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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/v1
kind: Memcached
metadata:
  name: mc1
  namespace: demo
spec:
  replicas: 1
  version: 1.6.22
  monitor:
    agent: prometheus.io/operator
    prometheus:
      serviceMonitor:
        labels:
          app: kubedb
        interval: 10s
  configSecret:
    name: mc-custom-config
  podTemplate:
    metadata:
      annotations:
        passMe: ToDatabasePod
    controller:
      annotations:
        passMe: ToDeployment
    spec:
      serviceAccountName: my-service-account
      schedulerName: my-scheduler
      nodeSelector:
        disktype: ssd
      imagePullSecrets:
      - name: myregistrykey
      containers:
      - name: memcached
        args:
        - "-u memcache"
        env:
        - name: TEST_ENV
          value: "value"
        resources:
          requests:
            memory: "64Mi"
            cpu: "250m"
          limits:
            memory: "128Mi"
            cpu: "500m"
  serviceTemplates:
  - alias: primary
    metadata:
      annotations:
        passMe: ToService
    spec:
      type: NodePort
      ports:
      - name:  http
        port:  9200
  deletionPolicy: Delete

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 PodDisruptionBudget 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 crds,

• 1.5.22
• 1.6.22
• 1.6.29

spec.monitor

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

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

spec.configSecret

spec.configSecret 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 Petset created for Memcached server.

KubeDB accept following fields to set in spec.podTemplate:

• metadata
  • annotations (pod’s annotation)
• controller
  • annotations (petset’s annotation)
• spec:
  • containers
  • volumes
  • podPlacementPolicy
  • initContainers
  • containers
  • podPlacementPolicy
  • imagePullSecrets
  • nodeSelector
  • serviceAccountName
  • schedulerName
  • tolerations
  • priorityClassName
  • priority
  • securityContext

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

You can check out the full list here. Uses of some field of spec.podTemplate is described below,

spec.podTemplate.spec.tolerations

The spec.podTemplate.spec.tolerations is an optional field. This can be used to specify the pod’s tolerations.

spec.podTemplate.spec.volumes

The spec.podTemplate.spec.volumes is an optional field. This can be used to provide the list of volumes that can be mounted by containers belonging to the pod.

spec.podTemplate.spec.podPlacementPolicy

spec.podTemplate.spec.podPlacementPolicy is an optional field. This can be used to provide the reference of the podPlacementPolicy. name of the podPlacementPolicy is referred under this attribute. This will be used by our Petset controller to place the db pods throughout the region, zone & nodes according to the policy. It utilizes kubernetes affinity & podTopologySpreadContraints feature to do so.

spec:
  podPlacementPolicy:
    name: default

spec.podTemplate.spec.containers

The spec.podTemplate.spec.containers can be used to provide the list containers and their configurations for to the database pod. some of the fields are described below,

spec.podTemplate.spec.containers[].name

The spec.podTemplate.spec.containers[].name field used to specify the name of the container specified as a DNS_LABEL. Each container in a pod must have a unique name (DNS_LABEL). Cannot be updated.

spec.podTemplate.spec.containers[].args

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

spec.podTemplate.spec.containers[].env

.env is an optional field that specifies the environment variables to pass to the Memcached containers.

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.containers[].resources

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

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:

• alias represents the identifier of the service. It has the following possible value:

  • primary is used for the primary service identification.
  • standby is used for the secondary service identification.
  • stats is used for the exporter service identification.

• metadata:

  • annotations

• spec:

  • type
  • ports
  • clusterIP
  • externalIPs
  • loadBalancerIP
  • loadBalancerSourceRanges
  • externalTrafficPolicy
  • healthCheckNodePort
  • sessionAffinityConfig

See here to understand these fields in details.

spec.deletionPolicy

deletionPolicy 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
• Delete (Default)
• WipeOut

When deletionPolicy 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.deletionPolicy 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.deletionPolicy KubeDB uses Delete termination policy by default.

spec.helathChecker

It defines the attributes for the health checker.

• spec.healthChecker.periodSeconds specifies how often to perform the health check.
• spec.healthChecker.timeoutSeconds specifies the number of seconds after which the probe times out.
• spec.healthChecker.failureThreshold specifies minimum consecutive failures for the healthChecker to be considered failed.
• spec.healthChecker.disableWriteCheck specifies whether to disable the writeCheck or not.

Know details about KubeDB Health checking from this blog post.

Next Steps

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