only ever have a single version inside each CRD and keeping this version
+perpetually backwards-compatible. As long as all consumers are aware that certain fields might not
+be set yet in older objects, this scheme works out generally fine.
+
+The agent will handle this scenario just fine by itself. Whenever a CRD is updated, it will reflect
+those changes back into a new `APIResourceSchema` and update the `APIExport`, making the changes
+immediately available to all consumers. Since the agent itself doesn't much care for the contents of
+objects, it itself is not affected by any structural changes in CRDs, as long as it is able to apply
+them on the underlying Kubernetes cluster.
+
+## Multi-Version CRDs
+
+Having multiple versions in a single CRD is immediately much more work, since in Kubernetes all
+versions of a CRD must be _losslessly_ convertible to every other version. Without CEL expressions
+or a dedicated conversion webhook this is practically impossible to achieve.
+
+At the moment kcp does not support CEL-based conversions, and there is no support for configuring a
+conversion webhook inside the Sync Agent either. This is because such a webhook would need to run
+very close to the kcp shards and it's simply out of scope for such a component to be described and
+deployed by the Sync Agent, let alone a trust nightmare for the kcp operators who would have to run
+foreign webhooks on their cluster.
+
+Since both conversion mechanisms are not usable in the current state of kcp and the Sync Agent,
+having multiple versions in a CRD can be difficult to manage.
+
+Generally the Sync Agent itself does not care much about the schemas of each CRD version or the
+convertibility between them. The synchronization works by using unstructured clients to the storage
+versison of the CRD on both sides (in kcp and on the service cluster). Which version is the storage
+version is up to the CRD author.
+
+When publishing multiple versions of a CRD
+
+* only those versions marked as `served` can be picked and
+* if no `storage` version is picked, the latest (highest) version will be chosen automatically as
+  the storage version in kcp.

docs/content/faq.md

@@ -17,18 +17,18 @@ Only if you have distinct API groups (and therefore also distinct `PublishedReso
 You cannot currently publish the same API group onto multiple kcp setups. See issue #13 for more
 information.
 
-## What happens when CRDs are updated?
-
-At the moment, nothing. `APIResourceSchemas` in kcp are immutable and the Sync Agent currently does
-not attempt to update existing schemas in an `APIExport`. If you add a _new_ CRD that you want to
-publish, that's fine, it will be added to the `APIExport`. But changes to existing CRDs require
-manual work.
-
-To trigger an update:
-
-* remove the `APIResourceSchema` from the `latestResourceSchemas`,
-* delete the `APIResourceSchema` object in kcp,
-* restart the api-syncagent
+## Can I have additional resources in APIExports, unmanaged by the Sync Agent?
+
+Yes, you can. The agent will only ever change those resourceSchemas that match group/resource of
+the configured `PublishedResources`. So if you configure the agent to publish
+`cert-manager.io/Certificate`, this would "claim" all resource schemas ending in
+`.certificates.cert-manager.io`. When updating the `APIExport`, the agent will only touch schemas
+with this suffix and leave all others alone.
+
+This is also used when a `PublishedResource` is deleted: Since the `APIResourceSchema` remains in kcp,
+but is no longer configured in the agent, the agent will simply ignore the schema in the `APIExport`.
+This allows for async cleanup processes to happen before an admin ultimately removes the old
+schema from the `APIExport`.
 
 ## Does the Sync Agent handle permission claims?
 

docs/content/publish-resources.md

@@ -18,8 +18,10 @@ For each of the CRDs on the service cluster that should be published, the servic
 `PublishedResource` object, which will contain both which CRD to publish, as well as numerous other
 important settings that influence the behaviour around handling the CRD.
 
-When publishing a resource (CRD), exactly one version is published. All others are ignored from the
-standpoint of the resource synchronization logic.
+When publishing a resource (CRD), service owners can choose to restrict it to a subset of available
+versions and even change API group, versions and names in transit (for example published a v1 from
+the service cluster as v1beta1 within kcp). This process of changing the identity of a CRD is called
+"projection" in the agent.
 
