If you want a replicated database to be as simple as possible to use, you should make it behave as if it were a consistent single-node database. Then users don’t have to worry about replication lag, conflicts, and other inconsistencies; it gives you the advantage of fault tolerance but without the complexity of having to think about multiple replicas.
This is the idea behind linearizability (also known as atomic consistency, strong consistency, immediate consistency, or external consistency). The exact definition of linearizability is quite subtle, and we will explore it in the rest of this section. The basic idea, though, is to make a system appear as if there is only one copy of the data, and all operations on it are atomic. With this guarantee, even though there may be multiple replicas in reality, the application does not need to worry about them.
In a linearizable system, as soon as one client successfully completes a write, all clients reading from the database must be able to see the value just written. Maintaining the illusion of a single copy of the data means guaranteeing that the value read is the most recent, up-to-date value and doesn’t come from a stale cache or replica. In other words, linearizability is a recency guarantee. To clarify this idea, let’s look at an example of a system that is not linearizable.
Figure 10-1 shows a nonlinearizable sports website. Aaliyah and Bryce are sitting in the same room, both checking their phones to see the outcome of a game their favorite team is playing. Just after the final score is announced, Aaliyah refreshes the page, sees the winner announced, and excitedly tells Bryce about it. Bryce incredulously hits reload on his own phone, but his request goes to a database replica that is lagging, so his phone shows that the game is still ongoing.
Diagram illustrating a nonlinearizable system where different clients see inconsistent sports match results, causing confusion.
Figure 10-1. This system is not linearizable, causing sports fans to be confused.
If Aaliyah and Bryce had hit reload at the same time, it would have been less surprising if they had gotten different query results, because they wouldn’t know at exactly what time their respective requests were processed by the server. However, Bryce knows that he hit the reload button (initiated his query) after he heard Aaliyah exclaim the final score, and therefore he expects his query result to be at least as recent as Aaliyah’s. The fact that his query returned a stale result is a violation of linearizability.