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MongoDB

What is MongoDB

MongoDB is a Kubernetes Custom Resource Definitions (CRD). It provides declarative configuration for MongoDB in a Kubernetes native way. You only need to describe the desired database configuration in a MongoDB object, and the KubeDB operator will create Kubernetes objects in the desired state for you.

MongoDB Spec

As with all other Kubernetes objects, a MongoDB needs apiVersion, kind, and metadata fields. It also needs a .spec section. Below is an example MongoDB object.

apiVersion: kubedb.com/v1alpha1
kind: MongoDB
metadata:
  name: mgo1
  namespace: demo
spec:
  version: "3.4-v2"
  replicas: 3
    replicaSet:
      name: rs0
      keyFile:
        secretName: mgo-replicaset-keyfile
  databaseSecret:
    secretName: mgo1-auth
  storageType: "Durable"
  storage:
    storageClassName: "standard"
    accessModes:
    - ReadWriteOnce
    resources:
      requests:
        storage: 1Gi
  init:
    scriptSource:
      configMap:
        name: mg-init-script
  backupSchedule:
    cronExpression: "@every 2m"
    storageSecretName: mg-snap-secret
    gcs:
      bucket: kubedb-qa
      prefix: demo
  monitor:
    agent: prometheus.io/coreos-operator
    prometheus:
      namespace: demo
      labels:
        app: kubedb
      interval: 10s
  configSource:
    configMap:
      name: mg-custom-config
  podTemplate:
    annotations:
      passMe: ToDatabasePod
    controller:
      annotations:
        passMe: ToStatefulSet
    spec:
      schedulerName: my-scheduler
      nodeSelector:
        disktype: ssd
      imagePullSecrets:
      - name: myregistrykey
      args:
      - --maxConns=100
      env:
      - name: MONGO_INITDB_DATABASE
        value: myDB
      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.version

spec.version is a required field specifying the name of the MongoDBVersion crd where the docker images are specified. Currently, when you install KubeDB, it creates the following MongoDBVersion crd,

• 3.4-v2, 3.4-v1, 3.4
• 3.6-v2, 3.6-v1, 3.6
• 4.0.5, 4.0
• 4.1.7

spec.replicas

spec.replicas the number of members in rs0 mongodb replicaset.

spec.replicaSet

spec.replicaSet represents the configuration for replicaset.

• name denotes the name of mongodb replicaset.

• KeyFileSecret (optional) is a secret name that contains keyfile (a random string)against key.txt key. Each mongod instances in the replica set uses the contents of the keyfile as the shared password for authenticating other members in the deployment. Only mongod instances with the correct keyfile can join the replica set. User can provide the KeyFileSecret by creating a secret with key key.txt. See here to create the string for KeyFileSecret. If KeyFileSecret is not given, KubeDB operator will generate a KeyFileSecret itself.

spec.databaseSecret

spec.databaseSecret is an optional field that points to a Secret used to hold credentials for mongodb superuser. If not set, KubeDB operator creates a new Secret {mongodb-object-name}-auth for storing the password for mongodb superuser for each MongoDB object. If you want to use an existing secret please specify that when creating the MongoDB object using spec.databaseSecret.secretName.

This secret contains a user key and a password key which contains the username and password respectively for mongodb superuser.

Example:

$ kubectl create secret generic mgo1-auth -n demo \
--from-literal=user=jhon-doe \
--from-literal=password=6q8u_2jMOW-OOZXk
secret "mgo1-auth" created

apiVersion: v1
data:
  password: NnE4dV8yak1PVy1PT1pYaw==
  user: amhvbi1kb2U=
kind: Secret
metadata:
  ...
  name: mgo1-auth
  namespace: demo
  ...
type: Opaque

spec.storageType

spec.storageType is an optional field that specifies the type of storage to use for database. It can be either Durable or Ephemeral. The default value of this field is Durable. If Ephemeral is used then KubeDB will create MongoDB database using emptyDir volume. In this case, you don’t have to specify spec.storage field.

spec.storage

Since 0.9.0-rc.0, If you set spec.storageType: to Durable, then spec.storage is a required field that specifies the StorageClass of PVCs dynamically allocated to store data for the database. This storage spec will be passed to the StatefulSet created by KubeDB operator to run database pods. You can specify any StorageClass available in your cluster with appropriate resource requests.

• spec.storage.storageClassName is the name of the StorageClass used to provision PVCs. PVCs don’t necessarily have to request a class. A PVC with its storageClassName set equal to "" is always interpreted to be requesting a PV with no class, so it can only be bound to PVs with no class (no annotation or one set equal to “”). A PVC with no storageClassName is not quite the same and is treated differently by the cluster depending on whether the DefaultStorageClass admission plugin is turned on.
• spec.storage.accessModes uses the same conventions as Kubernetes PVCs when requesting storage with specific access modes.
• spec.storage.resources can be used to request specific quantities of storage. This follows the same resource model used by PVCs.

