[TEP-0129] Move CRDs definition and update multi-tenancy docs accordingly

docs/developers/multi-tenant-support.md

@@ -1,5 +1,59 @@
 # Support for running in multi-tenant configuration
 
+Multi-tenancy for Tekton is intended as running multiple instances of the
+Tekton control plane in a single Kubernetes cluster, with each control plane
+responsible for reconciliation of the Tekton resources in a single namespace.
+
+This type of setup involves various aspects of Tekton that are normally
+cluster-wide or associated with namespaces:
+
+- Namespaces
+- Custom Resource Definitions (CRDs)
+- RBAC (Service Accounts, Roles and Bindings)
+- Validation and Mutation Webhooks
+- Conversion Webhook
+
+## Namespaces
+
+The Tekton control plance namespace resource is defined in a dedicated
+[sub-folder](/config/100-namespace/),  folder that includes
+all Tekton resources. When installing from source, the namespace folder can be
+skipped, and the namespace can be created beforehand instead.
+
+To customize the namespace where the control plan is installed, all namespaced
+resources under the [`config`](/config/) folder (or in the release file) must be updated.
+That can be achieved by search and replace, or using tools like `ko`, `sed` or `kustomize`.
+
+More details for the installation from source are available in the
+[root development document](/DEVELOPMENT.md#installing-into-custom-namespaces).
+
+The Tekton control plane by default watches all namespaces for Tekton resources.
+To change this behaviour, and limit the Tekton controller to a single namespace,
+add the `--namespace <namespace-name>` command line flag to the Tekton controller
+[deployment file](/config/controller.yaml).
+
+## Custom Resource Definitions
+
+Custom Resource Definitions are defined in a dedicated [sub-folder](/config/300-crds/),
+under the [`config`](/config/) folder that includes all Tekton resources.
+
+When installing from source, instead of applying resources recursively, apply only
+the content of the [`config`](/config/) folder, for instance:
+
+```sh
+ko apply -f config
+```
+
+Note that the namespace will have to be created beforehand.
+When installing from a release file, the [`yq(https://github.com/mikefarah/yq) tool
+comes in handy:
+
+```sh
+yq  e '. | select(.kind != "CustomResourceDefinition")' < release.yaml > release-nocrd.yaml
+```
+
+## RBAC
+
 In order to support potential multi-tenant configurations the roles of the
 controller are split into two:
 
@@ -10,4 +64,36 @@ By default the roles are cluster-scoped for backwards-compatibility and
 ease-of-use. If you want to start running a multi-tenant service you are able to
 bind `tekton-pipelines-controller-tenant-access` using a `RoleBinding` instead
 of a `ClusterRoleBinding`, thereby limiting the access that the controller has
-to specific tenant namespaces.
+to specific tenant namespaces.
+
+## Validation and Mutation Webhooks
+
+Validation and mutation webhooks configurations by default act with cluster scope.
+Normally only instance of the webhooks per cluster is enough to serve all instances
+of Tekton installed.
+
+If different versions of Tekton must coexist in the same cluster, they might require
+different webhooks. Webhook configurations can be namespaced by using the
+[`namespaceSelector`](https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/#matching-requests-namespaceselector).
+
+The name of the [configurations](/config/webhook.yaml) must also be adapted so that
+many instances may coexist, one per namespace.
+
+Note that running multiple versions of Tekton in a single cluster might work in
+certain cases, but it's not officially supported.
+
+## Conversion Webhook
+
+Kubernets only allows one specific version of all resources associated with a specific
+CRD being stored in etcd. For instance, all `Tasks` will be stored as `v1`, even if
+both `v1` and `v1beta1` are served by the API.
+
+The conversion webhook converts versions on the fly when a version other than the stored
+one is used. Tekton's conversion webhook is responsible for reconciling the CRD
+definitions. Since those are defined at cluster level, only once conversion webhook may
+exist in the cluster.
+
+The conversion webhook is bundled in a single binary with the other webhooks, so today
+it's not possible to run multiple validation and mutation webhooks and a single
+conversion webhook. [TEP-0129](https://github.com/tektoncd/community/blob/main/teps/0129-multiple-tekton-instances-per-cluster.md)
+will change that, once implemented.

docs/install.md

@@ -72,6 +72,9 @@ To install Tekton Pipelines on a Kubernetes cluster:
      kubectl apply --filename https://storage.googleapis.com/tekton-releases/pipeline/latest/release.notags.yaml
      ```
 
+Multi-tenant installation is only partially supported today, read the [guide](./developers/multi-tenant-support.md)
+for reference.
+
 1. Monitor the installation:
 
    ```bash