[Yandex Cloud documentation](../../index.md) > [Yandex Managed Service for PostgreSQL](../index.md) > Concepts > Resource relationships

# Resource relationships in Managed Service for PostgreSQL


The main entity Managed Service for PostgreSQL operates is a _database cluster_.

Each cluster consists of one or multiple _database hosts_, which are virtual machines with deployed DBMS servers. Cluster hosts may reside in different availability zones. You can learn more about Yandex Cloud geography in the [Platform overview](../../overview/concepts/geo-scope.md) section.

A cluster of two or more hosts is natively [highly available](high-availability.md) because one of its replica hosts will assume the master role upon the current master host's failure. To learn more about how the number of hosts affects cluster availability, see [this section](high-availability.md#host-configuration).

{% note info %}

A replica that has an explicitly specified replication thread source cannot take the master role. To learn more, see [Replication](replication.md).

{% endnote %}

The minimum number of hosts per cluster depends on the selected [disk type](storage.md). The default cluster configuration offered in the management console includes two hosts.

{% note warning %}

We do not recommend creating a cluster having a single host. While being cheaper, such a cluster is anything but highly available.

{% endnote %}

A cluster's computing capacity depends on its _host class_, which is the virtual machine template used to deploy the cluster's hosts. For a list of available host classes and their specs, see [Host classes](instance-types.md).

To manage connections and balance the cluster load, the service architecture features the _Odyssey connection pooler_. To learn more, see [Connection management](pooling.md).


You can access a cluster created in a folder from any VM in the same cloud network. For more information on how the network works, see [this Virtual Private Cloud guide](../../vpc/index.md).


{% note alert %}

The service can automatically access the system and diagnostic information of your databases to render monitoring charts and maintain your cluster health. It never accesses or uses the data you entered into the database manually.

{% endnote %}

Be mindful of what is managed by the service and what, by the Yandex Cloud customer. Understanding these zones of control makes you an efficient user of cloud resources and helps avoid potential database-related issues. For more information, see [Zones of control between managed database (MDB) service users and Yandex Cloud](../../overview/concepts/mdb-responsibilities.md).