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

# Resource relationships in Managed Service for OpenSearch


[OpenSearch](https://opensearch.org/) is a distributed system for data search and analysis. It uses the document data model, in which a document is a logical counterpart of a row in relational databases. Each document stores key-value pairs, e.g., in [JSON](https://www.json.org/json-en.html) format.

Document example:

>{
>  "name": "John Doe",
>  "gpa": 3.89,
>  "grad_year": 2022
>}

Each document added to OpenSearch goes into an [_index_](indexing.md). After indexing, document data will be available for [search and analysis](searching.md).

OpenSearch features [_OpenSearch Dashboards_](https://opensearch.org/docs/latest/dashboards/), a data visualization UI.

The main entity Managed Service for OpenSearch operates is an cluster.

Each cluster consists of one or more [_hosts_](host-roles.md), which are virtual machines with deployed OpenSearch. Cluster hosts may reside in different [availability zones](../../overview/concepts/geo-scope.md). The minimum number of hosts in a cluster depends on the selected cluster configuration and [high availability](high-availability.md) requirements.

When [creating a cluster](../operations/cluster-create.md), specify:

- [_Host classes_](instance-types.md): VM templates for deploying the cluster hosts.

- _Host groups_: Groups of multiple linked hosts with specific configuration and [roles](host-roles.md):

    - _DATA_: Enable index storage and request processing.
    - _MANAGER_: Ensure fault tolerance of all OpenSearch components.
    - _DASHBOARDS_: Provide access to OpenSearch Dashboards.

- _Environment_ where the cluster will be deployed:

    - `PRODUCTION`: For stable versions of your applications.
    - `PRESTABLE`: For testing purposes. The prestable environment is similar to the production environment and likewise covered by an SLA, but it is the first to get new features, improvements, and bug fixes. In the prestable environment, you can test new versions for compatibility with your application.


An OpenSearch cluster created in a folder can be accessed by all VMs connected to the same [cloud network](../../vpc/concepts/network.md) as the cluster.


{% 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).

## High availability of a cluster {#high-availability}

For a cluster to be highly available and covered by a [Service Level Agreement (SLA)](https://yandex.com/legal/cloud_sla_mdb/), the following conditions must be met:


1. The cluster must have two or more hosts with the `DATA` role.
1. The cluster must have three or more hosts with the `MANAGER` role.


[More on high-availability of clusters](high-availability.md).