doc: add design proposal for topology-aware cluster selection

Add a design document describing topology-aware multi-cluster volume
provisioning. This enables the CSI driver to dynamically select the
appropriate Ceph cluster at CreateVolume time based on the node's
topology zone.

The proposal introduces two configuration mechanisms:
- topologyDomainLabels field in config.json cluster entries
- clusterIDs StorageClass parameter (comma-separated list)

Ref: #5177

Signed-off-by: WMP <example@example.com>

Author: MarcinJanowski

docs/design/proposals/topology-aware-cluster-selection.md

@@ -0,0 +1,311 @@
+# Topology-Aware Multi-Cluster Volume Provisioning
+
+Currently Ceph-CSI supports only a single Ceph cluster per StorageClass. The
+`clusterID` parameter in the StorageClass is mandatory and points to exactly one
+cluster entry in `config.json`. This works well for single-cluster environments,
+but creates a significant limitation for distributed Kubernetes deployments
+spanning multiple geographic zones, each backed by a separate Ceph cluster.
+
+In such deployments administrators must create a separate StorageClass per
+zone/cluster, and application teams must manually select the correct
+StorageClass depending on where their workloads run. This defeats the purpose of
+Kubernetes topology-aware scheduling and creates operational overhead.
+
+Reference: https://github.com/ceph/ceph-csi/issues/5177
+
+## Problem
+
+Consider a Kubernetes cluster with nodes spread across two zones, each served
+by a separate Ceph cluster:
+
+- `zone-poland` with Ceph cluster `cluster-poland` (monitors: `10.0.1.1:6789`)
+- `zone-france` with Ceph cluster `cluster-france` (monitors: `10.0.2.1:6789`)
+
+Today, the administrator must create two StorageClasses:
+
+```yaml
+apiVersion: storage.k8s.io/v1
+kind: StorageClass
+metadata:
+  name: csi-rbd-poland
+provisioner: rbd.csi.ceph.com
+parameters:
+  clusterID: "cluster-poland"
+  pool: replicapool
+---
+apiVersion: storage.k8s.io/v1
+kind: StorageClass
+metadata:
+  name: csi-rbd-france
+provisioner: rbd.csi.ceph.com
+parameters:
+  clusterID: "cluster-france"
+  pool: replicapool
+```
+
+Application teams must then know which StorageClass to use based on where their
+pods will be scheduled. If a pod moves to a different zone, the PVC might point
+to a remote cluster, losing data locality.
+
+The goal is to have a **single StorageClass** that automatically selects the
+correct Ceph cluster based on the node's topology zone.
+
+## Proposed Solution
+
+### Configuration Changes
+
+#### config.json
+
+Each cluster entry in `config.json` gains an optional `topologyDomainLabels`
+field that maps Kubernetes topology label keys to their expected values:
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+data:
+  config.json: |-
+    [
+      {
+        "clusterID": "cluster-poland",
+        "topologyDomainLabels": {
+          "topology.kubernetes.io/zone": "zone-poland"
+        },
+        "monitors": [
+          "10.0.1.1:6789"
+        ],
+        "rbd": {
+          "radosNamespace": ""
+        },
+        "cephFS": {
+          "subvolumeGroup": "csi"
+        }
+      },
+      {
+        "clusterID": "cluster-france",
+        "topologyDomainLabels": {
+          "topology.kubernetes.io/zone": "zone-france"
+        },
+        "monitors": [
+          "10.0.2.1:6789"
+        ],
+        "rbd": {
+          "radosNamespace": ""
+        },
+        "cephFS": {
+          "subvolumeGroup": "csi"
+        }
+      }
+    ]
+metadata:
+  name: ceph-csi-config
+```
+
+Clusters without `topologyDomainLabels` are ignored during topology-based
+selection and continue to work exactly as before.
+
+#### StorageClass
+
+A new parameter `clusterIDs` is introduced as a comma-separated list of
+candidate cluster IDs. The StorageClass **must** use
+`volumeBindingMode: WaitForFirstConsumer` so that Kubernetes provides topology
+hints to the CSI driver via `AccessibilityRequirements` in the `CreateVolume`
