What are admin, config, and local ? in Mongo db Compass !

 In MongoDB, admin, config, and local are special system databases that serve specific purposes within the database ecosystem. Here's an explanation of each:

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1. admin Database

• Purpose: The admin database is used for administrative tasks and configuration. It serves as the central database for managing the MongoDB instance.
• Key Features:
  • It contains system collections for user authentication and role-based access control.
  • Admin users created here can have privileges across all databases in the MongoDB instance.
  • It's used to issue administrative commands (e.g., shutting down the server, modifying cluster configuration).
  • Some commands like db.shutdownServer(), db.serverStatus(), and replication commands require access to the admin database.
• Example: If a user is created in the admin database with root privileges, they can perform operations on all databases.

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2. config Database

• Purpose: The config database is used in a sharded cluster to store metadata and configuration details for the cluster.
• Key Features:
  • It contains information about shards, databases, and collections in a sharded setup.
  • Tracks chunk distribution and migrations between shards.
  • Helps the MongoDB query router (mongos) to determine where specific data resides.
  • It is crucial for sharded cluster operation.
• Example: When you use sharding, the config database contains collections such as config.chunks (to track data chunks) and config.databases (to track databases in the cluster).

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3. local Database

• Purpose: The local database stores data specific to a single server instance. This data is not replicated and is specific to the node.
• Key Features:
  • Used in replication to store the oplog (operation log), which is essential for keeping replicas in sync.
  • Contains metadata and temporary data for the local node.
  • Data stored here does not get replicated to other nodes in a replica set.
• Example:
  • In a replica set, the local.oplog.rs collection contains the operation log that records changes to the database.
  • It also contains the replSet configuration used for replication.

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Summary Table:

Database	Purpose	Key Use Cases
admin	Central administrative database	Managing users, roles, and issuing commands
config	Sharded cluster metadata	Metadata for sharding (chunks, shard info)
local	Node-specific data	Oplog for replication and local configuration

These databases are reserved for system use and are critical for MongoDB's functionality. You typically don't store application data in these databases.