Database Reference
In-Depth Information
Do be aware that running this after every write is unwise, because it'll force every write
to wait for the next journal sync. This means that all writes may take up to 100 ms to
return. So use this feature with care.
3
10.1.3
Data imports and exports
If you're migrating an existing system to MongoDB, or if you need to seed the data-
base with information from, say, a data warehouse, then you'll need an efficient
import method. You might also need a good export strategy, since you may have to
export data from MongoDB to external processing jobs. For example, exporting data
to Hadoop for batch processing has become a common practice.
4
There are two ways to import and export data with MongoDB. You can use the
included tools,
mongoimport
and
mongoexport
, or you can write a simple program
using one of the drivers.
5
MONGOIMPORT
AND
MONGOEXPORT
Bundled with MongoDB are two utilities for importing and exporting data:
mongoimport
and
mongoexport
. You can use
mongoimport
to import
JSON
,
CSV
, and
TSV
files. This is frequently useful for loading data from relational databases into
MongoDB:
$ mongoimport -d stocks -c values --type csv --headerline stocks.csv
In the example, you import a
CSV
file called
stocks.csv
into the
values
collection of
the
stocks
database. The
--headerline
flag indicates that the first line of the
CSV
contains the field names. You can see all the import options by running
mongoimport
--help
.
Use
mongoexport
to export all of a collection's data to a
JSON
or
CSV
file:
$ mongoexport -d stocks -c values -o stocks.csv
This command exports data to the file stocks.csv. As with its counterpart, you can see
the rest of
mongoexport
's command options by starting it with the
--help
flag.
C
USTOM
IMPORT
AND
EXPORT
SCRIPTS
You're likely to use MongoDB's import and export tools when the data you're dealing
with is relatively flat; once you introduce sub-documents and arrays, the
CSV
format
becomes awkward because it's not designed to represent nested data. When you need
to export a rich document to
CSV
or import a
CSV
to a rich MongoDB document, it
may be easier to build a custom tool instead. You can do this using any of the drivers.
For example, MongoDB users commonly write scripts that connect to a relational
database and then combine the data from two tables into a single collection.
3
Future releases of MongoDB promise finer-grained control over journal syncing. Consult the latest release
notes for details.
4
For this particular use case, an officially supported MongoDB-Hadoop adapter is available at
http://
5
Note that importing and exporting data is distinct from backups, which are covered later in the chapter.
