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Capped Collections

Create a Capped Collection

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  • About this Task
  • Steps
  • Create a Capped Collection with a Maximum Size
  • Create a Capped Collection with a Maximum Number of Documents
  • Learn More

To create a capped collection, specify the capped option to either the db.createCollection() method or the create command.

You must create capped collections explicitly. You cannot create a capped collection implicitly by inserting data into a non-existing collection.

When you create a capped collection you must specify the maximum size of the collection. MongoDB pre-allocates the specified storage for the collection. The size of the capped collection includes a small amount of space for internal overhead.

You can optionally specify a maximum number of documents for the collection. MongoDB removes older documents if the collection reaches the maximum size limit before it reaches the maximum document count.

About this Task

Generally, TTL (Time To Live) indexes offer better performance and more flexibility than capped collections. TTL indexes expire and remove data from normal collections based on the value of a date-typed field and a TTL value for the index.

Capped collections serialize write operations and therefore have worse concurrent insert, update, and delete performance than non-capped collections. Before you create a capped collection, consider if you can use a TTL index instead.

Steps

The following examples show you how to:

  • Create a Capped Collection with a Maximum Size

  • Create a Capped Collection with a Maximum Number of Documents

Create a Capped Collection with a Maximum Size

Create a capped collection called log that has a maximum size of 100,000 bytes:

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

Note

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.

Create a Capped Collection with a Maximum Number of Documents

Create a capped collection called log2 that has a maximum size of 5,242,880 bytes and can store a maximum of 5,000 documents:

db.createCollection(
   "log2",
   {
      capped: true,
      size: 5242880,
      max: 5000
   }
)

Important

The size field is always required, even when you specify the max number of documents.

Learn More

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