The <dbname>.system.* namespaces in MongoDB are special and contain database system information. System collections include:
<dbname>.system.namespaces lists all namespaces.
<dbname>.system.indexes lists all indexes. Additional namespace / index metadata exists in the database.ns files, and is opaque.
<dbname>.system.profile stores database profiling information.
<dbname>.system.users lists users who may access the database.
Additionally, in the local database only there is replication information in system collections, e.g., local.system.replset contains the replica set configuration.
Information on the structure of a stored document is stored within the document itself. See BSON .
There are several restrictions on manipulation of objects in the system collections. Inserting in system.indexes adds an index, but otherwise that table is immutable (the special drop index command updates it for you). system.users is modifiable. system.profile is droppable.
Note: $ is a reserved character. Do not use it in namespace names or within field names. Internal collections for indexes use the $character in their names. These collection store b-tree bucket data and are not in BSON format (thus direct querying is not possible).
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