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Charmed MongoDB Tutorials > Deploy a sharded cluster > 4. Add and remove shards

Add and remove shards

Shards spread data evenly throughout the cluster, making a database highly available. This part of the tutorial will teach you how to scale your cluster by adding and removing shards.

Summary

Disclaimer: This tutorial hosts all shards on the same machine. This should never be done in a production environment.

To enable high availability in a production environment, replicas should be hosted on different servers to maintain isolation.

Add shards

Sharded clusters cannot be scaled via juju the way replica sets can. So, before adding a shard to the cluster, we need to create it.

To create a new shard, deploy a Charmed MongoDB application with the shard config option role:

juju deploy  mongodb --config role="shard" shard2
Use juju status --watch 1s --relations to wait until shard2 is blocked due to missing relation. 
Model     Controller  Cloud/Region         Version  SLA          Timestamp
tutorial  overlord    localhost/localhost  3.1.7    unsupported  15:15:53+01:00

App            Version  Status  Scale  Charm    Channel  Rev  Exposed  Message     
config-server           active      1  mongodb  6/beta   149  no       Primary     
shard0                  active      1  mongodb  6/beta   149  no       Primary     
shard1                  active      1  mongodb  6/beta   149  no       Primary     
shard2                  blocked     1  mongodb  6/beta   149  no       missing relation to config server                                  

Unit              Workload  Agent      Machine  Public address  Ports            Message   
config-server/0*  active    idle       6        10.17.247.150   27017-27018/tcp  Primary   
shard0/0*         active    idle       7        10.17.247.50    27017/tcp        Primary   
shard1/0*         active    idle       8        10.17.247.214   27017/tcp        Primary   
shard2/0*         blocked   executing  9        10.17.247.144                    missing relation to config server                                

Machine  State    Address        Inst id        Base          AZ  Message
6        started  10.17.247.150  juju-3acea1-6  ubuntu@22.04      Running
7        started  10.17.247.50   juju-3acea1-7  ubuntu@22.04      Running
8        started  10.17.247.214  juju-3acea1-8  ubuntu@22.04      Running
9        started  10.17.247.144  juju-3acea1-9  ubuntu@22.04      Running           

Integration provider          Requirer                      Interface      Type     Message
config-server:config-server   shard0:sharding               shards         regular
config-server:config-server   shard1:sharding               shards         regular
config-server:database-peers  config-server:database-peers  mongodb-peers  peer
shard0:database-peers         shard0:database-peers         mongodb-peers  peer
shard1:database-peers         shard1:database-peers         mongodb-peers  peer
shard2:database-peers         shard2:database-peers         mongodb-peers  peer

Now that our shard is deployed, it is ready to be added to the cluster. For the shard to be added to the cluster it must be integrated with the config-server:

juju integrate config-server:config-server shard2:sharding
Use juju status --watch 1s --relations to watch the cluster until the new shard is active. 
Model     Controller  Cloud/Region         Version  SLA          Timestamp
tutorial  overlord    localhost/localhost  3.1.7    unsupported  15:29:18+01:00

App            Version  Status  Scale  Charm    Channel  Rev  Exposed  Message     
config-server           active      1  mongodb  6/beta   149  no       Primary
shard0                  active      1  mongodb  6/beta   149  no       Primary     
shard1                  active      1  mongodb  6/beta   149  no       Primary     
shard2                  active      1  mongodb  6/beta   149  no       Primary                           

Unit              Workload  Agent  Machine  Public address  Ports            Message       
config-server/0*  active    idle   6        10.17.247.150   27017-27018/tcp  Primary                                    
shard0/0*         active    idle   7        10.17.247.50    27017/tcp        Primary       
shard1/0*         active    idle   8        10.17.247.214   27017/tcp        Primary       
shard2/0*         active    idle   9        10.17.247.144   27017/tcp        Primary                                   

Machine  State    Address        Inst id        Base          AZ  Message
6        started  10.17.247.150  juju-3acea1-6  ubuntu@22.04      Running
7        started  10.17.247.50   juju-3acea1-7  ubuntu@22.04      Running
8        started  10.17.247.214  juju-3acea1-8  ubuntu@22.04      Running
9        started  10.17.247.144  juju-3acea1-9  ubuntu@22.04      Running           

