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MongoDB Limits and Thresholds

This document provides a collection of hard and soft limitations of the MongoDB system.

BSON Documents

BSON Document Size

The maximum BSON document size is 16 megabytes.

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

Nested Depth for BSON Documents

MongoDB supports no more than 100 levels of nesting for BSON documents.

Naming Restrictions

Database Name Case Sensitivity

Since database names are case insensitive in MongoDB, database names cannot differ only by the case of the characters.

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 have fewer than 64 characters.

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.)

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 the mongo shell or a similar method for your driver.

The maximum length of the collection namespace, which includes the database name, the dot (.) separator, and the collection name (i.e. <database>.<collection>), is 120 bytes.

Restrictions on Field Names
  • Field names cannot contain the null character.

  • Top-level field names cannot start with the dollar sign ($) character.

    Otherwise, starting in MongoDB 3.6, the server permits storage of field names that contain dots (i.e. .) and dollar signs (i.e. $).


    The MongoDB Query Language cannot always meaningfully express queries over documents whose field names contain these characters (see SERVER-30575).

    Until support is added in the query language, the use of $ and . in field names is not recommended and is not supported by the official MongoDB drivers.


Namespace Length

The maximum length of the collection namespace, which includes the database name, the dot (.) separator, and the collection name (i.e. <database>.<collection>), is 120 bytes.

Number of Namespaces

Changed in version 3.0.

For the MMAPv1 the number of namespaces is limited to the size of the namespace file divided by 628.

A 16 megabyte namespace file can support approximately 24,000 namespaces. Each collection and index is a namespace.

The WiredTiger storage engine is not subject to this limitation.

Size of Namespace File

Changed in version 3.0.

For the MMAPv1 storage engine, namespace files can be no larger than 2047 megabytes.

By default namespace files are 16 megabytes. You can configure the size using the nsSize option.

The WiredTiger storage engine is not subject to this limitation.


Index Key Limit

The total size of an index entry, which can include structural overhead depending on the BSON type, must be less than 1024 bytes.

Changed in version 2.6: MongoDB versions 2.6 and greater implement a stronger enforcement of the limit on index key:

  • MongoDB will not create an index on a collection if the index entry for an existing document exceeds the index key limit. Previous versions of MongoDB would create the index but not index such documents.

  • Reindexing operations will error if the index entry for an indexed field exceeds the index key limit. Reindexing operations occur as part of compact and repairDatabase commands as well as the db.collection.reIndex() method.

    Because these operations drop all the indexes from a collection and then recreate them sequentially, the error from the index key limit prevents these operations from rebuilding any remaining indexes for the collection and, in the case of the repairDatabase command, from continuing with the remainder of the process.

  • MongoDB will not insert into an indexed collection any document with an indexed field whose corresponding index entry would exceed the index key limit, and instead, will return an error. Previous versions of MongoDB would insert but not index such documents.

  • Updates to the indexed field will error if the updated value causes the index entry to exceed the index key limit.

    If an existing document contains an indexed field whose index entry exceeds the limit, any update that results in the relocation of that document on disk will error.

  • mongorestore and mongoimport will not insert documents that contain an indexed field whose corresponding index entry would exceed the index key limit.

  • In MongoDB 2.6, secondary members of replica sets will continue to replicate documents with an indexed field whose corresponding index entry exceeds the index key limit on initial sync but will print warnings in the logs.

    Secondary members also allow index build and rebuild operations on a collection that contains an indexed field whose corresponding index entry exceeds the index key limit but with warnings in the logs.

    With mixed version replica sets where the secondaries are version 2.6 and the primary is version 2.4, secondaries will replicate documents inserted or updated on the 2.4 primary, but will print error messages in the log if the documents contain an indexed field whose corresponding index entry exceeds the index key limit.

  • For existing sharded collections, chunk migration will fail if the chunk has a document that contains an indexed field whose index entry exceeds the index key limit.

Number of Indexes per Collection

A single collection can have no more than 64 indexes.

