Partition API

Partitions and PartitionSets build an API that allows service providers and cluster administrators to create shard Partitions. These partitions can then be leveraged: - for the deployment of the controllers of the service providers and other components. These APIs provide the information on shard topology required for the scheduling. - for grouping APIExport endpoints of multiple shards in common buckets, that is Partitions. These buckets can the be used to achieve geo proximity: controllers should communicate with API servers of shards in the same region. They can also be leveraged for scalability and load distribution, that is to adapt the number of deployment/ processes to the load pattern specific to the controllers of a service provider.

Partitions

The partitioning of Shards is done through labels and label selectors.

Here is an example of a Partition

kind: Partition
apiVersion: topology.kcp.io/v1alpha1
metadata:
    name: cloud-region-gcp-europe-xdfgs
    ownerReferences:
    ...
spec:
    selector:
     matchLabels:
       cloud: gcp
       region: europe
     matchExpressions:
     - key: country
       operator: NotIn
       values:
       - LI
  ```

`Partitions` can be referenced in [`APIExportEndpointSlices`](./quickstart-tenancy-and-apis.md)

## PartitionSets

`PartitionSets` is  an API for convenience. `PartitionSet` can be used to get `Partitions` automatically created based on dimensions that match the shard label keys. The `Partitions` are created in the same workspace as the `PartitionSet`. They can then be copied to the desired workspace for consumption, for instance, by an `APIExportEndpointSlice`.

Here is an example of a `PartitionSet`

```yaml
kind: PartitionSet
apiVersion: topology.kcp.io/v1alpha1
metadata:
    name: cloud-region
spec:
   dimensions:
   - region
   - cloud
   selectors:
     matchExpressions:
     - key: region
       operator: NotIn
       values:
       - Antarctica
       - Greenland
     - key: country
       operator: NotIn
       values:
       - NK
status:
   count: 10
...

It is to note that a Partition is created only if it matches at least one shard. With the provided example if there is no shard in the cloud provider aliyun in the region europe no Partition will be created for it.

An example of a Partition generated by this PartitionSet can be found above. The dimensions are translated into matchLabels with values specific to each Partition. An owner reference of the Partition will be set to the PartitionSet.