This topic describes the terms used in ApsaraDB for ClickHouse. This helps you understand ApsaraDB for ClickHouse.
region
A region is the geographic location where the server that runs your ApsaraDB for ClickHouse cluster resides. You must specify a region when you create an ApsaraDB for ClickHouse cluster. The region cannot be changed after the cluster is created.
zone
Zones are physical areas within a region. Each zone has an independent power grid and network. Zones are interconnected over an internal network. If clusters reside in the same zone, clusters can communicate with each other at a low network latency.
ApsaraDB for ClickHouse cluster
An ApsaraDB for ClickHouse cluster is a distributed database that runs on multiple physical ClickHouse servers. A ClickHouse server can have one or more replicas and one or more shards based on different specifications. An ApsaraDB for ClickHouse cluster can contain multiple logical database objects.
edition
- Double-replica Edition: For ApsaraDB for ClickHouse of the Double-replica Edition, each shard has a replica. If a shard fails, its replica can take over services from the failed shard.
- Single-replica Edition: For ApsaraDB for ClickHouse of the Single-replica Edition, each shard has no replica. If a shard in a cluster fails, the entire cluster becomes unavailable. The cluster cannot provide stable services until the shard is recovered.
- The number of resources that an ApsaraDB for ClickHouse cluster of the Double-replica Edition requires is twice the number of resources that an ApsaraDB for ClickHouse cluster of the Single-replica Edition requires. An ApsaraDB for ClickHouse cluster of the Double-replica Edition costs twice as much as an ApsaraDB for ClickHouse cluster of the Single-replica Edition.
- An ApsaraDB for ClickHouse cluster of the Single-replica Edition uses highly reliable disks to avoid data losses.
shard
In scenarios in which a large amount of data needs to be processed, the storage and computing resources of a single server may cause performance bottlenecks. To make data processing efficient, ApsaraDB for ClickHouse distributes a large amount of data to multiple servers. Each server stores and processes only part of the data. In this architecture, each server is called a shard.
replica
To ensure data security and high availability if an error occurs, ApsaraDB for ClickHouse provides replicas. Data on a server can be replicated to one or more servers.
database
A database is the object of the highest level in an ApsaraDB for ClickHouse cluster. A database can contain objects such as tables, columns, views, functions, and data types.
table
A table consists of rows and columns. Data is organized in a table.
| Type | Description | Difference |
|---|---|---|
| local table | When you write data to a local table, the data is written only to the server that stores the local table and cannot be distributed to other servers. |
|
| distributed table | A distributed table consists of a number of local tables. A distributed table abstracts local tables into a unified table and supports data write and query operations for external users. When data is written to a distributed table, the data is automatically distributed to each local table of the distributed table. When data in a distributed table is queried, each local table of the distributed table is queried. The query results of all the local tables are aggregated and returned. |
| Type | Description | Difference |
|---|---|---|
| non-replicated table | Each non-replicated table is stored on a single node and cannot be replicated to other nodes. Only one replica exists. |
|
| replicated table | Data in a replicated table is automatically replicated to multiple nodes. This indicates that a replicated table has multiple replicas. |