Index Name Length

Fully qualified index names, which includes the namespace and the dot separators (i.e. <database name>.<collection name>.$<index name>), cannot be longer than 128 characters.

By default, <index name> is the concatenation of the field names and index type. You can explicitly specify the <index name> to the createIndex() method to ensure that the fully qualified index name does not exceed the limit.

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.

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 field(s).

Geospatial Index

Geospatial indexes cannot cover a query.

Memory Usage in Foreground Index Builds

You can build one or more indexes on a collection with the database command createIndexes. The default limit on memory usage for a createIndexes operation is 500 megabytes. You can override this limit by setting the maxIndexBuildMemoryUsageMegabytes server parameter.

createIndexes uses a combination of memory and temporary files on disk to complete index builds. Once the memory limit is reached, createIndexes uses temporary disk files in a subdirectory named _tmp within the --dbpath directory for additional scratch space. The higher the memory limit is set, the faster the index build can complete, but be careful not to set this limit too high relative to available RAM or your system can run out of free memory.

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

An initial sync operation 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 foreground index builds on multiple collections in multiple databases simultaneously and potentially consume an amount of memory greater than the limit set in maxIndexBuildMemoryUsageMegabytes.


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 Build Indexes on Replica Sets.

Collation and Index Types

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


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


Maximum Number of Documents in a Capped Collection

If you specify a maximum number of documents for a capped collection using the max parameter to create, the limit must be less than 232 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.

Database Size

The MMAPv1 storage engine limits each database to no more than 16000 data files. This means that a single MMAPv1 database has a maximum size of 32TB. Setting the storage.mmapv1.smallFiles option reduces this limit to 8TB.

Data Size

Changed in version 3.0.

Using the MMAPv1 storage engine, a single mongod instance cannot manage a data set that exceeds maximum virtual memory address space provided by the underlying operating system.

Virtual Memory Limitations
Operating System Journaled Not Journaled
Linux 64 terabytes 128 terabytes
Windows Server 2012 R2 and Windows 8.1 64 terabytes 128 terabytes
Windows (otherwise) 4 terabytes 8 terabytes

The WiredTiger storage engine is not subject to this limitation.

Number of Collections in a Database

Changed in version 3.0.

For the MMAPv1 storage engine, the maximum number of collections in a database is a function of the size of the namespace file and the number of indexes of collections in the database.

The WiredTiger storage engine is not subject to this limitation.

See Number of Namespaces for more information.

Replica Sets

Number of Members of a Replica Set

Changed in version 3.0.0.

Replica sets can have up to 50 members. See Increased Number of Replica Set Members for more information about specific driver compatibility with large replica sets.

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

Changed in version 2.6.

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.

Sharded Clusters

Sharded clusters have the restrictions and thresholds described here.

Sharding Operational Restrictions

Operations Unavailable in Sharded Environments

The group does not work with sharding. Use mapReduce or aggregate instead.

Deprecated since version 3.0: db.eval() is deprecated.

db.eval() is incompatible with sharded collections. You may use db.eval() with un-sharded collections in a shard cluster.

$where does not permit references to the db object from the $where function. This is uncommon in un-sharded collections.

The geoSearch command is not supported in sharded environments.

Covered Queries in Sharded Clusters

Starting in MongoDB 3.0, an index cannot cover a query on a sharded collection when run against a mongos if the index does not contain the shard key, with the following exception for the _id index: If a query on a sharded collection only specifies a condition on the _id field and returns only the _id field, the _id index can cover the query when run against a mongos even if the _id field is not the shard key.

In previous versions, an index cannot cover a query on a sharded collection when run against a mongos.

Sharding Existing Collection Data Size

An existing collection can only be sharded if its size does not exceed specific limits. These limits can be estimated based on the average size of all shard key values, and the configured chunk size.


These limits only apply for the initial sharding operation. Sharded collections can grow to any size after successfully enabling sharding.

