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convertToCapped

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Definition

convertToCapped
Warning

Do Not Run This Command In Sharded Clusters

MongoDB does not support the convertToCapped command in a sharded cluster.

The convertToCapped command converts an existing, non-capped collection to a capped collection within the same database.

Syntax

The command has the following syntax:

db.runCommand(
   {
     convertToCapped: <collection>,
     size: <capped size>,
     writeConcern: <document>,
     comment: <any>
   }
)

Command Fields

The command takes the following fields:

FieldDescription
convertToCappedThe name of the existing collection to convert.
sizeThe maximum size, in bytes, for the capped collection.
writeConcernOptional. A document expressing the write concern of the drop command. Omit to use the default write concern.
commentOptional.
A user-provided comment to attach to this command. Once set, this comment appears alongside records of this command in the following locations: A comment can be any valid BSON type (string, integer, object, array, etc).
New in version 4.4.

convertToCapped takes an existing collection (<collection>) and transforms it into a capped collection with a maximum size in bytes, specified by the size argument (<capped size>).

During the conversion process, the convertToCapped command exhibits the following behavior:

  • MongoDB traverses the documents in the original collection in natural order and loads the documents into a new capped collection.
  • If the capped size specified for the capped collection is smaller than the size of the original uncapped collection, then MongoDB will overwrite documents in the capped collection based on insertion order, or first in, first out order.
  • Internally, to convert the collection, MongoDB uses the following procedure

    • cloneCollectionAsCapped command creates the capped collection and imports the data.
    • MongoDB drops the original collection.
    • renameCollection renames the new capped collection to the name of the original collection.
  • 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.
Warning

The convertToCapped will not recreate indexes from the original collection on the new collection, other than the index on the _id field. If you need indexes on this collection you will need to create these indexes after the conversion is complete.

Example

Convert a Collection

The following example uses db.collection.insertOne() to create an events collection, and db.collection.stats() to obtain information about the collection:

db.events.insertOne( { click: 'button-1', time: new Date() } )
db.events.stats()

MongoDB will return the following:

{
        "ns" : "test.events",
        ...
        "capped" : false,
        ...
}

To convert the events collection into a capped collection and view the updated collection information, run the following commands:

db.runCommand( { convertToCapped: 'events', size: 8192 } )
db.events.stats()

MongoDB will return the following:

{
     "ns" : "test.events",
     ...
     "capped" : true,
     "max" : NumberLong("9223372036854775807"),
     "maxSize" : 8192,
     ...
}

The convertToCapped will not recreate indexes from the original collection on the new collection, other than the index on the _id field. If you need indexes on this collection you will need to create these indexes after the conversion is complete.

Tip

See also:

create