> For the complete documentation index, see [llms.txt](https://docs.ionos.com/cloud/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.ionos.com/cloud/~/revisions/AMiYziCllTKB2y2lmONY/databases/mongodb/overview/high-availability.md).

# 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 IONOS 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 operations 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

You can change the write concern for single operations, for example due to performance reasons. For more information, see [<mark style="color:blue;">Write Acknowledgement</mark>](https://www.mongodb.com/docs/v5.0/reference/write-concern/#mongodb-writeconcern-writeconcern.-majority-).

### Read preferences

You can determine which instance to use for read operations by setting the read preference on the client side, if your client supports it. For more information, see [<mark style="color:blue;">Read Preference</mark>](https://www.mongodb.com/docs/v5.0/core/read-preference/).

If you read from the primary instance, you always get the most up-to-date data. You can spread the load by reading from secondary sources, but you might get stale data. However, you can get consistency guarantees using a [<mark style="color:blue;">Read Concern</mark>](https://www.mongodb.com/docs/v5.0/reference/read-concern-majority/#mongodb-readconcern-readconcern.-majority-).


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