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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. $).

    Important

    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.

MongoDB does not support duplicate field names

The MongoDB Query Language is undefined over documents with duplicate field names. BSON builders may support creating a BSON document with duplicate field names. While the BSON builder may not throw an error, inserting these documents into MongoDB is not supported even if the insert succeeds. For example, inserting a BSON document with duplicate field names through a MongoDB driver may result in the driver silently dropping the duplicate values prior to insertion.

Namespaces

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.

Indexes

Index Key Limit

Changed in version 4.2

Starting in version 4.2, MongoDB removes the Index Key Limit for featureCompatibilityVersion (fCV) set to "4.2" or greater.

For MongoDB 2.6 through MongoDB versions with fCV set to "4.0" or earlier, the total size of an index entry, which can include structural overhead depending on the BSON type, must be less than 1024 bytes.

When the Index Key Limit applies:

  • MongoDB will not create an index on a collection if the index entry for an existing document exceeds the index key limit.

  • Reindexing operations will error if the index entry for an indexed field exceeds the index key limit. Reindexing operations occur as part of the compact command 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.

  • 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

Changed in version 4.2

Starting in version 4.2, MongoDB removes the Index Name Length Limit for MongoDB versions with featureCompatibilityVersion (fCV) set to "4.2" or greater.

In previous versions of MongoDB or MongoDB versions with fCV set to "4.0" or earlier, fully qualified index names, which include the namespace and the dot separators (i.e. <database name>.<collection name>.$<index name>), cannot be longer than 127 bytes.

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 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 500 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 larger than 500 megabytes. However, setting this limit too high relative to the unused RAM on your system can result in memory errors.

Changed in version 4.2.

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 index builds on multiple collections in multiple databases simultaneously and potentially consume an amount of memory greater than the limit set in 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 Build Indexes on Replica Sets.

Collation and Index Types

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

Tip

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.

Data

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.

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. [1]

[1]Starting in MongoDB 4.0, 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.

Sharding Operational Restrictions

Operations Unavailable in Sharded Environments

$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.

Important

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)

Note

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 or multi: false option must include the shard key or the _id field in the query specification. update() and remove() operations specifying justOne or multi: false 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.

See

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 Selection is Immutable

Once you shard a collection, the selection of the shard key is immutable; i.e. you cannot select a different shard key for that collection.

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.
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.

Operations

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 megabytes of RAM. If a stage exceeds this limit, MongoDB will produce an error. To allow for the handling of large datasets, use the allowDiskUse option to enable aggregation pipeline stages to write data to temporary files.

Changed in version 3.4.

The $graphLookup stage must stay within the 100 megabyte memory limit. If allowDiskUse: true is specified for the aggregate() operation, the $graphLookup stage ignores the option. If there are other stages in the aggregate() operation, allowDiskUse: true option is in effect for these other stages.

Starting in MongoDB 4.2, the profiler log messages and diagnostic log messages includes a usedDisk indicator if any aggregation stage wrote data to temporary files due to memory restrictions.

Aggregation and Read Concern
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.

Multi-document Transactions

For multi-document transactions:

  • You can specify read/write (CRUD) operations on existing collections. The collections can be in different databases. For a list of CRUD operations, see CRUD Operations.
  • You cannot write to capped collections. (Starting in MongoDB 4.2)
  • You cannot read/write to collections in the config, admin, or local databases.
  • You cannot write to system.* collections.
  • You cannot return the supported operation’s query plan (i.e. explain).
  • For cursors created outside of a transaction, you cannot call getMore inside the transaction.
  • For cursors created in a transaction, you cannot call getMore outside the transaction.
  • Starting in MongoDB 4.2, you cannot specify killCursors as the first operation in a transaction.

The following operations are not allowed in transactions:

  • Operations that affect the database catalog, such as creating or dropping a collection or an index. For example, a transaction cannot include an insert operation that would result in the creation of a new collection.

    The listCollections and listIndexes commands and their helper methods are also excluded.

  • Non-CRUD and non-informational operations, such as createUser, getParameter, count, etc. and their helpers.

Transactions have a lifetime limit as specified by transactionLifetimeLimitSeconds. The default is 60 seconds.

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.

Views

The view definition pipeline cannot include the $out or the $merge stage. If the view definition includes nested pipeline (e.g. the view definition includes $lookup or $facet stage), this restriction applies to the nested pipelines as well.

Views have the following operation restrictions:

Sessions

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.

Shell

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.