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Storage Autoscaling of a MongoDB Replicaset Database

This guide will show you how to use KubeDB to autoscale the storage of a MongoDB Replicaset database.

Before You Begin

At first, you need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster.
Install KubeDB Provisioner, Ops-manager and Autoscaler operator in your cluster following the steps here.
Install Metrics Server from here
Install Prometheus from here
You must have a StorageClass that supports volume expansion.
You should be familiar with the following KubeDB concepts:

To keep everything isolated, we are going to use a separate namespace called demo throughout this tutorial.

$ kubectl create ns demo
namespace/demo created

Note: YAML files used in this tutorial are stored in docs/examples/mongodb directory of kubedb/docs repository.

Storage Autoscaling of ReplicaSet Database

At first verify that your cluster has a storage class, that supports volume expansion. Let’s check,

$ kubectl get storageclass
NAME                  PROVISIONER             RECLAIMPOLICY   VOLUMEBINDINGMODE      ALLOWVOLUMEEXPANSION   AGE
standard (default)    rancher.io/local-path   Delete          WaitForFirstConsumer   false                  9h
topolvm-provisioner   topolvm.cybozu.com      Delete          WaitForFirstConsumer   true                   9h

We can see from the output the topolvm-provisioner storage class has ALLOWVOLUMEEXPANSION field as true. So, this storage class supports volume expansion. We can use it. You can install topolvm from here

Now, we are going to deploy a MongoDB replicaset using a supported version by KubeDB operator. Then we are going to apply MongoDBAutoscaler to set up autoscaling.

Deploy MongoDB replicaset

In this section, we are going to deploy a MongoDB replicaset database with version 4.2.3. Then, in the next section we will set up autoscaling for this database using MongoDBAutoscaler CRD. Below is the YAML of the MongoDB CR that we are going to create,

apiVersion: kubedb.com/v1alpha2
kind: MongoDB
metadata:
  name: mg-rs
  namespace: demo
spec:
  version: "4.2.3"
  replicaSet:
    name: "replicaset"
  replicas: 3
  storageType: Durable
  storage:
    storageClassName: topolvm-provisioner
    resources:
      requests:
        storage: 1Gi
  terminationPolicy: WipeOut

Let’s create the MongoDB CRO we have shown above,

$ kubectl create -f https://github.com/kubedb/docs/raw/v2023.06.19/docs/examples/mongodb/autoscaling/storage/mg-rs.yaml
mongodb.kubedb.com/mg-rs created

Now, wait until mg-rs has status Ready. i.e,

$ kubectl get mg -n demo
NAME      VERSION    STATUS    AGE
mg-rs     4.2.3      Ready     2m53s

Let’s check volume size from statefulset, and from the persistent volume,

$ kubectl get sts -n demo mg-rs -o json | jq '.spec.volumeClaimTemplates[].spec.resources.requests.storage'
"1Gi"

$ kubectl get pv -n demo
NAME                                       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                  STORAGECLASS          REASON   AGE
pvc-b16daa50-83fc-4d25-b553-4a25f13166d5   1Gi        RWO            Delete           Bound    demo/datadir-mg-rs-0   topolvm-provisioner            2m12s
pvc-d4616bef-359d-4b73-ab9f-38c24aaaec8c   1Gi        RWO            Delete           Bound    demo/datadir-mg-rs-1   topolvm-provisioner            61s
pvc-ead21204-3dc7-453c-8121-d2fe48b1c3e2   1Gi        RWO            Delete           Bound    demo/datadir-mg-rs-2   topolvm-provisioner            18s

You can see the statefulset has 1GB storage, and the capacity of all the persistent volume is also 1GB.

We are now ready to apply the MongoDBAutoscaler CRO to set up storage autoscaling for this database.

Storage Autoscaling

Here, we are going to set up storage autoscaling using a MongoDBAutoscaler Object.

Create MongoDBAutoscaler Object

In order to set up vertical autoscaling for this replicaset database, we have to create a MongoDBAutoscaler CRO with our desired configuration. Below is the YAML of the MongoDBAutoscaler object that we are going to create,

apiVersion: autoscaling.kubedb.com/v1alpha1
kind: MongoDBAutoscaler
metadata:
  name: mg-as-rs
  namespace: demo
spec:
  databaseRef:
    name: mg-rs
  storage:
    replicaSet:
      trigger: "On"
      usageThreshold: 60
      scalingThreshold: 50

Here,

spec.databaseRef.name specifies that we are performing vertical scaling operation on mg-rs database.
spec.storage.replicaSet.trigger specifies that storage autoscaling is enabled for this database.
spec.storage.replicaSet.usageThreshold specifies storage usage threshold, if storage usage exceeds 60% then storage autoscaling will be triggered.
spec.storage.replicaSet.scalingThreshold specifies the scaling threshold. Storage will be scaled to 50% of the current amount.
It has another field spec.storage.replicaSet.expansionMode to set the opsRequest volumeExpansionMode, which support two values: Online & Offline. Default value is Online.

