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Exposing the cluster

The Operator provides entry points for accessing the database by client applications in several scenarios. In either way the cluster is exposed with regular Kubernetes Service objects , configured by the Operator.

This document describes the usage of Custom Resource manifest options to expose clusters deployed with the Operator.

Using single entry point in a sharded cluster

If Percona Server for MongoDB sharding mode is turned on (default behavior), then database cluster runs special mongos Pods - query routers, which acts as an entry point for client applications:

image

By default, a ClusterIP type Service is created (this is controlled by sharding.mongos.expose.exposeType). The Service works in a round-robin fashion between all the mongos Pods.

The URI looks like this (taking into account the need for a proper password obtained from the Secret, and a proper namespace name instead of the <namespace name> placeholder):

$ mongo "mongodb://userAdmin:userAdminPassword@my-cluster-name-mongos.<namespace name>.svc.cluster.local/admin?ssl=false"

You can get the actual Service endpoints by running the following command:

$ kubectl get psmdb
Expected output
NAME              ENDPOINT                                             STATUS   AGE
my-cluster-name   my-cluster-name-mongos.default.svc.cluster.local     ready    85m

Warning

A ClusterIP Service endpoint is only reachable inside Kubernetes. If you need to connect from the outside, you need to expose the mongos Pods by using the NodePort or Load Balancer Service types. See the Connecting from outside Kubernetes section below for details.

Accessing replica set Pods

If Percona Server for MongoDB sharding mode mode is turned off, the application needs to connect to all the MongoDB Pods of the replica set:

image

When Kubernetes creates Pods, each Pod has an IP address in the internal virtual network of the cluster. Creating and destroying Pods is a dynamic process, therefore binding communication between Pods to specific IP addresses would cause problems as things change over time as a result of the cluster scaling, maintenance, etc. Due to this changing environment, you should connect to Percona Server for MongoDB by using Kubernetes internal DNS names in the URI.

By default, a ClusterIP type Service is created (this is controlled by replsets.expose.exposeType). The Service works in a round-robin fashion between all the mongod Pods of the replica set.

In this case, the URI looks like this (taking into account the need for a proper password obtained from the Secret, and a proper namespace name instead of the <namespace name> placeholder):

$ mongodb://databaseAdmin:databaseAdminPassword@my-cluster-name-rs0.<namespace name>.svc.cluster.local/admin?replicaSet=rs0&ssl=false"

You can get the actual Service endpoints by running the following command:

$ kubectl get psmdb
Expected output
NAME              ENDPOINT                                             STATUS   AGE
my-cluster-name   my-cluster-name-rs0.default.svc.cluster.local        ready    2m19s

Warning

A ClusterIP Service endpoint is only reachable inside Kubernetes. If you need to connect from the outside, you need to expose the mongod Pods by using the NodePort or Load Balancer Service types. See the Connecting from outside Kubernetes section below for details.

Connecting from outside Kubernetes

If connecting to a cluster from outside Kubernetes, you cannot reach the Pods using the Kubernetes internal DNS names. To make the Pods accessible, Percona Operator for MongoDB can create Kubernetes Services .

  • set expose.enabled option to true to allow exposing the Pods via Services,
  • set expose.exposeType option specifying the type of Service to be used:
    • ClusterIP - expose the Pod with an internal static IP address. This variant makes the Service reachable only from within the Kubernetes cluster.
    • NodePort - expose the Pod on each Kubernetes Node’s IP address at a static port. A ClusterIP Service, to which the Node port will be routed, is automatically created in this variant. As an advantage, the Service will be reachable from outside the cluster by Node address and port number, however the address will be bound to a specific Kubernetes Node.
    • LoadBalancer - expose the Pod externally using a cloud provider’s load balancer. Both ClusterIP and NodePort Services are automatically created in this variant

If the NodePort type is used, the URI looks like this:

mongodb://databaseAdmin:databaseAdminPassword@<node1>:<port1>,<node2>:<port2>,<node3>:<port3>/admin?replicaSet=rs0&ssl=false

All Node addresses should be directly reachable by the application.

Service per Pod

To make all database Pods accessible, Percona Operator for MongoDB can assign a Kubernetes Service to each Pod. Particularly, the Service per Pod option allows the application to take care of Cursor tracking instead of relying on a single Service. This solves the problem of CursorNotFound errors when the Service transparently cycles between the mongos instances while client is still iterating the cursor on some large collection.

