# Scale a TiDB cluster

## Overview

The capacity of a TiDB cluster can be increased or reduced without affecting online services.

Note:

If your TiDB cluster is deployed using Ansible, see Scale the TiDB Cluster Using TiDB-Ansible.

The following part shows you how to add or delete PD, TiKV or TiDB nodes.

About pd-ctl usage, refer to PD Control User Guide.

## PD

Assume we have three PD servers with the following details:

Name ClientUrls PeerUrls
pd1 http://host1:2379 http://host1:2380
pd2 http://host2:2379 http://host2:2380
pd3 http://host3:2379 http://host3:2380

Get the information about the existing PD nodes through pd-ctl:

./pd-ctl -u http://host1:2379
>> member


Add a new PD server to the current PD cluster by using the parameter join. To add pd4, you just need to specify the client url of any PD server in the PD cluster in the parameter --join, like:

./bin/pd-server --name=pd4 \
--client-urls="http://host4:2379" \
--peer-urls="http://host4:2380" \
--join="http://host1:2379"


### Delete a node dynamically

Delete pd4 through pd-ctl:

./pd-ctl -u http://host1:2379
>> member delete pd4


### Migrate a node dynamically

If you want to migrate a node to a new machine, you need to, first of all, add a node on the new machine and then delete the node on the old machine. As you can just migrate one node at a time, if you want to migrate multiple nodes, you need to repeat the above steps until you have migrated all nodes. After completing each step, you can verify the process by checking the information of all nodes.

## TiKV

Get the information about the existing TiKV nodes through pd-ctl:

./pd-ctl -u http://host1:2379
>> store


It is very easy to add a new TiKV server dynamically. You just need to start a TiKV server on the new machine. The newly started TiKV server will automatically register in the existing PD of the cluster. To reduce the pressure of the existing TiKV servers, PD loads balance automatically, which means PD gradually migrates some data to the new TiKV server.

### Delete a node dynamically

To delete (make it offline) a TiKV server safely, you need to inform PD in advance. After that, PD is able to migrate the data on this TiKV server to other TiKV servers, ensuring that data have enough replicas.

Assume that you need to delete the TiKV server with a store id 1, you can complete this through pd-ctl:

./pd-ctl -u http://host1:2379
>> store delete 1


Then you can check the state of this TiKV:

./pd-ctl -u http://host1:2379
>> store 1
{
"store": {
"id": 1,
"state": 1,
"state_name": "Offline"
},
"status": {
...
}
}


You can verify the state of this store using state_name:

• state_name=Up: This store is in service.
• state_name=Disconnected: The heartbeats of this store cannot be detected currently, which might be caused by a failure or network interruption.
• state_name=Down: PD does not receive heartbeats from the TiKV store for more than an hour (the time can be configured using max-down-time). At this time, PD adds a replica for the data on this store.
• state_name=Offline: This store is shutting down, but the store is still in service.
• state_name=Tombstone: This store is shut down and has no data on it, so the instance can be deleted.

### Migrate a node dynamically

To migrate TiKV servers to a new machine, you also need to add nodes on the new machine and then make all nodes on the old machine offline. In the process of migration, you can add all machines in the new cluster to the existing cluster, then make old nodes offline one by one. To verify whether a node has been made offline, you can check the state information of the node in process. After verifying, you can make the next node offline.

## TiDB

TiDB is a stateless server, which means it can be added or deleted directly. It should be noted that if you deploy a proxy (such as HAProxy) in front of TiDB, you need to update the proxy configuration and reload it.