New to KubeDB? Please start here.
What is Memcached
Memcached is a Kubernetes
Custom Resource Definitions (CRD). It provides declarative configuration for Memcached in a Kubernetes native way. You only need to describe the desired database configuration in a Memcached object, and the KubeDB operator will create Kubernetes objects in the desired state for you.
As with all other Kubernetes objects, a Memcached needs
metadata fields. It also needs a
.spec section. Below is an example of a Memcached object.
apiVersion: kubedb.com/v1alpha1 kind: Memcached metadata: name: mc1 namespace: demo spec: replicas: 3 version: 1.5.3-v1 monitor: agent: coreos-prometheus-operator prometheus: namespace: demo labels: app: kubedb interval: 10s configSource: configMap: name: mc-custom-config podTemplate: annotations: passMe: ToDatabasePod controller: annotations: passMe: ToDeployment spec: schedulerName: my-scheduler nodeSelector: disktype: ssd imagePullSecrets: - name: myregistrykey args: - "-u memcache" env: - name: TEST_ENV value: "value" resources: requests: memory: "64Mi" cpu: "250m" limits: memory: "128Mi" cpu: "500m" serviceTemplate: annotations: passMe: ToService spec: type: NodePort ports: - name: http port: 9200 targetPort: http terminationPolicy: Pause updateStrategy: type: RollingUpdate
spec.replicas is an optional field that specifies the number of desired Instances/Replicas of Memcached server. If you do not specify .spec.replicas, then it defaults to 1.
spec.version is a required field specifying the name of the MemcachedVersion crd where the docker images are specified. Currently, when you install KubeDB, it creates the following
Memcached managed by KubeDB can be monitored with builtin-Prometheus and CoreOS-Prometheus operator out-of-the-box. To learn more,
spec.configSource is an optional field that allows users to provide custom configuration for Memcached. This field accepts a
VolumeSource. So you can use any kubernetes supported volume source such as
azureDisk etc. To learn more about how to use a custom configuration file see here.
KubeDB allows providing a template for database pod through
spec.podTemplate. KubeDB operator will pass the information provided in
spec.podTemplate to the Deployment created for Memcached server.
KubeDB accept following fields to set in
- annotations (pod’s annotation)
- annotations (statefulset’s annotation)
Uses of some field of
spec.podTemplate is described below,
spec.podTemplate.spec.args is an optional field. This can be used to provide additional arguments to database installation.
spec.env is an optional field that specifies the environment variables to pass to the Memcached docker image.
Note that, KubeDB does not allow to update the environment variables. If you try to update environment variables, KubeDB operator will reject the request with following error,
Error from server (BadRequest): error when applying patch: ... for: "./mc.yaml": admission webhook "memcached.validators.kubedb.com" denied the request: precondition failed for: ... At least one of the following was changed: apiVersion kind name namespace spec.podTemplate.spec.nodeSelector spec.podTemplate.spec.env
KubeDB provides the flexibility of deploying Memcached server from a private Docker registry. To learn how to deploym Memcached from a private registry, please visit here.
spec.nodeSelector is an optional field that specifies a map of key-value pairs. For the pod to be eligible to run on a node, the node must have each of the indicated key-value pairs as labels (it can have additional labels as well). To learn more, see here .
spec.resources is an optional field. This can be used to request compute resources required by the database pods. To learn more, visit here.
You can also provide a template for the services created by KubeDB operator for Memcached server through
spec.serviceTemplate. This will allow you to set the type and other properties of the services.
KubeDB allows following fields to set in
You can specify update strategy of Deployment created by KubeDB for Memcached server thorough
spec.updateStrategy field. The default value of this field is
RollingUpdate. In future, we will use this field to determine how automatic migration from old KubeDB version to new one should behave.
terminationPolicy gives flexibility whether to
nullify(reject) the delete operation of
Memcached crd or which resources KubeDB should keep or delete when you delete
Memcached crd. KubeDB provides following four termination policies:
- Pause (
DoNotTerminate, KubeDB takes advantage of
ValidationWebhook feature in Kubernetes 1.9.0 or later clusters to implement
DoNotTerminate feature. If admission webhook is enabled,
DoNotTerminate prevents users from deleting the database as long as the
spec.terminationPolicy is set to
Following table show what KubeDB does when you delete Memcached crd for different termination policies,
|1. Block Delete operation||✓||✗||✗||✗|
|2. Create Dormant Database||✗||✓||✗||✗|
|3. Delete StatefulSet||✗||✓||✓||✓|
|4. Delete Services||✗||✓||✓||✓|
If you don’t specify
spec.terminationPolicy KubeDB uses
Pause termination policy by default.