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Constraints and uniqueness guarantees

Uniqueness constraints are common in databases: for example, a username or email address must uniquely identify one user, and in a file storage service there cannot be two files with the same path and filename. If you want to enforce this constraint as the data is written (such that if two people try to concurrently create a user or a file with the same name, one of them will be returned an error), you need linearizability.

iii. Strictly speaking, ZooKeeper and etcd provide linearizable writes, but reads may be stale, since by default they can be served by any one of the replicas. You can optionally request a linearizable read: etcd calls this a quorum read [16], and in ZooKeeper you need to call sync() before the read [15]; see “Implementing linearizable storage using total order broadcast” on page 350.

This situation is actually similar to a lock: when a user registers for your service, you can think of them acquiring a “lock” on their chosen username. The operation is also very similar to an atomic compare-and-set, setting the username to the ID of the user who claimed it, provided that the username is not already taken.

Similar issues arise if you want to ensure that a bank account balance never goes negative, or that you don’t sell more items than you have in stock in the warehouse, or that two people don’t concurrently book the same seat on a flight or in a theater. These constraints all require there to be a single up-to-date value (the account balance, the stock level, the seat occupancy) that all nodes agree on.

In real applications, it is sometimes acceptable to treat such constraints loosely (for example, if a flight is overbooked, you can move customers to a different flight and offer them compensation for the inconvenience). In such cases, linearizability may not be needed, and we will discuss such loosely interpreted constraints in “Timeliness and Integrity” on page 524.

However, a hard uniqueness constraint, such as the one you typically find in relational databases, requires linearizability. Other kinds of constraints, such as foreign key or attribute constraints, can be implemented without requiring linearizability [19].

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