+request.
+
+```yaml
+apiVersion: storage.k8s.io/v1
+kind: StorageClass
+metadata:
+  name: csi-rbd-topology
+provisioner: rbd.csi.ceph.com
+parameters:
+  clusterIDs: "cluster-poland,cluster-france"
+  pool: replicapool
+  imageFeatures: layering
+  csi.storage.k8s.io/provisioner-secret-name: csi-rbd-secret
+  csi.storage.k8s.io/provisioner-secret-namespace: ceph-system
+volumeBindingMode: WaitForFirstConsumer
+reclaimPolicy: Delete
+```
+
+> **Note:** The existing `clusterID` parameter continues to work as before.
+> When `clusterID` is present, it takes priority and the topology-based
+> selection is not used. The `clusterIDs` parameter is only consulted when
+> `clusterID` is absent.
+
+### How PV Creation Works
+
+Topology-aware cluster selection relies on the Kubernetes topology mechanism
+built into the CSI specification. Understanding how topology information flows
+from nodes to the `CreateVolume` call is key to understanding the design.
+
+#### Topology Discovery
+
+When the CSI node plugin (DaemonSet) starts on each node, Kubernetes calls
+`NodeGetInfo`. The driver reads the node's Kubernetes labels (configured via
+the `--domainlabels` flag) and returns them as `AccessibleTopology` segments.
+Kubernetes stores this information in the `CSINode` object.
+
+For example, a node with the label `topology.kubernetes.io/zone=zone-poland`
+reports:
+
+```json
+{
+  "accessible_topology": {
+    "segments": {
+      "topology.kubernetes.io/zone": "zone-poland"
+    }
+  }
+}
+```
+
+#### WaitForFirstConsumer Binding
+
+The StorageClass **must** use `volumeBindingMode: WaitForFirstConsumer`. This
+tells Kubernetes to delay volume provisioning until a pod consuming the PVC is
+scheduled to a specific node. Without this, Kubernetes calls `CreateVolume`
+immediately (with `Immediate` binding) and does not know which node the pod
+will run on — so no `AccessibilityRequirements` are provided and topology-based
+selection cannot work.
+
+#### AccessibilityRequirements: Preferred vs Requisite
+
+When Kubernetes calls `CreateVolume` after scheduling the pod, it includes
+`AccessibilityRequirements` with two lists of topologies:
+
+- **Preferred** — an ordered list of topologies where the volume should ideally
+  be created. The first entry is the topology of the node where the pod was
+  scheduled. This is what we use for data locality — placing storage close to
+  compute.
+
+- **Requisite** — a list of all topologies where the volume is allowed to be
+  created (hard constraints). This includes all nodes that have capacity to
+  serve the volume.
+
+For example, when a pod is scheduled on a node in `zone-poland` in a cluster
+that also has nodes in `zone-france`:
+
+```
+Preferred: [zone-poland]                  ← the pod's node
+Requisite: [zone-poland, zone-france]     ← all eligible zones
+```
+
+The driver checks Preferred first (for data locality), and falls back to
+Requisite only if no Preferred topology matches any cluster.
+
+#### End-to-End Flow
+
+When a pod is scheduled on a node in `zone-poland` and requests a PVC from the
+topology-aware StorageClass, the following happens:
+
+1. Kubernetes sees `volumeBindingMode: WaitForFirstConsumer` and delays
+   provisioning until the pod is scheduled to a specific node.
+
+2. Once the pod is bound to a node, Kubernetes calls `CreateVolume` with
+   `AccessibilityRequirements` containing the node's topology segments
+   (e.g. `topology.kubernetes.io/zone: zone-poland`).
+
+3. The CSI driver first tries to resolve `clusterID` from the StorageClass
+   parameters. Since it is not present, the driver falls back to
+   topology-based cluster selection.
+
+4. The driver parses the `clusterIDs` parameter to get the list of candidate
+   clusters: `["cluster-poland", "cluster-france"]`.
+
+5. For each candidate, the driver reads the `topologyDomainLabels` from
+   `config.json` and matches them against the `AccessibilityRequirements`.
