Help Center/ Document Database Service/ Service Overview/ Basic Concepts
Updated on 2025-11-03 GMT+08:00
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Basic Concepts

Instances

  • An instance is a basic management unit of DDS. One or more databases can be created in a single DDS instance, and one or more collections can be created in each database.
  • 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, and multiple dds mongos and shard nodes. The following diagram shows the node relationships.

Figure 1 Diagram of node relationships
  • dds mongos
    A dds mongos is a router for reading and writing data, providing a unified interface for accessing DB instances.
    • 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 configuration settings for DB instances and consists of one 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 shard nodes. You can specify the quantity.
    • Each shard node is deployed as a replica set to ensure data redundancy and high reliability.
    • You cannot connect to a shard node 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.

  • Regions are defined by their 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. There are universal regions and dedicated regions. A universal region serves all tenants, while a dedicated region serves specific tenants.
  • An AZ contains one or more physical data centers. Each AZ has independent cooling, fire extinguishing, moisture control, and electrical facilities. Within an AZ, computing, network, storage, and other resources are logically divided into multiple clusters. AZs within a region are interconnected by optical fibers for high-availability networking.

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:

  • Location

    For lower network latency and quick resource access, select the region nearest to your target users. Regions within the Chinese mainland have the same infrastructure, BGP network quality, as well as resource operations and configurations. If your target users are on the Chinese mainland, you can select any region in the Chinese mainland.

  • Resource price

    Prices may vary by region. 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.