Integration provider          Requirer                      Interface      Type     Message
config-server:config-server   shard0:sharding               shards         regular
config-server:config-server   shard1:sharding               shards         regular
config-server:config-server   shard2:sharding               shards         regular
config-server:database-peers  config-server:database-peers  mongodb-peers  peer
shard0:database-peers         shard0:database-peers         mongodb-peers  peer
shard1:database-peers         shard1:database-peers         mongodb-peers  peer            
shard2:database-peers         shard2:database-peers         mongodb-peers  peer

To verify that the shard is present in the cluster, we can check the cluster configuration through the mongos router inside config-server and listing shards like we did in 4. Access a sharded cluster | Connect via MongoDB URI.

To summarize:

  • echo $URI to get your URI
  • juju ssh config-server/0 to enter the config-server
  • charmed-mongodb.mongosh <your-URI> to enter the mongodb shell
  • sh.status()

This will confirm that shard shard2 is in the cluster configuration:

shards
[
  {
    _id: 'shard0',
    host: 'shard0/10.17.247.50:27017',
    state: 1,
    topologyTime: Timestamp({ t: 1708523939, i: 7 })
  },
  {
    _id: 'shard1',
    host: 'shard1/10.17.247.214:27017',
    state: 1,
    topologyTime: Timestamp({ t: 1708523945, i: 5 })
  },
  {
    _id: 'shard2',
    host: 'shard2/10.17.247.144:27017',
    state: 1,
    topologyTime: Timestamp({ t: 1708525386, i: 4 })
  }
]

...

When you’re ready to leave the MongoDB shell, just type exit. You will be back in the host of Charmed MongoDB (mongodb/0).

Exit this host by once again typing exit. Now you will be in your original shell where you can interact with Juju and LXD.

Remove shards

To remove a shard from Charmed MongoDB, we remove the integration from the shard and the config-server. This signals to the cluster that it is time to drain the shard and remove it from the config.

First, break the integration:

juju remove-relation config-server:config-server shard2:sharding
Watch juju status --watch 1s --relations until shard2 has finished being drained from the cluster and is no longer listed as a requirer of the config-server integration provider. 
Model     Controller  Cloud/Region         Version  SLA          Timestamp
tutorial  overlord    localhost/localhost  3.1.7    unsupported  15:52:10+01:00

App            Version  Status  Scale  Charm    Channel  Rev  Exposed  Message     
config-server           active      1  mongodb  6/beta   149  no       Primary                                  
shard0                  active      1  mongodb  6/beta   149  no       Primary     
shard1                  active      1  mongodb  6/beta   149  no       Primary     
shard2                  active      1  mongodb  6/beta   149  no       Primary                                      

Unit              Workload  Agent  Machine  Public address  Ports            Message       
config-server/0*  active    idle   6        10.17.247.150   27017-27018/tcp  Primary                                    
shard0/0*         active    idle   7        10.17.247.50    27017/tcp        Primary       
shard1/0*         active    idle   8        10.17.247.214   27017/tcp        Primary       
shard2/0*         active    idle   9        10.17.247.144   27017/tcp        Primary                                          

Machine  State    Address        Inst id        Base          AZ  Message
6        started  10.17.247.150  juju-3acea1-6  ubuntu@22.04      Running
7        started  10.17.247.50   juju-3acea1-7  ubuntu@22.04      Running
8        started  10.17.247.214  juju-3acea1-8  ubuntu@22.04      Running
9        started  10.17.247.144  juju-3acea1-9  ubuntu@22.04      Running           

Integration provider          Requirer                      Interface      Type     Message
config-server:config-server   shard0:sharding               shards         regular
config-server:config-server   shard1:sharding               shards         regular
config-server:database-peers  config-server:database-peers  mongodb-peers  peer   
shard0:database-peers         shard0:database-peers         mongodb-peers  peer
shard1:database-peers         shard1:database-peers         mongodb-peers  peer
shard2:database-peers         shard2:database-peers         mongodb-peers  peer

Now that the shard has been drained and removed from the configuration, it is safe to remove it altogether. To do so, run:

juju remove-application shard2

Watch the cluster with juju status --watch 1s --relations until shard2 is no longer listed anywhere.

Next Step: 5. Manage passwords

Previous 3. Access a sharded cluster Next 5. Manage passwords

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