+   All labels defined in the cluster's `topologyDomainLabels` must be present
+   and have matching values in the topology segments.
+
+6. Preferred topologies (from the CO's scheduling preference) are checked
+   first. If no match is found, requisite topologies (hard constraints) are
+   checked as a fallback.
+
+7. The first matching cluster is selected. In this example, `cluster-poland`
+   matches because its `topologyDomainLabels` contain
+   `topology.kubernetes.io/zone: zone-poland`, which matches the node's zone.
+
+8. The selected `clusterID` is used to resolve monitors from `config.json`.
+   The driver connects to the Ceph cluster in Poland and creates the RBD image
+   (or CephFS subvolume) there.
+
+9. The selected `clusterID` is encoded into the `volumeHandle`, so all
+   subsequent operations (NodeStage, ExpandVolume, DeleteVolume) resolve the
+   correct cluster automatically, without needing topology selection again.
+
+### Multi-Dimensional Topology
+
+The `topologyDomainLabels` field supports multiple labels for multi-dimensional
+matching. For example, a cluster can be associated with both a region and a
+zone:
+
+```json
+{
+  "clusterID": "cluster-poland-az1",
+  "topologyDomainLabels": {
+    "topology.kubernetes.io/region": "europe",
+    "topology.kubernetes.io/zone": "poland-az1"
+  }
+}
+```
+
+All labels must match for the cluster to be selected.
+
+## Impact on Existing Operations
+
+The topology-based cluster selection only affects the `CreateVolume` operation.
+All other CSI operations are unaffected because the `volumeHandle` already
+contains the selected `clusterID`:
+
+- **NodeStageVolume / NodePublishVolume** — the node plugin decodes the
+  `clusterID` from the `volumeHandle` and connects to the correct cluster.
+  No topology resolution needed.
+
+- **DeleteVolume / ControllerExpandVolume** — the controller decodes the
+  `clusterID` from the `volumeHandle`. Same behavior as today.
+
+- **CreateSnapshot** — uses the source volume's `clusterID`.
+
+The provisioner pod (Deployment) must have network access to monitors of all
+Ceph clusters listed in `config.json`. This is already the case when multiple
+clusters are configured today. The node plugin pods (DaemonSet) also mount the
+same `ceph-csi-config` ConfigMap and can connect to any cluster whose volumes
+they need to mount.
+
+Connection lifecycle is unchanged — the driver uses the existing connection pool
+(`conn_pool.go`) which manages connections by `monitors|user|keyfile`
+combination and auto-recycles unused connections.
+
+## Backward Compatibility
+
+- Existing `config.json` entries without `topologyDomainLabels` work unchanged.
+  The new field uses `omitempty` in JSON serialization.
+
+- StorageClasses with a single `clusterID` parameter use the existing fast
+  path. The topology selection code is never reached.
+
+- The `clusterIDs` parameter is purely additive. No existing parameters or
+  validation rules are removed.
+
+- Volumes created with topology-based selection are indistinguishable from
+  volumes created with an explicit `clusterID` — the `volumeHandle` format is
+  identical.
+
+## Limitations
+
+- `volumeBindingMode: WaitForFirstConsumer` is required when using `clusterIDs`.
+  With `Immediate` binding, Kubernetes does not provide
+  `AccessibilityRequirements` and the driver cannot determine the target
+  topology.
+
+- The pool name must be the same across all candidate clusters (since a single
+  `pool` parameter is specified in the StorageClass). If pools have different
+  names, the existing `topologyConstrainedPools` mechanism can be combined with
+  this feature in a future iteration.
+
+## Future Work
+
+- Make `clusterID` fully optional when `clusterIDs` is provided (currently both
+  are accepted, but at least one is required).
+- Combine topology-based cluster selection with `topologyConstrainedPools` for
+  selecting both cluster and pool based on topology.
+- Add E2E tests with a multi-cluster topology setup.