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The local
Database![](/docs/v5.2/assets/link.svg)
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Overview![](/docs/v5.2/assets/link.svg)
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.
Collections on all mongod
Instances![](/docs/v5.2/assets/link.svg)
local.startup_log
On startup, each
mongod
instance inserts a document intostartup_log
with diagnostic information about themongod
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" : "<string>", "hostname" : "<string>", "startTime" : ISODate("<date>"), "startTimeLocal" : "<string>", "cmdLine" : { "dbpath" : "<path>", "<option>" : <value> }, "pid" : <number>, "buildinfo" : { "version" : "<string>", "gitVersion" : "<string>", "sysInfo" : "<string>", "loaderFlags" : "<string>", "compilerFlags" : "<string>", "allocator" : "<string>", "versionArray" : [ <num>, <num>, <...> ], "javascriptEngine" : "<string>", "bits" : <number>, "debug" : <boolean>, "maxBsonObjectSize" : <number> } } Documents in the
startup_log
collection contain the following fields: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.buildinfo
An embedded document that reports information about the build environment and settings used to compile this
mongod
. This is the same output asbuildInfo
. SeebuildInfo
.
Collections on Replica Set Members![](/docs/v5.2/assets/link.svg)
local.system.replset
local.system.replset
holds the replica set's configuration object as its single document. To view the object's configuration information, issuers.conf()
frommongosh
. 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 theoplogSizeMB
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.
Restrictions![](/docs/v5.2/assets/link.svg)
- Multi-Document Transactions on
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.