To learn how to configure spec.storage, please visit the links below:

• https://kubernetes.io/docs/concepts/storage/persistent-volumes/#persistentvolumeclaims

spec.init

spec.init is an optional section that can be used to initialize a newly created MongoDB database. MongoDB databases can be initialized in one of two ways:

1. Initialize from Script
2. Initialize from Snapshot

Initialize via Script

To initialize a MongoDB database using a script (shell script, js script), set the spec.init.scriptSource section when creating a MongoDB object. It will execute files alphabetically with extensions .sh and .js that are found in the repository. ScriptSource must have following information:

• VolumeSource: Where your script is loaded from.

Below is an example showing how a script from a configMap can be used to initialize a MongoDB database.

apiVersion: kubedb.com/v1alpha1
kind: MongoDB
metadata:
  name: mgo1
spec:
  version: 3.4-v2
  init:
    scriptSource:
      configMap:
        name: mongodb-init-script

In the above example, KubeDB operator will launch a Job to execute all js script of mongodb-init-script in alphabetical order once StatefulSet pods are running. For more details tutorial on how to initialize from script, please visit here.

Initialize from Snapshots

To initialize from prior snapshots, set the spec.init.snapshotSource section when creating a MongoDB object. In this case, SnapshotSource must have following information:

• name: Name of the Snapshot
• namespace: Namespace of the Snapshot

apiVersion: kubedb.com/v1alpha1
kind: MongoDB
metadata:
  name: mgo1
spec:
  version: 3.4-v2
  init:
    snapshotSource:
      name: "snapshot-xyz"
      namespace: "demo"

In the above example, MongoDB database will be initialized from Snapshot snapshot-xyz in demo namespace. Here, KubeDB operator will launch a Job to initialize MongoDB once StatefulSet pods are running.

For more details tutorial on how to initialize from snapshot, please visit here.

spec.backupSchedule

KubeDB supports taking periodic snapshots for MongoDB database. This is an optional section in .spec. When spec.backupSchedule section is added, KubeDB operator immediately takes a backup to validate this information. After that, at each tick kubeDB operator creates a Snapshot object. This triggers operator to create a Job to take backup. I f used, set the various sub-fields accordingly.

• spec.backupSchedule.cronExpression is a required cron expression. This specifies the schedule for backup operations.
• spec.backupSchedule.{storage} is a required field that is used as the destination for storing snapshot data. KubeDB supports cloud storage providers like S3, GCS, Azure and OpenStack Swift. It also supports any locally mounted Kubernetes volumes, like NFS, Ceph, etc. Only one backend can be used at a time. To learn how to configure this, please visit here.

You can also specify a template for pod of backup job through spec.backupSchedule.podTemplate. KubeDB will use the information you have provided in podTemplate to create the backup job. KubeDB accept following fields to set in spec.backupSchedule.podTemplate:

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

spec.monitor

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

• Monitor MongoDB with builtin Prometheus
• Monitor MongoDB with CoreOS Prometheus operator

spec.configSource

spec.configSource is an optional field that allows users to provide custom configuration for MongoDB. This field accepts a VolumeSource. You can use any kubernetes supported volume source such as configMap, secret, azureDisk etc.

Please note that, the configfile name needs to be mongod.conf for mongodb.

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 StatefulSet created for MongoDB database.

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
  • 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. To learn about available args of mongod, visit here.

spec.podTemplate.spec.env

spec.podTemplate.spec.env is an optional field that specifies the environment variables to pass to the MongoDB docker image. To know about supported environment variables, please visit here.

Note that, KubeDB does not allow MONGO_INITDB_ROOT_USERNAME and MONGO_INITDB_ROOT_PASSWORD environment variables to set in spec.podTemplate.spec.env. If you want to use custom superuser and password, please use spec.databaseSecret instead described earlier.

If you try to set MONGO_INITDB_ROOT_USERNAME or MONGO_INITDB_ROOT_PASSWORD environment variable in MongoDB crd, Kubed operator will reject the request with following error,

Error from server (Forbidden): error when creating "./mongodb.yaml": admission webhook "mongodb.validators.kubedb.com" denied the request: environment variable MONGO_INITDB_ROOT_USERNAME is forbidden to use in MongoDB spec

Also, note that KubeDB does not allow to update the environment variables as updating them does not have any effect once the database is created. 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: "./mongodb.yaml": admission webhook "mongodb.validators.kubedb.com" denied the request: precondition failed for:
...At least one of the following was changed:
    apiVersion
    kind
    name
    namespace
    spec.ReplicaSet
    spec.databaseSecret
    spec.init
    spec.storageType
    spec.storage
    spec.podTemplate.spec.nodeSelector
    spec.podTemplate.spec.env

spec.podTemplate.spec.imagePullSecret

KubeDB provides the flexibility of deploying MongoDB database from a private Docker registry. spec.podTemplate.spec.imagePullSecrets is an optional field that points to secrets to be used for pulling docker image if you are using a private docker registry. To learn how to deploy MongoDB from a private registry, please visit here.

spec.podTemplate.spec.nodeSelector

spec.podTemplate.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.resources

spec.podTemplate.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 MongoDB database 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 StatefulSet created by KubeDB for MongoDB database 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 MongoDB crd or which resources KubeDB should keep or delete when you delete MongoDB 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 MongoDB 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 MongoDB database here.
• See the list of supported storage providers for snapshots here.
• Want to hack on KubeDB? Check our contribution guidelines.