The Power of NoSQL and MongoDB - Data Modeling for MongoDB - page 19

be in an inconsistent or changing state. [1] As an Amazon Seller, for example, when
I check my book inventory levels, there is usually a warning in bold red letters say-
ing the data I am viewing is incomplete or more than 24 hours old and therefore
may not be accurate.

• Soft-state . Soft-state means the NoSQL database plays “catch up,” updating the

database continuously even with changes that occurred from earlier in the day. Al-
though no one might have purchased a topic from me between 2 and 3 am, for
example, there might still be transactions occurring during this time that adjust my
inventory levels for purchases from the prior day.

• Eventual consistency . Recall that “consistent” means that the data accurately re-

flects any changes up to a certain point in time [2] . With NoSQL, the system will
eventually become consistent once it stops receiving input. It is acceptable for the
results not to be 100% current, assuming the accuracy eventually catches up at the
end of the week or month. Even though inventory adjustments might occur minutes
or hours after topic purchases, eventually the inventory levels I am viewing will be
accurate.

F OUR T YPES OF N O SQL D ATABASES

Here is a sketch for the traditional RDBMS structure for storing author-and title-related in-
formation:

The RDBMS structure stores like information together such as authors in the Author table
and titles in the Title table. The lines connecting these tables represent the rules (also
known as “database constraints”) that enforce how many values of one table can connect to
how many values of the other table. For example, the line between Author and Title would
capture that an author can write many titles and a title can be written by many authors. If
we wanted to know who wrote a particular title, we would need to join Title with Author .