# High Availability

## Cluster options

To guarantee partition tolerance, only odd numbers of cluster members are allowed. For the playground edition, you can only have 1 replica. All other editions allow the use of 3 replicas. Soon, we will allow more than 3 replicas per cluster.

The secondary instances are automatically highly available and replicate your data between instances. One instance is the primary, which accepts write transactions, and the others are secondary, which can optionally be used for read operations.

## Replication

The instances in an <code class="expression">space.vars.ionos\_cloud</code> MongoDB cluster are members of the same replica set, so all secondary instances replicate the data from the primary instance.

By default, the write concern before acknowledging a write operation is set to `"majority"` with `j: true`. The term `"majority"` means the write operation must be propagated to the majority of instances. In a three-instance cluster, for example, at least two instances must have the operation before the primary acknowledges it. The `j` option also requires that the change has already been persisted to the on-disk journal of these instances.

If data is not replicated to the majority, it may be lost in the event of a primary instance failure and subsequent rollback.

## Changing the commit guarantees per transaction

For example, you can change the write concern for single operations due to performance reasons. For more information, see [<mark style="color:blue;">Write Acknowledgement</mark>](https://www.mongodb.com/docs/v7.0/reference/write-concern/#mongodb-writeconcern-writeconcern.-majority-).

## Read preferences

If your client supports it, you can determine which instance to use for read operations by setting the read preference on the client side. For more information, see [<mark style="color:blue;">Read Preference</mark>](https://www.mongodb.com/docs/v7.0/core/read-preference/).

Reading from the primary server ensures you have the most current data. Although reading from secondary servers can distribute the load, they may provide outdated information. However, you can get consistency guarantees using a [<mark style="color:blue;">Read Concern</mark>](https://www.mongodb.com/docs/v7.0/reference/read-concern-majority/#mongodb-readconcern-readconcern.-majority-).


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