Database Administration - The Definitive Guide to MongoDB - page 194

The --drop option tells the mongorestore utility to discard each collection in the database before restoring it.
Consequently, the backed-up data replaces the data currently in the database. If you were to choose not to use the --drop
option, the restored data would be appended to the end of each collection, which would result in duplicated items.
Let's examine what happened in this example more closely.
By default, the mongodump utility connects to the local database using the default port, and pulls out all of the data
associated with each database and collection and stores them in a predefined folder structure.
The default folder structure created by mongodump takes this form:

./dump/[databasename]/[collectionname].bson

The database system used in the example consists of a single database called blog . The blog database contains
three collections: authors , posts , and tagcloud .
The mongodump saves the data it retrieves from the database server in .bson files that are just a copy of the internal
BSON format that MongoDB uses internally to store documents. You can also see the indexes on each collection
being restored in the preceding example. The MongoDB server maintains indexes, and it records index definitions
for each collection, which are stored in the metadata.json files. It is these metadata files that allow you to rebuild the
indexes when restoring from a backup.
Once you have dumped the database, you can archive and store the folder on any online or offline media, such as
CD, USB drive, tape, or S3 format.

■ the mongodump utility does not empty the contents of the output directory before it writes the backup files to it.
if you have existing contents in this directory, they won't be removed unless they match the name of a file
( collectionname .bson ) that mongodump has been instructed to back up. this is good if you want to add multiple
collection dumps to the same dump directory; however, it could cause problems if you use the same dump directory each
time you back up data but don't clear it out. For example, assume you have a database that you back up regularly, and at
some point you decide to delete a collection from this database. Unless you either clear out the directory where you are
performing your backup or manually remove the file associated with the deleted collection, then the next time you restore
the data, the deleted collection will reappear. Unless you want to overlay data in your backups, you should make sure
you clear out the destination directory before you use mongodump .

Note

Backing Up a Single Database
When you have multiple applications running on the same server, often you may find yourself wanting to back up
each database individually, rather than all at once, as in the preceding example.
With mongodump , you can do this by adding the -d database_name option to the command line. This causes
mongodump to create the ./dump folder; however, this folder will contain only the backup files for a single database.