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The local Database


Every mongod instance has its own local database, which stores data used in the replication process, and other instance-specific data. The local database is invisible to replication: collections in the local database are not replicated.

When running with authentication (i.e. auth), authenticating against the local database is equivalent to authenticating against the admin database. This authentication gives access to all databases.

In replication, the local database store stores internal replication data for each member of a replica set. The local database contains the following collections used for replication:

Collections on Replica Set Members


local.system.replset holds the replica set’s configuration object as its single document. To view the object’s configuration information, issue rs.conf() from the mongo shell. You can also query this collection directly. is the capped collection that holds the oplog. You set its size at creation using the oplogSize setting. To resize the oplog after replica set initiation, use the Change the Size of the Oplog procedure. For additional information, see the Oplog Internals topic in this document and the Oplog topic in the Replica Set Fundamental Concepts document.


This contains an object used internally by replica sets to track replication status.


This contains information about each member of the set and the latest point in time that this member has synced to. If this collection becomes out of date, you can refresh it by dropping the collection and allowing MongoDB to automatically refresh it during normal replication:


Collections used in Master/Slave Replication

In master/slave replication, the local database contains the following collections:

  • On the master:


    This is the oplog for the master-slave configuration.


    This contains information about each slave.

  • On each slave:


    This contains information about the slave’s master server.