System Collections

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Synopsis

Collections
Database-Specific Collections

Synopsis

MongoDB stores system information in collections that use the <database>.system.* namespace, which MongoDB reserves for internal use. Do not create collections that begin with system.

MongoDB also stores some additional instance-local metadata in the local database local.replset.* namespace, specifically for replication purposes and in the config database for sessions information.

Collections

System collections include these collections stored in the admin database:

admin.system.roles: The admin.system.roles collection stores custom roles that administrators create and assign to users to provide access to specific resources.

admin.system.users: The admin.system.users collection stores the user's authentication credentials as well as any roles assigned to the user. Users may define authorization roles in the admin.system.roles collection.

admin.system.version: The admin.system.version collection stores metadata to support internal operations. Do not modify this collection unless specifically instructed to in this documentation or by a MongoDB support engineer.

System collections include these collections stored in the config database:

config.system.indexBuilds: The indexBuilds collection stores information related to in-progress index builds.

config.system.preimages: Stores previous versions of modified documents from collections with the changeStreamPreAndPostImages option enabled. The config.system.preimages collection is automatically purged when the pre-images expire. For details, see changeStreamOptions.preAndPostImages.expireAfterSeconds.

Database-Specific Collections

System collections also include these collections stored directly in each database:

<database>.system.buckets

Stores the underlying data associated to a time series collection. The system.buckets collection stores data in an optimized format and schema for an efficient representation of the persisted time series data.

Do not modify this collection.

Important

When you upgrade to MongoDB 8.0, if you have any system.buckets collections that are not time-series collections, you might need to drop or rename those collections before you upgrade, depending on your 8.0 patch release:

MongoDB 8.0.5 and later: You do not need to drop system.buckets collections that aren't time-series collections before you upgrade. However, you must drop or rename them after you complete your upgrade.
MongoDB 8.0.4 and earlier: You must drop or rename system.buckets collections that aren't time-series collections before you upgrade. All system.buckets collections must have valid time series options configured before you upgrade to versions 8.0.0 - 8.0.4.

To determine whether you have system.buckets collections that are not time-series collections, use the db.getCollectionInfos() method with a filter:

db.getCollectionInfos(
   {
      $and: [
         { name: { $regex: /^system\.buckets/ } },
         { 'options.timeseries': { $exists: false } }
      ]
   }
)

<database>.system.profile: The <database>.system.profile collection stores database profiling information. For information on profiling, see Database Profiler.

<database>.system.js: The <database>.system.js collection stores special JavaScript code for use in server side JavaScript. See Store a JavaScript Function on the Server for more information.

<database>.system.views

The <database>.system.views collection contains information about each view in the database.

Starting in MongoDB 5.0, for featureCompatibilityVersion set to "5.0" or greater, users can no longer write directly to the <database>.system.views collection.

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