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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.
On startup, each
mongodinstance inserts a document into
startup_logwith diagnostic information about the
mongodinstance itself and host information.
startup_logis a capped collection. This information is primarily useful for diagnostic purposes.
For example, the following is a prototype of a document from the
Documents in the
startup_logcollection contain the following fields:
A UTC ISODate value that reflects when the server started.
A string that reports the
startTimein the system's local time zone.
An embedded document that reports the
mongodruntime options and their values.
local.system.replsetholds the replica set's configuration object as its single document. To view the object's configuration information, issue
mongosh. You can also query this collection directly.
local.oplog.rsis the capped collection that holds the oplog. You set its size at creation using the
oplogSizeMBsetting. 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.
Starting in MongoDB 4.0, the oplog can grow past its configured size limit to avoid deleting the
majority commit point.
Starting in MongoDB 5.0, it is no longer possible to perform manual write operations to the oplog on a cluster running as a replica set. Performing write operations to the oplog when running as a standalone instance should only be done with guidance from MongoDB Support.
- Multi-Document Transactions on
- You cannot perform read/write operations to the collections in the
localdatabase inside a multi-document transaction.
- Retryable Writes against
You cannot perform write operations to collections in the
localdatabase with retryable writes enabled.
The official MongoDB 4.2-series drivers enable retryable writes by default. Applications which write to the
localdatabase will encounter write errors upon upgrading to 4.2-series drivers unless retryable writes are explicitly disabled.