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Benjamin Dreux February 08, 2014
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GridStore

GridFS is a scalable MongoDB filesystem for storing and retrieving large files. The default limit for a MongoDB record is 16MB, so to store data that is larger than this limit, GridFS can be used. GridFS shards the data into smaller chunks automatically. See MongoDB documentation for details.

GridStore is a single file inside GridFS that can be managed by the script.

Open GridStore

Opening a GridStore (a single file in GridFS) is a bit similar to opening a database. At first you need to create a GridStore object and then open it.

  var gs = new mongodb.GridStore(db, filename, mode[, options])

Where

  • db is the database object
  • filename is the name of the file in GridFS that needs to be accessed/created
  • mode indicated the operation, can be one of:
    • "r" (Read): Looks for the file information in fs.files collection, or creates a new id for this object.
    • "w" (Write): Erases all chunks if the file already exist.
    • "w+" (Append): Finds the last chunk, and keeps writing after it.
  • options can be used to specify some metadata for the file, for example content_type, metadata and chunk_size

Example:

  var gs = new mongodb.GridStore(db, "test.png", "w", {
    "content_type": "image/png",
    "metadata":{
        "author": "Daniel"
    },
    "chunk_size": 1024*4
  });

When GridStore object is created, it needs to be opened.

  gs.open(callback);

callback gets two parameters - and error object (if error occured) and the GridStore object.

Opened GridStore object has a set of useful properties

  • gs.length - length of the file in bytes
  • gs.contentType - the content type for the file
  • gs.uploadDate - when the file was uploaded
  • gs.metadata - metadata that was saved with the file
  • gs.chunkSize - chunk size

Example

  gs.open(function(err, gs){
      console.log("this file was uploaded at "+gs.uploadDate);
  });

Writing to GridStore

Writing can be done with write

  gs.write(data, callback)

where data is a Buffer or a string, callback gets two parameters - an error object (if error occured) and result value which indicates if the write was successful or not.

While the GridStore is not closed, every write is appended to the opened GridStore.

Writing a file to GridStore

This function opens the GridStore, streams the contents of the file into GridStore, and closes the GridStore.

  gs.writeFile( file, callback )

where

  • file is a file descriptor, or a string file path
  • callback is a function with two parameters - error object (if error occured) and the GridStore object.

Reading from GridStore

Reading from GridStore can be done with read

  gs.read([size], callback)

where

  • size is the length of the data to be read
  • callback is a callback function with two parameters - error object (if an error occured) and data (binary string)

Streaming from GridStore

You can stream data as it comes from the database using stream

  gs.stream([autoclose=false])

where

  • autoclose If true current GridStore will be closed when EOF and 'close' event will be fired

The function returns read stream based on this GridStore file. It supports the events 'read', 'error', 'close' and 'end'.

Delete a GridStore

GridStore files can be unlinked with unlink

  mongodb.GridStore.unlink(db, name, callback)

Where

  • db is the database object
  • name is either the name of a GridStore object or an array of GridStore object names
  • callback is the callback function

Closing the GridStore

GridStore needs to be closed after usage. This can be done with close

  gs.close(callback)

Check the existance of a GridStore file

Checking if a file exists in GridFS can be done with exist

  mongodb.GridStore.exist(db, filename, callback)

Where

  • db is the database object
  • filename is the name of the file to be checked or a regular expression
  • callback is a callback function with two parameters - an error object (if an error occured) and a boolean value indicating if the file exists or not

Seeking in a GridStore

Seeking can be done with seek

  gs.seek(position, callback);

This function moves the internal pointer to the specified position.

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