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Enforcing Constraints

Let’s think about correctness in the context of the ideas around unbundling databases (“Unbundling Databases” on page 499). We saw that end-to-end duplicate suppression can be achieved with a request ID that is passed all the way from the client to the database that records the write. What about other kinds of constraints?

In particular, let’s focus on uniqueness constraints—such as the one we relied on in Example 12-2. In “Constraints and uniqueness guarantees” on page 330 we saw several other examples of application features that need to enforce uniqueness: a username or email address must uniquely identify a user, a file storage service cannot have more than one file with the same name, and two people cannot book the same seat on a flight or in a theater.

Other kinds of constraints are very similar: for example, ensuring that an account balance never goes negative, that you don’t sell more items than you have in stock in the warehouse, or that a meeting room does not have overlapping bookings. Techniques that enforce uniqueness can often be used for these kinds of constraints as well.

 
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