# Basic Concepts

## Region

The geographical location of the server where you purchase the cloud data warehouse UClickHouse. You can choose the Region where the UClickHouse cluster is located according to your needs.

## Zone

A physically isolated area with separate power and network within the same region. Intranet communication is possible between zones, and network latency within a zone is smaller.

### UClickHouse Cluster

The cloud data warehouse UClickHouse is a distributed database composed of multiple Clickhouse Server nodes. Depending on the purchased specifications, UClickHouse nodes may contain one or two replicas. A UClickHouse cluster may contain one or more shards.

## Replica

To ensure data safety and high availability of cluster services, UClickHouse provides a replica mechanism that redundantly stores data from a single-node server on two or more servers.

- Double Replica: Each node contains two replicas. When one replica service is unavailable, another replica of the same shard can continue to provide service.
- Single Replica: Each node has only one replica; when this replica is unavailable, the entire cluster becomes unavailable.

<blockquote>
Note:
  <ol>
  <li>The double replica mode has high availability features. Every two replica nodes form a shard, and when one node within the shard is unavailable, the other node can continue to provide service capability (requires using the replica table engine).</li>
  <li>The single replica mode does not commit to SLA and is recommended only for test/development environments. For production environments, the double replica mode is recommended.</li>
  </ol>
</blockquote>

## Shard

In scenarios with ultra-large scale data, the storage and computing resources of a single machine become a bottleneck for data processing and analysis. UClickHouse disperses the massive data across multiple servers (nodes). Each server (node) only stores and processes a portion of the massive data. In single replica mode, a shard corresponds to a node. In double replica mode, a shard corresponds to two nodes. Each server (node) is referred to as a shard.

## Compute Specifications

The resource configuration of the UClickHouse cluster. You can choose any of the two specifications to create, according to your needs.