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document-database-service/umn/source/introduction/basic_concepts/cluster.rst
proposalbot 264dadb4c1 Changes to dds_umn from docs/doc-exports#731 (Added version 4.2. 
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dds_01_0002.html

Cluster

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

Figure 1 Diagram of node relationships

mongos

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

  • Each DB instance has 2 to 32 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.

config

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.

shard

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

  • Each cluster instance has 2 to 32 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.