This feature can be enabled for both sharded and non-sharded clusters by setting the sharding.mongos.expose.servicePerPod Custom Resource option to true in the deploy/cr.yaml file.

If this feature is enabled with the exposeType: NodePort, the created Services look like this:

$ kubectl get svc
NAME                       TYPE           CLUSTER-IP      EXTERNAL-IP    PORT(S)                      AGE
my-cluster-name-mongos-0   NodePort       10.38.158.103   <none>         27017:31689/TCP              12s
my-cluster-name-mongos-1   NodePort       10.38.155.250   <none>         27017:31389/TCP              12s
...

Controlling hostnames in replset configuration

Starting from v1.14, the Operator configures replica set members using local fully-qualified domain names (FQDN), which are resolvable and available only from inside the Kubernetes cluster. Exposing the replica set using the options described above will not affect hostname usage in the replica set configuration.

Note

Before v1.14, the Operator used the exposed IP addresses in the replica set configuration in the case of the exposed replica set.

It is still possible to restore the old behavior. For example, it may be useful to have the replica set configured with external IP addresses for multi-cluster deployments. The clusterServiceDNSMode field in the Custom Resource controls this Operator behavior. You can set clusterServiceDNSMode to one of the following values:

  1. Internal: Use local FQDNs (i.e., cluster1-rs0-0.cluster1-rs0.psmdb.svc.cluster.local) in replica set configuration even if the replica set is exposed. This is the default value.
  2. ServiceMesh: Use a special FQDN using the Pod name (i.e., cluster1-rs0-0.psmdb.svc.cluster.local), assuming it’s resolvable and available in all clusters.
  3. External: Use exposed IP in replica set configuration if replica set is exposed; else, use local FQDN. This copies the behavior of the Operator v1.13.

If backups are enabled in your cluster, you need to restart replset and config servers after changing clusterServiceDNSMode. This option changes the hostnames inside the replset configuration and running pbm-agents don’t discover the change until they’re restarted. You may have errors in backup-agent container logs and your backups may not work until you restarted the agents.

Restart can be done manually with the kubectl rollout restart sts <clusterName>-<replsetName> command executed for each replica set in the spec.replsets; also, if sharding enabled, do the same for config servers with kubectl rollout restart sts <clusterName>-cfg. Alternatively, you can simply restart your cluster.

Warning

You should be careful with the clusterServiceDNSMode=External variant. Using IP addresses instead of DNS hostnames is discouraged in MongoDB. IP addresses make reconfiguration and recovery more complicated, and are generally problematic in scenarios where IP addresses change. In particular, if you delete and recreate the cluster with clusterServiceDNSMode=External without deleting its volumes (having delete-psmdb-pvc finalizer unset), your cluster will crash and there will be no straightforward way to recover it.

Exposing replica set with split-horizon DNS

Split-horizon DNS provides each replica set Pod with a set of DNS URIs for external usage. This allows to communicate with replica set Pods both from inside the Kubernetes cluster and from outside of Kubernetes.

Split-horizon can be configured via the replset.horizons subsection in the Custom Resource options. Set it in the deploy/cr.yaml configuration file as follows:

    ...
    replsets:
      - name: rs0
        expose:
          enabled: true
          exposeType: LoadBalancer
        horizons:
          cluster1-rs0-0:
            external: rs0-0.mycluster.xyz
            external-2: rs0-0.mycluster2.xyz
          cluster1-rs0-1:
            external: rs0-1.mycluster.xyz
            external-2: rs0-1.mycluster2.xyz
          cluster1-rs0-2:
            external: rs0-2.mycluster.xyz
            external-2: rs0-2.mycluster2.xyz

URIs for external usage are specified as key-value pairs, where the key is an arbitrary name and the value is the actual URI. The URI may include a port number. If nothing is set, the default MongoDB port will be used.

Split horizon has following limitations:

  • connecting with horizon domains is only supported if client connects using TLS certificates, and these TLS certificates need to be generated manually
  • duplicating domain names in horizons is not allowed by MongoDB
  • using IP addresses in horizons is not allowed by MongoDB
  • horizons should be set for all Pods of a replica set or not set at all
  • horizons should be configured on an existing cluster (creating a new cluster with pre-configured horizons is currently not supported)

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Last update: 2024-06-07