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GridFS Reference

GridFS stores files in two collections:

GridFS places the collections in a common bucket by prefixing each with the bucket name. By default, GridFS uses two collections with names prefixed by fs bucket:

  • fs.files
  • fs.chunks

You can choose a different bucket name than fs, and create multiple buckets in a single database.

See also

GridFS for more information about GridFS.

The chunks Collection

Each document in the chunks collection represents a distinct chunk of a file as represented in the GridFS store. The following is a prototype document from the chunks collection.:

{
  "_id" : <ObjectId>,
  "files_id" : <ObjectId>,
  "n" : <num>,
  "data" : <binary>
}

A document from the chunks collection contains the following fields:

chunks._id

The unique ObjectId of the chunk.

chunks.files_id

The _id of the “parent” document, as specified in the files collection.

chunks.n

The sequence number of the chunk. GridFS numbers all chunks, starting with 0.

chunks.data

The chunk’s payload as a BSON binary type.

The chunks collection uses a compound index on files_id and n, as described in GridFS Index.

The files Collection

Each document in the files collection represents a file in the GridFS store. Consider the following prototype of a document in the files collection:

{
  "_id" : <ObjectId>,
  "length" : <num>,
  "chunkSize" : <num>,
  "uploadDate" : <timestamp>,
  "md5" : <hash>,

  "filename" : <string>,
  "contentType" : <string>,
  "aliases" : <string array>,
  "metadata" : <dataObject>,
}

Documents in the files collection contain some or all of the following fields. Applications may create additional arbitrary fields:

files._id

The unique ID for this document. The _id is of the data type you chose for the original document. The default type for MongoDB documents is BSON ObjectId.

files.length

The size of the document in bytes.

files.chunkSize

The size of each chunk. GridFS divides the document into chunks of the size specified here. The default size is 255 kilobytes.

Changed in version 2.4.10: The default chunk size changed from 256k to 255k.

files.uploadDate

The date the document was first stored by GridFS. This value has the Date type.

files.md5

An MD5 hash returned by the filemd5 command. This value has the String type.

files.filename

Optional. A human-readable name for the document.

files.contentType

Optional. A valid MIME type for the document.

files.aliases

Optional. An array of alias strings.

files.metadata

Optional. Any additional information you want to store.