MongoDB Applied Design Patterns

Chapter 3. Mimicking Transactional Behavior

Relational database schemas often rely on the existence of atomic multistatement transactions to ensure data consistency: either all of the statements in a group succeed, or all of the statements fail, moving the database from one self-consistent state to another. When trying to scale relational databases over multiple physical servers, however, transactions must use a two-phase commit protocol, which significantly slows down transactions that may span multiple servers. MongoDB, in not allowing multidocument atomic transactions, effectively side-steps this problem and substitutes another one: how to maintain consistency in the absence of transactions.

In this chapter, we’ll explore how MongoDB’s document model and its atomic update operations enable an approach that maintains consistency where a relational database would use a transaction. We’ll also look at how we can use an approach known as compensation to mimic the transactional behavior of relational databases.

The Relational Approach to Consistency

One of the goals of relational database normalization is the ability to make atomic changes to a single row, which maintains the domain-level consistency of your data model. Although normalization goes a long way toward such consistency enforcement, there are some types of consistency requirements that are difficult or impossible to express in a single SQL statement:

• Deleting a row in a one-to-many relationship should also delete the many ...