Use the following formulas to calculate the theoretical maximum collection size.

maxSplits = 16777216 (bytes) / <average size of shard key values in bytes>
maxCollectionSize (MB) = maxSplits * (chunkSize / 2)


The maximum BSON document size is 16MB or 16777216 bytes.

All conversions should use base-2 scale, e.g. 1024 kilobytes = 1 megabyte.

If maxCollectionSize is less than or nearly equal to the target collection, increase the chunk size to ensure successful initial sharding. If there is doubt as to whether the result of the calculation is too ‘close’ to the target collection size, it is likely better to increase the chunk size.

After successful initial sharding, you can reduce the chunk size as needed. If you later reduce the chunk size, it may take time for all chunks to split to the new size. See Modify Chunk Size in a Sharded Cluster for instructions on modifying chunk size.

This table illustrates the approximate maximum collection sizes using the formulas described above:

Average Size of Shard Key Values 512 bytes 256 bytes 128 bytes 64 bytes
Maximum Number of Splits 32,768 65,536 131,072 262,144
Max Collection Size (64 MB Chunk Size) 1 TB 2 TB 4 TB 8 TB
Max Collection Size (128 MB Chunk Size) 2 TB 4 TB 8 TB 16 TB
Max Collection Size (256 MB Chunk Size) 4 TB 8 TB 16 TB 32 TB
Single Document Modification Operations in Sharded Collections

All update() and remove() operations for a sharded collection that specify the justOne option must include the shard key or the _id field in the query specification. update() and remove() operations specifying justOne in a sharded collection which do not contain either the shard key or the _id field return an error.

Unique Indexes in Sharded Collections

MongoDB does not support unique indexes across shards, except when the unique index contains the full shard key as a prefix of the index. In these situations MongoDB will enforce uniqueness across the full key, not a single field.


Unique Constraints on Arbitrary Fields for an alternate approach.

Maximum Number of Documents Per Chunk to Migrate

Changed in version 3.4.11.

MongoDB cannot move a chunk if the number of documents in the chunk is greater than 1.3 times the result of dividing the configured chunk size by the average document size. db.collection.stats() includes the avgObjSize field, which represents the average document size in the collection.

Shard Key Limitations

Shard Key Size

A shard key cannot exceed 512 bytes.

Shard Key Index Type

A shard key index can be an ascending index on the shard key, a compound index that start with the shard key and specify ascending order for the shard key, or a hashed index.

A shard key index cannot be an index that specifies a multikey index, a text index or a geospatial index on the shard key fields.

Shard Key is Immutable

If you must change a shard key:

  • Dump all data from MongoDB into an external format.
  • Drop the original sharded collection.
  • Configure sharding using the new shard key.
  • Pre-split the shard key range to ensure initial even distribution.
  • Restore the dumped data into MongoDB.
Shard Key Value in a Document is Immutable

Once you shard a collection, the shard key and the shard key values are immutable; i.e.

  • You cannot select a different shard key for that collection.
  • You cannot update the values of the shard key fields.
Monotonically Increasing Shard Keys Can Limit Insert Throughput

For clusters with high insert volumes, a shard keys with monotonically increasing and decreasing keys can affect insert throughput. If your shard key is the _id field, be aware that the default values of the _id fields are ObjectIds which have generally increasing values.

When inserting documents with monotonically increasing shard keys, all inserts belong to the same chunk on a single shard. The system eventually divides the chunk range that receives all write operations and migrates its contents to distribute data more evenly. However, at any moment the cluster directs insert operations only to a single shard, which creates an insert throughput bottleneck.

If the operations on the cluster are predominately read operations and updates, this limitation may not affect the cluster.

To avoid this constraint, use a hashed shard key or select a field that does not increase or decrease monotonically.

Hashed shard keys and hashed indexes store hashes of keys with ascending values.


Sort Operations

If MongoDB cannot use an index to get documents in the requested sort order, the combined size of all documents in the sort operation, plus a small overhead, must be less than 32 megabytes.

