What is capped collection?

Capped collection is a fixed size collection, that automatically overwrites its oldest entries when it reaches its maximum size.

Options

Filed	Type	Description
capped	Boolean	Optional. ‘true’ indicates it is a capped collection. For capped collections, you must also set maximum size in ‘size’ field
autoIndexId	Boolean	Optional. Specify false to disable the automatic creation of an index on the _id field.
size	Number	Optional. Specifies a maximum size in bytes for a capped collection. The size field is required for capped collections, and ignored for other collections.
max	Number	Optional. The maximum number of documents allowed in the capped collection. The size limit takes precedence over this limit. If a capped collection reaches its maximum size before it reaches the maximum number of documents, MongoDB removes old documents. If you prefer to use this limit, ensure that the size limit, which is required, is sufficient to contain the documents limit.
usePowerOf2Sizes	boolean	Optional. It is the default allocation strategy for all collections. Set to false to disable the usePowerOf2Sizes allocation strategy for this collection. By setting usePowerOf2Sizes, you ensure that MongoDB will allocate space for documents in sizes that are powers of 2. Smalles allocation for a document is 32. So it grows like 32, 64, 128, 256….

For example

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

{ "ok" : 1 }

Above command creates a collection named address with a maximum size of 5 megabytes and a maximum of 5000 documents.

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