MongoDB Limits and Thresholds

MongoDB Atlas Limitations

The following limitations apply only to deployments hosted in MongoDB Atlas. If any of these limits present a problem for your organization, contact Atlas support.

BSON Documents

BSON Document Size

The maximum BSON document size is 16 mebibytes.

The maximum document size helps ensure that a single document cannot use an excessive amount of RAM or, during transmission, an excessive amount of bandwidth. To store documents larger than the maximum size, MongoDB provides the GridFS API. For more information about GridFS, see mongofiles and the documentation for your driver

Nested Depth for BSON Documents

MongoDB supports no more than 100 levels of nesting for BSON documents. Each object or array adds a level.

Naming Restrictions

Use of Case in Database Names

Do not rely on case to distinguish between databases. For example, you cannot use two databases with names like, salesData and SalesData.

After you create a database in MongoDB, you must use consistent capitalization when you refer to it. For example, if you create the salesData database, do not refer to it using alternate capitalization such as salesdata or SalesData.

Restrictions on Database Names for Windows

For MongoDB deployments running on Windows, database names cannot contain any of the following characters:

/\. "$*<>:|?

Also database names cannot contain the null character.

Restrictions on Database Names for Unix and Linux Systems

For MongoDB deployments running on Unix and Linux systems, database names cannot contain any of the following characters:

/\. "$

Also database names cannot contain the null character.

Length of Database Names

Database names cannot be empty and must be less than 64 bytes.

Restriction on Collection Names

Collection names should begin with an underscore or a letter character, and cannot:

• contain the $.

• be an empty string (e.g. "").

• contain the null character.

• begin with the system. prefix. (Reserved for internal use.)

• contain .system..

If your collection name includes special characters, such as the underscore character, or begins with numbers, then to access the collection use the db.getCollection() method in mongosh or a similar method for your driver.

Namespace Length:

The namespace length limit for unsharded collections and views is 255 bytes, and 235 bytes for sharded collections. For a collection or a view, the namespace includes the database name, the dot (.) separator, and the collection/view name (e.g. <database>.<collection>).

Restrictions on Field Names

• Field names cannot contain the null character.

• The server permits storage of field names that contain dots (.) and dollar signs ($).

• MongodB 5.0 adds improved support for the use of ($) and (.) in field names. There are some restrictions. See Field Name Considerations for more details.

• Each field name must be unique within the document. You must not store documents with duplicate fields because MongoDB CRUD operations might behave unexpectedly if a document has duplicate fields.

Restrictions on _id

The field name _id is reserved for use as a primary key; its value must be unique in the collection, is immutable, and may be of any type other than an array or regex. If the _id contains subfields, the subfield names cannot begin with a ($) symbol.

Naming Warnings

Namespaces

Namespace Length

The namespace length limit for unsharded collections and views is 255 bytes, and 235 bytes for sharded collections. For a collection or a view, the namespace includes the database name, the dot (.) separator, and the collection/view name (e.g. <database>.<collection>).

See also:

Naming Restrictions

Indexes

Number of Indexes per Collection

A single collection can have no more than 64 indexes.

Number of Indexed Fields in a Compound Index

There can be no more than 32 fields in a compound index.

Queries cannot use both text and Geospatial Indexes

You cannot combine the $text query, which requires a special text index, with a query operator that requires a different type of special index. For example you cannot combine $text query with the $near operator.

Fields with 2dsphere Indexes can only hold Geometries

Fields with 2dsphere indexes must hold geometry data in the form of coordinate pairs or GeoJSON data. If you attempt to insert a document with non-geometry data in a 2dsphere indexed field, or build a 2dsphere index on a collection where the indexed field has non-geometry data, the operation will fail.

See also:

The unique indexes limit in Sharding Operational Restrictions.

Limited Number of 2dsphere index keys

To generate keys for a 2dsphere index, mongod maps GeoJSON shapes to an internal representation. The resulting internal representation may be a large array of values.

When mongod generates index keys on a field that holds an array, mongod generates an index key for each array element. For compound indexes, mongod calculates the cartesian product of the sets of keys that are generated for each field. If both sets are large, then calculating the cartesian product could cause the operation to exceed memory limits.

indexMaxNumGeneratedKeysPerDocument limits the maximum number of keys generated for a single document to prevent out of memory errors. The default is 100000 index keys per document. It is possible to raise the limit, but if an operation requires more keys than the indexMaxNumGeneratedKeysPerDocument parameter specifies, the operation will fail.

NaN values returned from Covered Queries by the WiredTiger Storage Engine are always of type double

If the value of a field returned from a query that is covered by an index is NaN, the type of that NaN value is always double.

Multikey Index

Multikey indexes cannot cover queries over array fields.

Geospatial Index

Geospatial indexes can't cover a query.

Memory Usage in Index Builds

createIndexes supports building one or more indexes on a collection. createIndexes uses a combination of memory and temporary files on disk to complete index builds. The default limit on memory usage for createIndexes is 200 megabytes, shared between all indexes built using a single createIndexes command. Once the memory limit is reached, createIndexes uses temporary disk files in a subdirectory named _tmp within the --dbpath directory to complete the build.

You can override the memory limit by setting the maxIndexBuildMemoryUsageMegabytes server parameter. Setting a higher memory limit may result in faster completion of index builds. However, setting this limit too high relative to the unused RAM on your system can result in memory exhaustion and server shutdown.

• For feature compatibility version (fcv) "4.2" and later, the index build memory limit applies to all index builds.

Index builds may be initiated either by a user command such as createIndexes or by an administrative process such as an initial sync. Both are subject to the limit set by maxIndexBuildMemoryUsageMegabytes.

An initial sync populates only one collection at a time and has no risk of exceeding the memory limit. However, it is possible for a user to start index builds on multiple collections in multiple databases simultaneously and potentially consume an amount of memory greater than the limit set by maxIndexBuildMemoryUsageMegabytes.

Tip

To minimize the impact of building an index on replica sets and sharded clusters with replica set shards, use a rolling index build procedure as described on Rolling Index Builds on Replica Sets.

Collation and Index Types

The following index types only support simple binary comparison and do not support collation:

• 2d indexes

Tip

To create a text or 2d index on a collection that has a non-simple collation, you must explicitly specify {collation: {locale: "simple"} } when creating the index.

Hidden Indexes

• You cannot hidden index.

Sorts

Maximum Number of Sort Keys

• You can sort on a maximum of 32 keys.

• Providing a sort pattern with duplicate fields causes an error.

Data

Maximum Number of Documents in a Capped Collection

If you specify the maximum number of documents in a capped collection with create's max parameter, the value must be less than 2 ³¹ documents.

If you do not specify a maximum number of documents when creating a capped collection, there is no limit on the number of documents.

Replica Sets

Number of Members of a Replica Set

Replica sets can have up to 50 members.

Number of Voting Members of a Replica Set

Replica sets can have up to 7 voting members. For replica sets with more than 7 total members, see Non-Voting Members.

Maximum Size of Auto-Created Oplog

If you do not explicitly specify an oplog size (i.e. with oplogSizeMB or --oplogSize) MongoDB will create an oplog that is no larger than 50 gigabytes. [4]

[4]	The oplog can grow past its configured size limit to avoid deleting the majority commit point.

Sharded Clusters

Sharded clusters have the restrictions and thresholds described here.