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.
In replication, the local database store stores internal replication data for each member of a replica set. The local stores the following collections:
Changed in version 2.4: When running with authentication (i.e. authorization), authenticating to the local database is not equivalent to authenticating to the admin database. In previous versions, authenticating to the local database provided access to all databases.
Collection on all mongod Instances¶
On startup, each mongod instance inserts a document into startup_log with diagnostic information about the mongod instance itself and host information. startup_log is a capped collection. This information is primarily useful for diagnostic purposes.
Consider the following prototype of a document from the startup_log collection:
Documents in the startup_log collection contain the following fields:
Includes the system hostname and a millisecond epoch value.
The system’s hostname.
A string that reports the startTime in the system’s local time zone.
The process identifier for this process.
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.
local.oplog.rs is the capped collection that holds the oplog. You set its size at creation using the oplogSizeMB setting. To resize the oplog after replica set initiation, use the Change the Size of the Oplog procedure. For additional information, see the Oplog Size section.
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 If this collection becomes out of date, you can refresh it by dropping the collection and allowing MongoDB to automatically refresh it during normal replication: