New to KubeDB? Please start here.

Memcached

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.

Memcached Spec

As with all other Kubernetes objects, a Memcached needs apiVersion, kind, and 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:
      serviceAccountName: my-service-account
      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

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.

KubeDB uses Pod Disruption Budget to ensure that majority of these replicas are available during voluntary disruptions so that quorum is maintained.

spec.version

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 MemcachedVersion crd,

• 1.5.4, 1.5.4-v1, 1.5, 1.5-v1

spec.monitor

Memcached managed by KubeDB can be monitored with builtin-Prometheus and CoreOS-Prometheus operator out-of-the-box. To learn more,

• Monitor Memcached with builtin Prometheus
• Monitor Memcached with CoreOS Prometheus operator

spec.configSource

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 configMap, secret, azureDisk etc. To learn more about how to use a custom configuration file see here.

spec.podTemplate

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 spec.podTemplate:

• metadata
  • annotations (pod’s annotation)
• controller
  • annotations (statefulset’s annotation)
• spec:
  • args
  • env
  • resources
  • initContainers
  • imagePullSecrets
  • nodeSelector
  • affinity
  • serviceAccountName
  • schedulerName
  • tolerations
  • priorityClassName
  • priority
  • securityContext
  • livenessProbe
  • readinessProbe
  • lifecycle

Uses of some field of spec.podTemplate is described below,

spec.podTemplate.spec.args

spec.podTemplate.spec.args is an optional field. This can be used to provide additional arguments to database installation.

spec.podTemplate.spec.env

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

spec.podTemplate.spec.imagePullSecrets

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.podTemplate.spec.nodeSelector

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.podTemplate.spec.serviceAccountName

serviceAccountName is an optional field supported by KubeDB Operator (version 0.13.0 and higher) that can be used to specify a custom service account to fine tune role based access control.

If this field is left empty, the KubeDB operator will create a service account name matching Memcached crd name. Role and RoleBinding that provide necessary access permissions will also be generated automatically for this service account.

If a service account name is given, but there’s no existing service account by that name, the KubeDB operator will create one, and Role and RoleBinding that provide necessary access permissions will also be generated for this service account.

If a service account name is given, and there’s an existing service account by that name, the KubeDB operator will use that existing service account. Since this service account is not managed by KubeDB, users are responsible for providing necessary access permissions manually. Follow the guide here to grant necessary permissions in this scenario.

spec.podTemplate.spec.resources

spec.resources is an optional field. This can be used to request compute resources required by the database pods. To learn more, visit here.

spec.serviceTemplate

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 spec.serviceTemplate:

• metadata:
  • annotations
• spec:
  • type
  • ports
  • clusterIP
  • externalIPs
  • loadBalancerIP
  • loadBalancerSourceRanges
  • externalTrafficPolicy
  • healthCheckNodePort

spec.updateStrategy

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.

spec.terminationPolicy

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:

• DoNotTerminate
• Pause (Default)
• Delete
• WipeOut

When, terminationPolicy is 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 DoNotTerminate.

Following table show what KubeDB does when you delete Memcached crd for different termination policies,

If you don’t specify spec.terminationPolicy KubeDB uses Pause termination policy by default.

Next Steps

• Learn how to use KubeDB to run a Memcached server here.
• Want to hack on KubeDB? Check our contribution guidelines.