Aggregation Pipeline Operation

Pipeline stages have a limit of 100 MiB (100 * 1024 * 1024 bytes) of RAM. If a stage exceeds this limit, MongoDB will produce an error. To allow for the handling of large datasets, you can set the allowDiskUse option in the aggregate() method. The allowDiskUse option enables most aggregation pipeline operations to write data to a temporary file. The exceptions to the allowDiskUse option are the following aggregation operations; these operations must stay within the memory restriction limit:

  • $graphLookup stage
  • $addToSet accumulator expression used in the $group stage (Starting in version 3.6.17)
  • $push accumulator expression used in the $group stage (Starting in version 3.6.17)

If the pipeline includes other stages that observe allowDiskUse: true in the aggregate() operation, allowDiskUse: true option is in effect for these other stages.

2d Geospatial queries cannot use the $or operator
Geospatial Queries

For spherical queries, use the 2dsphere index result.

The use of 2d index for spherical queries may lead to incorrect results, such as the use of the 2d index for spherical queries that wrap around the poles.

Geospatial Coordinates
  • Valid longitude values are between -180 and 180, both inclusive.
  • Valid latitude values are between -90 and 90, both inclusive.
Area of GeoJSON Polygons

For $geoIntersects or $geoWithin, if you specify a single-ringed polygon that has an area greater than a single hemisphere, include the custom MongoDB coordinate reference system in the $geometry expression; otherwise, $geoIntersects or $geoWithin queries for the complementary geometry. For all other GeoJSON polygons with areas greater than a hemisphere, $geoIntersects or $geoWithin queries for the complementary geometry.

Write Command Batch Limit Size

100,000 writes are allowed in a single batch operation, defined by a single request to the server.

Changed in version 3.6: The limit raises from 1,000 to 100,000 writes. This limit also applies to legacy OP_INSERT messages.

The Bulk() operations in the mongo shell and comparable methods in the drivers do not have this limit.


In addition to being read-only, views have the following operation restrictions:


Sessions and $external Username Limit

Changed in version 3.6.3: To use sessions with $external authentication users (i.e. Kerberos, LDAP, x.509 users), the usernames cannot be greater than 10k bytes.

Session Idle Timeout

Sessions that receive no read or write operations for 30 minutes or that are not refreshed using refreshSessions within this threshold are marked as expired and can be closed by the MongoDB server at any time. Closing a session kills any in-progress operations and open cursors associated with the session. This includes cursors configured with noCursorTimeout or a maxTimeMS greater than 30 minutes.

Consider an application that issues a db.collection.find(). The server returns a cursor along with a batch of documents defined by the cursor.batchSize() of the find(). The session refreshes each time the application requests a new batch of documents from the server. However, if the application takes longer than 30 minutes to process the current batch of documents, the session is marked as expired and closed. When the application requests the next batch of documents, the server returns an error as the cursor was killed when the session was closed.

For operations that return a cursor, if the cursor may be idle for longer than 30 minutes, issue the operation within an explicit session using Session.startSession() and periodically refresh the session using the refreshSessions command. For example:

var session = db.getMongo().startSession()
var sessionId = session.getSessionId().id

var cursor = session.getDatabase("examples").getCollection("data").find().noCursorTimeout()
var refreshTimestamp = new Date() // take note of time at operation start

while (cursor.hasNext()) {

  // Check if more than 5 minutes have passed since the last refresh
  if ( (new Date()-refreshTimestamp)/1000 > 300 ) {
    print("refreshing session")
    db.adminCommand({"refreshSessions" : [sessionId]})
    refreshTimestamp = new Date()

  // process cursor normally


In the example operation, the db.collection.find() method is associated with an explicit session. The cursor is configured with noCursorTimeout() to prevent the server from closing the cursor if idle. The while loop includes a block that uses refreshSessions to refresh the session every 5 minutes. Since the session will never exceed the 30 minute idle timeout, the cursor can remain open indefinitely.

For MongoDB drivers, defer to the driver documentation for instructions and syntax for creating sessions.


The mongo shell prompt has a limit of 4095 codepoints for each line. If you enter a line with more than 4095 codepoints, the shell will truncate it.