Let’s create the MongoDBAutoscaler CR we have shown above,

$ kubectl apply -f https://github.com/kubedb/docs/raw/v2023.06.19/docs/examples/mongodb/autoscaling/storage/mg-as-rs.yaml
mongodbautoscaler.autoscaling.kubedb.com/mg-as-rs created

Storage Autoscaling is set up successfully

Let’s check that the mongodbautoscaler resource is created successfully,

$ kubectl get mongodbautoscaler -n demo
NAME       AGE
mg-as-rs   20s

$ kubectl describe mongodbautoscaler mg-as-rs -n demo
Name:         mg-as-rs
Namespace:    demo
Labels:       <none>
Annotations:  <none>
API Version:  autoscaling.kubedb.com/v1alpha1
Kind:         MongoDBAutoscaler
Metadata:
  Creation Timestamp:  2021-03-08T14:11:46Z
  Generation:          1
  Managed Fields:
    API Version:  autoscaling.kubedb.com/v1alpha1
    Fields Type:  FieldsV1
    fieldsV1:
      f:metadata:
        f:annotations:
          .:
          f:kubectl.kubernetes.io/last-applied-configuration:
      f:spec:
        .:
        f:databaseRef:
          .:
          f:name:
        f:storage:
          .:
          f:replicaSet:
            .:
            f:scalingThreshold:
            f:trigger:
            f:usageThreshold:
    Manager:         kubectl-client-side-apply
    Operation:       Update
    Time:            2021-03-08T14:11:46Z
  Resource Version:  152149
  Self Link:         /apis/autoscaling.kubedb.com/v1alpha1/namespaces/demo/mongodbautoscalers/mg-as-rs
  UID:               a0dab64d-e7c4-4819-8ffe-360c70231577
Spec:
  Database Ref:
    Name:  mg-rs
  Storage:
    Replica Set:
      Scaling Threshold:  50
      Trigger:            On
      Usage Threshold:    60
Events:                   <none>

So, the mongodbautoscaler resource is created successfully.

Now, for this demo, we are going to manually fill up the persistent volume to exceed the usageThreshold using dd command to see if storage autoscaling is working or not.

Let’s exec into the database pod and fill the database volume using the following commands:

$ kubectl exec -it -n demo mg-rs-0 -- bash
root@mg-rs-0:/# df -h /data/db
Filesystem                                         Size  Used Avail Use% Mounted on
/dev/topolvm/760cb655-91fe-4497-ab4a-a771aa53ece4 1014M  335M  680M  33% /data/db
root@mg-rs-0:/# dd if=/dev/zero of=/data/db/file.img bs=500M count=1
1+0 records in
1+0 records out
524288000 bytes (524 MB, 500 MiB) copied, 0.482378 s, 1.1 GB/s
root@mg-rs-0:/# df -h /data/db
Filesystem                                         Size  Used Avail Use% Mounted on
/dev/topolvm/760cb655-91fe-4497-ab4a-a771aa53ece4 1014M  835M  180M  83% /data/db

So, from the above output we can see that the storage usage is 83%, which exceeded the usageThreshold 60%.

Let’s watch the mongodbopsrequest in the demo namespace to see if any mongodbopsrequest object is created. After some time you’ll see that a mongodbopsrequest of type VolumeExpansion will be created based on the scalingThreshold.

$ watch kubectl get mongodbopsrequest -n demo
Every 2.0s: kubectl get mongodbopsrequest -n demo
NAME                TYPE              STATUS        AGE
mops-mg-rs-mft11m   VolumeExpansion   Progressing   10s

Let’s wait for the ops request to become successful.

$ watch kubectl get mongodbopsrequest -n demo
Every 2.0s: kubectl get mongodbopsrequest -n demo
NAME                TYPE              STATUS        AGE
mops-mg-rs-mft11m   VolumeExpansion   Successful    97s

We can see from the above output that the MongoDBOpsRequest has succeeded. If we describe the MongoDBOpsRequest we will get an overview of the steps that were followed to expand the volume of the database.

