Kristina Chodorow - 50 Tips and Tricks for MongoDB Developers

Copyright 2011 Kristina Chodorow

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Chapter 1. Application Design Tips

Tip #1: Duplicate data for speed, reference data for integrity

Data used by multiple documents can either be embedded (denormalized) or referenced (normalized). Denormalization isnt better than normalization and visa versa: each have their own trade-offs and you should choose to do whatever will work best with your application.

Denormalization can lead to inconsistent data: suppose you want to change the apple to a pear in . If you change the value in one document but the application crashes before you can update the other documents, your database will have two different values for fruit floating around.

Figure 1-1. A normalized schema. The fruit field is stored in the food collection and referenced by the documents in the meals collection.

Inconsistency isnt great, but the level of not-greatness depends on what youre storing. For many applications, brief periods of inconsistency are OK: if someone changes his username, it might not matter that old posts show up with his old username for a few hours. If its not OK to have inconsistent values even briefly, you should go with normalization.