 All published resources together form the APIExport. When a service is enabled in a workspace
 (i.e. it is bound to it), users can manage objects for the projected resources described by the
@@ -46,11 +48,18 @@ spec:
   resource:
     kind: Certificate
     apiGroup: cert-manager.io
-    version: v1
+    versions: [v1]
 ```
 
 However, you will most likely apply more configuration and use features described below.
 
+You always have to select at least one version, and all selected versions must be marked as `served`
+on the service cluster. If the storage version is selected to be published, it stays the storage
+version in kcp. If no storage version is selected, the latest selected version becomes the storage
+version.
+
+For more information refer to the [API lifecycle](api-lifecycle.md).
+
 ### Filtering
 
 The Sync Agent can be instructed to only work on a subset of resources in kcp. This can be restricted
@@ -70,16 +79,18 @@ spec:
         foo: bar
 ```
 
+The configuration above would mean the agent only synchronizes objects from `my-app` namespaces (in
+each of the kcp workspaces) that also have a `foo=bar` label on them.
+
 ### Schema
 
-**Warning:** The actual CRD schema is always copied verbatim. All projections <!--, mutations -->
-etc. have to take into account that the resource contents must be expressible without changes to the
-schema, so you cannot define entirely new fields in an object that are not defined by the original
-CRD.
+**Warning:** The actual CRD schema is always copied verbatim. All projections, mutations etc. have
+to take into account that the resource contents must be expressible without changes to the schema,
+so you cannot define entirely new fields in an object that are not defined by the original CRD.
 
 ### Projection
 
-For stronger separation of concerns and to enable whitelabelling of services, the type meta for
+For stronger separation of concerns and to enable whitelabelling of services, the type meta for CRDs
 can be projected, i.e. changed between the local service cluster and kcp. You could for example
 rename `Certificate` from cert-manager to `Sertifikat` inside kcp.
 
@@ -103,18 +114,22 @@ metadata:
 spec:
   resource: ...
   projection:
-    version: v1beta1
+    # all of these options are optional
     kind: Sertifikat
     plural: Sertifikater
     shortNames: [serts]
+    versions:
+      # old version => new version;
+      # this must not map multiple versions to the same new version.
+      v1: v1beta1
     # categories: [management]
     # scope: Namespaced # change only when you know what you're doing
 ```
 
 Consumers (end users) in kcp would then ultimately see projected names only. Note that GVK
 projection applies only to the synced object itself and has no effect on the contents of these
 objects. To change the contents, use external solutions like Crossplane to transform objects.
-<!-- To change the contents, use *Mutations*. -->
+To change the contents, use *Mutations*.
 
 ### (Re-)Naming
 
@@ -274,7 +289,7 @@ spec:
   resource:
     kind: Certificate
     apiGroup: cert-manager.io
-    version: v1
+    versions: [v1]
 
   naming:
     # this is where our CA and Issuer live in this example
@@ -360,7 +375,7 @@ spec:
   resource:
     kind: Certificate
     apiGroup: cert-manager.io
-    version: v1
+    versions: [v1]
 
   naming:
     namespace: kube-system
@@ -445,7 +460,7 @@ spec:
   resource:
     kind: Certificate
     apiGroup: cert-manager.io
-    version: v1
+    versions: [v1]
 
   naming:
     # this is where our CA and Issuer live in this example

docs/mkdocs.yml

@@ -12,7 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-
 site_name: api-syncagent
 repo_url: https://github.com/kcp-dev/api-syncagent
 repo_name: kcp-dev/api-syncagent
@@ -24,6 +23,7 @@ nav:
   - Getting Started: getting-started.md
   - Publishing Resources: publish-resources.md
   - Consuming Services: consuming-services.md
+  - API Lifecycle: api-lifecycle.md
   - FAQ: faq.md
   - Release Process: releasing.md
 

internal/controller/apiexport/controller.go

@@ -19,6 +19,7 @@ package apiexport
 import (
 	"context"
 	"fmt"
+	"slices"
 
 	"github.com/kcp-dev/logicalcluster/v3"
 	"go.uber.org/zap"
@@ -121,12 +122,9 @@ func (r *Reconciler) reconcile(ctx context.Context) error {
 	}
 
 	// filter out those PRs that have not yet been processed into an ARS
-	filteredPubResources := []syncagentv1alpha1.PublishedResource{}
-	for i, pubResource := range pubResources.Items {
-		if pubResource.Status.ResourceSchemaName != "" {
-			filteredPubResources = append(filteredPubResources, pubResources.Items[i])
-		}
-	}
+	filteredPubResources := slices.DeleteFunc(pubResources.Items, func(pr syncagentv1alpha1.PublishedResource) bool {
+		return pr.Status.ResourceSchemaName == ""
+	})
 
 	// for each PR, we note down the created ARS and also the GVKs of related resources
 	arsList := sets.New[string]()