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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.
local.startup_log
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.
For example, the following is a prototype of a document from the startup_log
collection:
{ "_id" : "", "hostname" : "", "startTime" : ISODate[""], "startTimeLocal" : "", "cmdLine" : { "dbpath" : "", "" : }, "pid" : , "buildinfo" : { "version" : "", "gitVersion" : "", "sysInfo" : "", "loaderFlags" : "", "compilerFlags" : "", "allocator" : "", "versionArray" : [ , , ], "javascriptEngine" : "", "bits" : , "debug" : , "maxBsonObjectSize" : } }
Documents
in the startup_log
collection contain the following fields:
local.startup_log._id
Includes the system hostname and a millisecond epoch value.
local.startup_log.hostname
The system's hostname.
local.startup_log.startTime
A UTC ISODate value that reflects when the server started.
local.startup_log.startTimeLocal
A string that reports the startTime
in the system's local time zone.
local.startup_log.cmdLine
An embedded document that reports the mongod
runtime options and their values.
local.startup_log.pid
The process identifier for this process.
local.startup_log.buildinfo
An embedded document that reports information about the build environment and settings used to compile this mongod
. This is the same
output as buildInfo
. See buildInfo
.
local.system.replset
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 mongosh
. You can also query this collection directly.
local.oplog.rs
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.
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.
local.replset.minvalid
This contains an object used internally by replica sets to track replication status.
local
You cannot perform read/write operations to the collections
in the local
database inside a multi-document transaction.Retryable Writes against local
You cannot perform write operations to collections in the local
database with retryable writes enabled.
Important
The official
MongoDB 4.2-series drivers enable retryable writes by default. Applications which write to the local
database will encounter write errors upon upgrading to 4.2-series drivers unless retryable writes are explicitly disabled.
To disable retryable writes, specify retryWrites=false
in the
connection string for the MongoDB cluster.