# Scale a MongoDB Cluster Horizontally

DBaaS allows you to scale up and down the number of database replicas. Increased replica count may result in a cluster with improved availability, performance (capacity to handle more data reads), and fault tolerance for upgrades. A new IP address must be provided for each new instance. To do so, send a `PATCH` request with the new `instances` count (supported values are: 1, 3):

## Endpoint

Use the following endpoint to request the new instance count: `https://api.ionos.com/databases/mongodb/clusters/{clusterId}`

{% hint style="info" %}
**Note:** The sample cluster UUID is `d02de413-d5af-4104-a6f9-3a3c2766ee61`.
{% endhint %}

## Request

```bash
curl --include \
    --request PATCH \
    --user "clientname@ionos.com:Mb2.r5oHf-0t" \
    --header "Content-Type: application/json" \
    --data-binary '{
      "properties": {
        "instances": 3,
        "connections": [
          {
            "datacenterId": "b72ebcf5-6158-4ddb-8f65-31b20d82b6c7",
            "lanId": "2",
            "cidrList": [
              "10.7.222.3/24",
              "10.7.222.4/24",
              "10.7.222.5/24"
            ]
          }
       ]
      }
    }' 
```

## Response

```json
{
  "type": "cluster",
  "id": "d02de413-d5af-4104-a6f9-3a3c2766ee61",
  "metadata": {
    "state": "BUSY",
    "health": "HEALTHY",
    "createdDate": "2020-12-10T12:37:50.000Z",
    "createdBy": "clientname@ionos.com",
    "createdByUserId": "012342f-411f-1eeb-9d07-046c59cc737e"
  },
  "properties": {
    "type": "replicaset",
    "edition": "enterprise",
    "cores": 2,
    "ram": 2048,
    "storageSize": 10240,
    "storageType": "HDD",
    "displayName": "Represents database name",
    "location": "de/fra",
    "connections": [
      {
        "datacenterId": "b72ebcf5-6158-4ddb-8f65-31b20d82b6c7",
        "lanId": "2",
        "cidrList": [
          "10.1.1.3/24",
          "10.1.1.4/24",
          "10.1.1.5/24"
        ]
      }
    ],
    "instances": 3,
    "maintenanceWindow": {
      "time": "16:30:59",
      "dayOfTheWeek": "Monday"
    },
    "connectionString": "mongodb+srv://m-65f4a879f126e3c4.mongodb.de-fra.ionos.com"
  }
}
```

The patch example takes a previous cluster with one replica and adds two more replicas to it. The two new IP addresses are added to the connection properties `cidrList`, and the instances property receives the total number of replicas.

New servers are provisioned and added one at a time to the pool of replicas, implying an incremental rollout.

Downgrading a cluster follows the same pattern, with replica sets being removed one at a time and the desired replica set is reached.

{% hint style="info" %}
**Note:** Only **MongoDB Business** edition provides an option to downscale the number of instances. Scaling down may result in one or more failovers and the disruption of open connections.
{% endhint %}

For more information, see [<mark style="color:blue;">Replica Set Elections</mark>](https://www.mongodb.com/docs/manual/core/replica-set-elections/).