$ kubectl describe mongodbopsrequest -n demo mops-mg-rs-mft11m
Name:         mops-mg-rs-mft11m
Namespace:    demo
Labels:       app.kubernetes.io/component=database
              app.kubernetes.io/instance=mg-rs
              app.kubernetes.io/managed-by=kubedb.com
              app.kubernetes.io/name=mongodbs.kubedb.com
Annotations:  <none>
API Version:  ops.kubedb.com/v1alpha1
Kind:         MongoDBOpsRequest
Metadata:
  Creation Timestamp:  2021-03-08T14:15:52Z
  Generation:          1
  Managed Fields:
    API Version:  ops.kubedb.com/v1alpha1
    Fields Type:  FieldsV1
    fieldsV1:
      f:metadata:
        f:labels:
          .:
          f:app.kubernetes.io/component:
          f:app.kubernetes.io/instance:
          f:app.kubernetes.io/managed-by:
          f:app.kubernetes.io/name:
        f:ownerReferences:
      f:spec:
        .:
        f:databaseRef:
          .:
          f:name:
        f:type:
        f:volumeExpansion:
          .:
          f:replicaSet:
    Manager:      kubedb-autoscaler
    Operation:    Update
    Time:         2021-03-08T14:15:52Z
    API Version:  ops.kubedb.com/v1alpha1
    Fields Type:  FieldsV1
    fieldsV1:
      f:status:
        .:
        f:conditions:
        f:observedGeneration:
        f:phase:
    Manager:    kubedb-enterprise
    Operation:  Update
    Time:       2021-03-08T14:15:52Z
  Owner References:
    API Version:           autoscaling.kubedb.com/v1alpha1
    Block Owner Deletion:  true
    Controller:            true
    Kind:                  MongoDBAutoscaler
    Name:                  mg-as-rs
    UID:                   a0dab64d-e7c4-4819-8ffe-360c70231577
  Resource Version:        153496
  Self Link:               /apis/ops.kubedb.com/v1alpha1/namespaces/demo/mongodbopsrequests/mops-mg-rs-mft11m
  UID:                     84567b84-6de4-4658-b0d2-2c374e03e63d
Spec:
  Database Ref:
    Name:  mg-rs
  Type:    VolumeExpansion
  Volume Expansion:
    Replica Set:  1594884096
Status:
  Conditions:
    Last Transition Time:  2021-03-08T14:15:52Z
    Message:               MongoDB ops request is expanding volume of database
    Observed Generation:   1
    Reason:                VolumeExpansion
    Status:                True
    Type:                  VolumeExpansion
    Last Transition Time:  2021-03-08T14:17:02Z
    Message:               Successfully Expanded Volume
    Observed Generation:   1
    Reason:                ReplicasetVolumeExpansion
    Status:                True
    Type:                  ReplicasetVolumeExpansion
    Last Transition Time:  2021-03-08T14:17:07Z
    Message:               Successfully Expanded Volume
    Observed Generation:   1
    Reason:                
    Status:                True
    Type:                  
    Last Transition Time:  2021-03-08T14:17:12Z
    Message:               StatefulSet is recreated
    Observed Generation:   1
    Reason:                ReadyStatefulSets
    Status:                True
    Type:                  ReadyStatefulSets
    Last Transition Time:  2021-03-08T14:17:12Z
    Message:               Successfully Expanded Volume
    Observed Generation:   1
    Reason:                Successful
    Status:                True
    Type:                  Successful
  Observed Generation:     1
  Phase:                   Successful
Events:
  Type    Reason                     Age    From                        Message
  ----    ------                     ----   ----                        -------
  Normal  PauseDatabase              2m36s  KubeDB Ops-manager operator  Pausing MongoDB demo/mg-rs
  Normal  PauseDatabase              2m36s  KubeDB Ops-manager operator  Successfully paused MongoDB demo/mg-rs
  Normal  ReplicasetVolumeExpansion  86s    KubeDB Ops-manager operator  Successfully Expanded Volume
  Normal                             81s    KubeDB Ops-manager operator  Successfully Expanded Volume
  Normal  ResumeDatabase             81s    KubeDB Ops-manager operator  Resuming MongoDB demo/mg-rs
  Normal  ResumeDatabase             81s    KubeDB Ops-manager operator  Successfully resumed MongoDB demo/mg-rs
  Normal  ReadyStatefulSets          76s    KubeDB Ops-manager operator  StatefulSet is recreated
  Normal  Successful                 76s    KubeDB Ops-manager operator  Successfully Expanded Volume

Now, we are going to verify from the Statefulset, and the Persistent Volume whether the volume of the replicaset database has expanded to meet the desired state, Let’s check,

$ kubectl get sts -n demo mg-rs -o json | jq '.spec.volumeClaimTemplates[].spec.resources.requests.storage'
"1594884096"
$ kubectl get pv -n demo
NAME                                       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                  STORAGECLASS          REASON   AGE
pvc-b16daa50-83fc-4d25-b553-4a25f13166d5   2Gi        RWO            Delete           Bound    demo/datadir-mg-rs-0   topolvm-provisioner            11m
pvc-d4616bef-359d-4b73-ab9f-38c24aaaec8c   2Gi        RWO            Delete           Bound    demo/datadir-mg-rs-1   topolvm-provisioner            10m
pvc-ead21204-3dc7-453c-8121-d2fe48b1c3e2   2Gi        RWO            Delete           Bound    demo/datadir-mg-rs-2   topolvm-provisioner            9m52s

The above output verifies that we have successfully autoscaled the volume of the MongoDB replicaset database.

Cleaning Up

To clean up the Kubernetes resources created by this tutorial, run:

kubectl delete mg -n demo mg-rs
kubectl delete mongodbautoscaler -n demo mg-as-rs

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