Help Center/ Document Database Service/ Service Overview/ Basic Concepts
Updated on 2026-05-09 GMT+08:00
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Basic Concepts

Instances

  • The smallest management unit of DDS is an instance. An instance contains at least one database, and each database has one or more collections.
  • You can create and manage DB instances of various types and versions on the management console. For details about DB instance types, versions, and specifications, see Instances.
  • Resources, such as the CPU, memory, and I/O, of each DDS DB instance are isolated from each other.

Clusters

Each DDS cluster consists of a config node, multiple dds mongos nodes, and multiple shards. The following diagram shows the node relationships.

Figure 1 Diagram of node relationships
Table 1 Component description

Component

Description

dds mongos
A dds mongos acts as a router for handling queries and write operations, serving as the single entry point for instance access.
  • Each DB instance has 2 to 16 dds mongos nodes. You can specify the quantity.
  • A dds mongos reads configuration settings from the config node and allocates read and write requests to shard nodes. You can connect to a dds mongos directly.

config
  • A config stores instance configuration settings and is deployed as a replica set.
  • You cannot connect to a config node directly.

shard
Shards are used to store user data.
  • Each Community Edition instance has 2 to 16 shards. You can specify the quantity.
  • Each shard is deployed as a replica set to ensure data redundancy and high reliability.
  • You cannot connect to a shard directly.

Database Parameter Templates

A database parameter template is a collection of configuration parameters and values and can be applied to multiple DB instances.

Regions and AZs

  • Concept
    A region and availability zone (AZ) identify the location of a data center. You can create resources in a specific region and AZ.
    Table 2 Explanations

    Concept

    Explanation

    Region

    Regions are divided based on geographical location and network latency. Public services, such as Elastic Cloud Server (ECS), Elastic Volume Service (EVS), Object Storage Service (OBS), Virtual Private Cloud (VPC), Elastic IP (EIP), and Image Management Service (IMS), are shared within the same region. Regions are classified into universal regions and dedicated regions. A universal region provides universal cloud services for common tenants. A dedicated region provides specific services for specific tenants.

    AZ

    An AZ contains one or more physical data centers. Each AZ has independent cooling, fire extinguishing, moisture-proof, and electricity facilities. Within an AZ, compute, network, storage, and other resources are logically divided into multiple clusters. AZs within a region are interconnected using high-speed optical fibers to allow you to build cross-AZ high-availability systems.
    Figure 2 shows the relationship between regions and AZs.
    Figure 2 Regions and AZs

    Huawei Cloud provides services in many regions around the world. You can select a region and an AZ as needed. For more information, see Huawei Cloud Global Regions.

  • Selecting a Region

    When selecting a region, consider the following factors:

    Table 3 Selecting a region

    Factor

    Reason

    Location

    It is recommended that you select the closest region for lower network latency and quick access. Regions within the Chinese mainland provide the same infrastructure, BGP network quality, as well as resource operations and configurations. Therefore, if your target users are on the Chinese mainland, you do not need to consider the network latency differences when selecting a region.

    Resource price

    Prices may vary in different regions. For details, see Product Pricing Details.

DB Connections

The number of connections is the number of databases that can be simultaneously connected to an application. The number of connections is irrelevant to the maximum number of users allowed by your applications or websites.

  • For a cluster instance, the number of connections is the number of connections between the client and the dds mongos nodes.
  • For a replica set instance, the number of connections is the number of connections between the client and the primary and secondary nodes.
  • For a single-node instance, the number of connections is the number of connections between the client and the node.

Memory

The maximum available memory that can be used by a DDS DB instance.

Storage Space

The storage space in GB specified when you purchase a DDS DB instance.

  • Both your collection data and the data required for the operation of your DB instance such as system database data and indexes occupy the storage space on your purchased DB instance.
  • Ensure that your DDS DB instance has sufficient storage space, or your DB instance may be read-only. If your DB instance is read-only due to insufficient storage space, you can scale up the storage space.