DB/MongoDB

mongodb - GridFS

linuxism 2013. 6. 18. 12:49



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 256k. 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.

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 relevantdriver 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"));



출처 - http://docs.mongodb.org/manual/core/gridfs/






When should I use GridFS?

For documents in a MongoDB collection, you should always use GridFS for storing files larger than 16 MB.

In some situations, storing large files may be more efficient in a MongoDB database than on a system-level filesystem.

  • If your filesystem limits the number of files in a directory, you can use GridFS to store as many files as needed.
  • When you want to keep your files and metadata automatically synced and deployed across a number of systems and facilities. When usinggeographically distributed replica sets MongoDB can distribute files and their metadata automatically to a number of mongod instances and facilities.
  • When you want to access information from portions of large files without having to load whole files into memory, you can use GridFS to recall sections of files without reading the entire file into memory.

Do not use GridFS if you need to update the content of the entire file atomically. As an alternative you can store multiple versions of each file and specify the current version of the file in the metadata. You can update the metadata field that indicates “latest” status in an atomic update after uploading the new version of the file, and later remove previous versions if needed.

Furthermore, if your files are all smaller the 16 MB BSON Document Size limit, consider storing the file manually within a single document. You may use the BinData data type to store the binary data. See your drivers documentation for details on using BinData.

For more information on GridFS, see GridFS.



출처 - http://docs.mongodb.org/manual/faq/developers/#faq-developers-when-to-use-gridfs

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