Percona Server for MongoDB Sharding

About sharding

Sharding provides horizontal database scaling, distributing data across multiple MongoDB Pods. It is useful for large data sets when a single machine’s overall processing speed or storage capacity turns out to be not enough. Sharding allows splitting data across several machines with a special routing of each request to the necessary subset of data (so-called shard).

A MongoDB Sharding involves the following components:

  • shard - a replica set which contains a subset of data stored in the database (similar to a traditional MongoDB replica set),
  • mongos - a query router, which acts as an entry point for client applications,
  • config servers - a replica set to store metadata and configuration settings for the sharded database cluster.


Percona Server for MongoDB 1.6.0 supports only one shard of a MongoDB cluster; still, this limited sharding support allows using mongos as an entry point instead of provisioning a load-balancer per replica set node.

Turning sharding on and off

Sharding is controlled by the sharding section of the deploy/cr.yaml configuration file and is turned on by default.

To enable sharding, set the sharding.enabled key to true (this will turn existing MongoDB replica set nodes into sharded ones). To disable sharding, set the sharding.enabled key to false.

When sharding is turned on, the Operator runs replica sets with config servers and mongos instances. Their number is controlled by configsvrReplSet.size and mongos.size keys, respectively.


Config servers for now can properly work only with WiredTiger engine, and sharded MongoDB nodes can use either WiredTiger or InMemory one.

Checking connectivity to sharded and non-sharded cluster

With sharding turned on, you have mongos service as an entry point to access your database. If you do not use sharding, you have to access mongod processes of your replica set.

  1. Run percona-client and connect its console output to your terminal (running it may require some time to deploy the corresponding Pod):

    kubectl run -i --rm --tty percona-client --image=percona/percona-server-mongodb:4.4.2-4 --restart=Never -- bash -il
  2. Find the password for the admin user, which you will need to access the cluster. Use kubectl get secrets to see the list of Secrets objects (by default Secrets object you are interested in has my-cluster-name-secrets name). Then kubectl get secret my-cluster-name-secrets -o yaml will return the YAML file with generated secrets, including the MONGODB_USER_ADMIN and MONGODB_USER_ADMIN_PASSWORD strings:


    Here the actual login name and password are base64-encoded, and echo 'aDAzQ0pCY3NSWEZ2ZUIzS1I=' | base64 --decode will bring it back to a human-readable form.

  3. Now run mongo tool in the percona-client command shell using the login (which is normally userAdmin) and password obtained from the secret.

    • If sharding is turned on, the command will look as follows (with your database cluster namespace instead of the <namespace name> placeholder).

      mongo "mongodb://userAdmin:userAdminPassword@my-cluster-name-mongos.<namespace name>.svc.cluster.local/admin?ssl=false"
    • If sharding is turned off, the command will look as follows (with your database cluster namespace instead of the <namespace name> placeholder).

      mongo "mongodb+srv://userAdmin:userAdminPassword@my-cluster-name-rs0.<namespace name>.svc.cluster.local/admin?replicaSet=rs0&ssl=false"

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