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Capped Collections

On this page

  • Overview
  • Behavior
  • Insertion Order
  • Automatic Removal of Oldest Documents
  • Oplog Collection
  • _id Index
  • Restrictions and Recommendations
  • Reads
  • Updates
  • Sharding
  • Query Efficiency
  • Aggregation $out
  • Transactions
  • Stable API
  • Procedures
  • Create a Capped Collection
  • Query a Capped Collection
  • Check if a Collection is Capped
  • Convert a Collection to Capped
  • Change a Capped Collection's Size
  • Change the Maximum Number of Documents in a Capped Collection
  • Tailable Cursor

Capped collections are fixed-size collections that support high-throughput operations that insert and retrieve documents based on insertion order. Capped collections work in a way similar to circular buffers: once a collection fills its allocated space, it makes room for new documents by overwriting the oldest documents in the collection.

See createCollection() or create for more information on creating capped collections.


As an alternative to capped collections, consider MongoDB's TTL (Time To Live) indexes. As described in Expire Data from Collections by Setting TTL, these indexes allow you to expire and remove data from normal collections based on the value of a date-typed field and a TTL value for the index.

TTL indexes are not compatible with capped collections.

Capped collections guarantee preservation of the insertion order. As a result, queries do not need an index to return documents in insertion order. Without this indexing overhead, capped collections can support higher insertion throughput.

To make room for new documents, capped collections automatically remove the oldest documents in the collection without requiring scripts or explicit remove operations.

Consider the following potential use cases for capped collections:

  • Store log information generated by high-volume systems. Inserting documents in a capped collection without an index is close to the speed of writing log information directly to a file system. Furthermore, the built-in first-in-first-out property maintains the order of events, while managing storage use. For example, the oplog uses a capped collection.

  • Cache small amounts of data in a capped collections. Since caches are read rather than write heavy, you would either need to ensure that this collection always remains in the working set (i.e. in RAM) or accept some write penalty for the required index or indexes.

The collection that stores a log of the operations in a replica set uses a capped collection.

Starting in MongoDB 4.0, unlike other capped collections, the oplog can grow past its configured size limit to avoid deleting the majority commit point.


MongoDB rounds the capped size of the oplog up to the nearest integer multiple of 256, in bytes.

Capped collections have an _id field and an index on the _id field by default.

Starting in MongoDB 5.0, you cannot use read concern "snapshot" when reading from a capped collection.

If you plan to update documents in a capped collection, create an index so that these update operations do not require a collection scan.

You cannot shard a capped collection.

Use natural ordering to retrieve the most recently inserted elements from the collection efficiently. This is similar to using the tail command on a log file.

The aggregation pipeline stage $out cannot write results to a capped collection.

You cannot write to capped collections in transactions.

Capped collections are not supported in Stable API V1.

You must create capped collections explicitly using the db.createCollection() method, which is a mongosh helper for the create command. When creating a capped collection you must specify the maximum size of the collection in bytes, which MongoDB will pre-allocate for the collection. The size of the capped collection includes a small amount of space for internal overhead.

db.createCollection( "log", { capped: true, size: 100000 } )


The value that you provide for the size field must be greater than 0 and less than or equal to 1024^5 (1 PB). MongoDB rounds the size of all capped collections up to the nearest integer multiple of 256, in bytes.

Additionally, you may also specify a maximum number of documents for the collection using the max field as in the following document:

db.createCollection("log", { capped : true, size : 5242880, max :
5000 } )


The size argument is always required, even when you specify the max number of documents. MongoDB removes older documents if a collection reaches the maximum size limit before it reaches the maximum document count.



If you perform a find() on a capped collection with no ordering specified, MongoDB guarantees that the ordering of results is the same as the insertion order.

To retrieve documents in reverse insertion order, issue find() along with the sort() method with the $natural parameter set to -1, as shown in the following example:

db.cappedCollection.find().sort( { $natural: -1 } )

Use the isCapped() method to determine if a collection is capped, as follows:


You can convert a non-capped collection to a capped collection with the convertToCapped command:

db.runCommand({"convertToCapped": "mycoll", size: 100000});

The size parameter specifies the size of the capped collection in bytes.

This holds a database exclusive lock for the duration of the operation. Other operations which lock the same database will be blocked until the operation completes. See What locks are taken by some common client operations? for operations that lock the database.

New in version 6.0.

You can resize a capped collection using the collMod command's cappedSize option to set the cappedSize in bytes. cappedSize must be greater than 0 and less than or equal to 1024^5 (1 PB).


Before you can resize a capped collection, you must have already set the featureCompatibilityVersion to at least version "6.0".

For example, the following command sets the maximum size of the "log" capped collection to 100000 bytes:

db.runCommand( { collMod: "log", cappedSize: 100000 } )

New in version 6.0.

To change the maximum number of documents in a capped collection, use the collMod command's cappedMax option. If cappedMax is less than or equal to 0, there is no maximum document limit. If cappedMax is less than the current number of documents in the collection, MongoDB removes the excess documents on the next insert operation.

For example, the following command sets the maximum number of documents in the "log" capped collection to 500:

db.runCommand( { collMod: "log", cappedMax: 500 } )

You can use a tailable cursor with capped collections. Similar to the Unix tail -f command, the tailable cursor "tails" the end of a capped collection. As new documents are inserted into the capped collection, you can use the tailable cursor to continue retrieving documents.

See Tailable Cursors for information on creating a tailable cursor.

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