Database Reference
In-Depth Information
Database
Tables
Rows
Figure 3-2.
A typical relational database model
As you can see, the general structure is the same between the two types of databases;
nevertheless, you do not use them in even remotely similar manners. There are several
types of collections in MongoDB. The default collection type is expandable in size: the
more data you add to it, the larger it becomes. It's also possible to define collections that
are
capped
. These
capped collections
can only contain a certain amount of data before
the oldest document is replaced by a newer document (you will learn more about these
collections in Chapter 4).
Every collection in MongoDB has a unique name. This name should begin with
a letter, or optionally, an underscore (
_
) when created using the
createCollection
function. The name can contain numbers and letters; however, the
$
symbol is reserved
by MongoDB. Similarly, using an empty string (“ ”) is not allowed; the null character
cannot be used in the name and it cannot start with the “
system
.” string. Generally, it's
recommended that you keep the collection's name simple and short (to around nine
characters or so); however, the maximum number of allowed characters in a collection
name is 118, minus the number of charaters in the database and the additional separating
period character. Obviously, there isn't much practical reason to create such a long name.
The above mentioned combination of databasename “period” collection name is
called a namespace. A single database has a default limit of 24,000 namespaces. Each
collection accounts for at least two namespaces: one for the collection itself and one
more for the default _id index created in the collection. If you were to add more indexes
per collection, however, another namespace would be used. In theory, this means that
each database can have up to 12,000 collections by default, assuming each collection only
carries one index. However, this limit on the number of namespaces can be increased
by providing the
nssize
parameter when executing the MongoDB service application
(
mongod
).
