# Scale MySQL on Kubernetes and OpenShift

One of the great advantages brought by Kubernetes and the OpenShift
platform is the ease of an application scaling. Scaling an application
results in adding resources or Pods and scheduling them to available
Kubernetes nodes.

Scaling can be vertical and horizontal. Vertical scaling adds more
compute or storage resources to MySQL nodes; horizontal scaling is
about adding more nodes to the cluster.

## Vertical scaling

### Scale compute

There are multiple components that Operator deploys and manages: Percona 
XtraDB Cluster (PXC), HAProxy or ProxySQL, etc. To add or reduce CPU or Memory 
you need to edit corresponding sections in the Custom Resource. We follow 
the structure for `requests` and `limits` that Kubernetes [provides](https://kubernetes.io/docs/concepts/configuration/manage-resources-containers/).

To add more resources to your MySQL nodes in PXC edit the following section in
the Custom Resource:

```yaml
spec:
...
  pxc:
    ...
    resources:
      requests: 
        memory: 4G
        cpu: 2
      limits:
        memory: 4G
        cpu: 2
```

Use our reference documentation for the [Custom Resource options](operator.md#operator-custom-resource-options) 
for more details about other components.

### Scale storage

Kubernetes manages storage with a PersistentVolume (PV), a segment of
storage supplied by the administrator, and a PersistentVolumeClaim
(PVC), a request for storage from a user. In Kubernetes v1.11 the
feature was added to allow a user to increase the size of an existing
PVC object (considered stable since Kubernetes v1.24).
The user cannot shrink the size of an existing PVC object.

Starting from the version 1.14.0, the Operator allows to scale Percona XtraDB
Cluster storage automatically by changing the appropriate Custom Resource
option, if the volume type supports PVCs expansion.

#### Automated scaling with Volume Expansion capability

Certain volume types support PVCs expansion (exact details about
PVCs and the supported volume types can be found in
[Kubernetes documentation](https://kubernetes.io/docs/concepts/storage/persistent-volumes/#expanding-persistent-volumes-claims)).

You can run the following command to check if your storage supports the
expansion capability:

``` {.bash data-prompt="$" }
$ kubectl describe sc <storage class name> | grep allowVolumeExpansion
```

??? example "Expected output"

    ``` {.text .no-copy}
    allowVolumeExpansion: true
    ```

The Operator versions 1.14.0 and higher will automatically expand such storage
for you when you change the
`pxc.volumeSpec.persistentVolumeClaim.resources.requests.storage` option in the
Custom Resource.

!!! warning

    Automated storage scaling by the Operator is in a technical preview stage
    and is not recommended for production environments.

For example, you can do it by editing and applying the `deploy/cr.yaml` file:

``` {.text .no-copy}
spec:
...
  pxc:
    ...
    volumeSpec:
      persistentVolumeClaim:
        resources:
          requests:
            storage: <NEW STORAGE SIZE>
```

Apply changes as usual:

``` {.bash data-prompt="$" }
$ kubectl apply -f cr.yaml
```

#### Manual scaling without Volume Expansion capability

Manual scaling is the way to go if you version of the Operator is older than
1.14.0, your volumes have type which does not support Volume Expansion, or you
just do not rely on automated scaling.

You will need to delete Pods one by one and their persistent volumes to resync 
the data to the new volumes. **This can also be used to shrink the storage.**

1. Update the Custom Resource with the new storage size by editing and applying
    the `deploy/cr.yaml` file:

    ``` {.text .no-copy}
    spec:
    ...
      pxc:
        ...
        volumeSpec:
          persistentVolumeClaim:
            resources:
              requests:
                storage: <NEW STORAGE SIZE>
    ```

    Apply the Custom Resource update in a usual way:

    ``` {.bash data-prompt="$" }
    $ kubectl apply -f deploy/cr.yaml
    ```

2. Delete the StatefulSet with the `orphan` option

    ``` {.bash data-prompt="$" }
    $ kubectl delete sts <statefulset-name> --cascade=orphan
    ```

    The Pods will not go down and the Operator is going to recreate
    the StatefulSet:

    ``` {.bash data-prompt="$" }
    $ kubectl get sts <statefulset-name>
    ```

    ??? example "Expected output"

        ``` {.text .no-copy}
        cluster1-pxc       3/3     39s
        ```

3. Scale up the cluster (Optional)

    Changing the storage size would require us to terminate the Pods, which 
    decreases the computational power of the cluster and might cause performance 
    issues. To improve performance during the operation we are going to 
    change the size of the cluster from 3 to 5 nodes:

    ```yaml
    ...
    spec:
    ...
      pxc:
        ...
        size: 5
    ```
    
    Apply the change:
    
    ``` {.bash data-prompt="$" }
    $ kubectl apply -f deploy/cr.yaml
    ```

    New Pods will already have new storage:
    
    ``` {.bash data-prompt="$" }
    $ kubectl get pvc
    ```

    ??? example "Expected output"

        ``` {.text .no-copy}
        NAME                     STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
        datadir-cluster1-pxc-0   Bound    pvc-90f0633b-0938-4b66-a695-556bb8a9e943   10Gi       RWO            standard       110m
        datadir-cluster1-pxc-1   Bound    pvc-7409ea83-15b6-448f-a6a0-12a139e2f5cc   10Gi       RWO            standard       109m
        datadir-cluster1-pxc-2   Bound    pvc-90f0b2f8-9bba-4262-904c-1740fdd5511b   10Gi       RWO            standard       108m
        datadir-cluster1-pxc-3   Bound    pvc-439bee13-3b57-4582-b342-98281aca50ba   19Gi       RWO            standard       49m
        datadir-cluster1-pxc-4   Bound    pvc-2d4f3a60-4ec4-48a0-96cd-5243e2f05234   19Gi       RWO            standard       47m
        ```

4. Delete PVCs and Pods with old storage size one by one. Wait for data to sync 
    before you proceeding to the next node.

    ``` {.bash data-prompt="$" }
    $ kubectl delete pvc <PVC NAME>
    $ kubectl delete pod <POD NAME>
    ```
    The new PVC is going to be created along with the Pod. 

## Horizontal scaling

Size of the cluster is controlled by a [size key](operator.md#pxc-size) in the [Custom Resource options](operator.md#operator-custom-resource-options) configuration. That’s why scaling the cluster needs
nothing more but changing this option and applying the updated
configuration file. This may be done in a specifically saved config:

```yaml
spec:
...
  pxc:
    ...
    size: 5
```
    
Apply the change:
    
``` {.bash data-prompt="$" }
$ kubectl apply -f deploy/cr.yaml
```

Alternatively, you cana do it on the fly, using the following command:

``` {.bash data-prompt="$" }
$ kubectl scale --replicas=5 pxc/<CLUSTER NAME>
```

In this example we have changed the size of the Percona XtraDB Cluster
to `5` instances.

## Automated scaling

To automate horizontal scaling it is possible to use [Horizontal 
Pod Autoscaler (HPA)](https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale/). 
It will scale the Custom Resource itself, letting Operator to deal 
with everything else.

It is also possible to use [Kuvernetes Event-driven Autoscaling (KEDA)](https://keda.sh/), 
where you can apply more sophisticated logic for decision making on scaling.

For now it is not possible to use Vertical Pod Autoscaler (VPA) with 
the Operator due to the limitations it introduces for objects with 
owner references.