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GridFS

GridFS is a specification for storing and retrieving files that exceed the BSON-document size limit of 16MB.

Instead of storing a file in a single document, GridFS divides a file into parts, or chunks, [1] and stores each of those chunks as a separate document. By default GridFS limits chunk size to 255k. GridFS uses two collections to store files. One collection stores the file chunks, and the other stores file metadata.

When you query a GridFS store for a file, the driver or client will reassemble the chunks as needed. You can perform range queries on files stored through GridFS. You also can access information from arbitrary sections of files, which allows you to “skip” into the middle of a video or audio file.

GridFS is useful not only for storing files that exceed 16MB but also for storing any files for which you want access without having to load the entire file into memory. For more information on the indications of GridFS, see When should I use GridFS?.

[1]The use of the term chunks in the context of GridFS is not related to the use of the term chunks in the context of sharding.

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

Implement GridFS

To store and retrieve files using GridFS, use either of the following:

  • A MongoDB driver. See the drivers documentation for information on using GridFS with your driver.
  • The mongofiles command-line tool in the mongo shell. See mongofiles.

GridFS Collections

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.

Each document in the chunks collection represents a distinct chunk of a file as represented in the GridFS store. Each chunk is identified by its unique ObjectId stored in its _id field.

For descriptions of all fields in the chunks and files collections, see GridFS Reference.

GridFS Index

GridFS uses a unique, compound index on the chunks collection for the files_id and n fields. The files_id field contains the _id of the chunk’s “parent” document. The n field contains the sequence number of the chunk. GridFS numbers all chunks, starting with 0. For descriptions of the documents and fields in the chunks collection, see GridFS Reference.

The GridFS index allows efficient retrieval of chunks using the files_id and n values, as shown in the following example:

cursor = db.fs.chunks.find({files_id: myFileID}).sort({n:1});

See the relevant driver documentation for the specific behavior of your GridFS application. If your driver does not create this index, issue the following operation using the mongo shell:

db.fs.chunks.ensureIndex( { files_id: 1, n: 1 }, { unique: true } );

Example Interface

The following is an example of the GridFS interface in Java. The example is for demonstration purposes only. For API specifics, see the relevant driver documentation.

By default, the interface must support the default GridFS bucket, named fs, as in the following:

// returns default GridFS bucket (i.e. "fs" collection)
GridFS myFS = new GridFS(myDatabase);

// saves the file to "fs" GridFS bucket
myFS.createFile(new File("/tmp/largething.mpg"));

Optionally, interfaces may support other additional GridFS buckets as in the following example:

// returns GridFS bucket named "contracts"
GridFS myContracts = new GridFS(myDatabase, "contracts");

// retrieve GridFS object "smithco"
GridFSDBFile file = myContracts.findOne("smithco");

// saves the GridFS file to the file system
file.writeTo(new File("/tmp/smithco.pdf"));