• Document Database Service

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  2. Document Database Service
  3. User Guide
  4. Introduction
  5. Basic Concepts
  6. Cluster


DDS cluster instances consist of mongos, config, and shard nodes. The following diagram shows the node relationships.

Figure 1 Diagram of node relationships


A mongos is a router for reading and writing data, providing a unified interface for accessing DB instances.

  • Each DB instance has 2 to 12 mongos. You can specify the quantity.
  • A mongos reads configuration settings from configs and allocates read and write requests to shards. You can connect to a mongos directly.


A config stores configuration settings for DB instances and consists of one replica set.

  • The availability of a config is the prerequisite to deploying a DB instance or modifying the instance information.
  • You cannot connect to a config directly.


In the cluster instance, shards are used to store user data.

  • Each DB instance has 2 to 12 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.