Designing Data-Intensive Applications: The Big Ideas Behind Reliable, Scalable, and Maintainable Systems

by Martin Kleppmann

Transactions are an abstraction layer that allows an application to pretend that certain concurrency problems and certain kinds of hardware and software faults don’t exist. A large class of errors is reduced down to a simple transaction abort, and the application just needs to try again. In this chapter we saw many examples of problems that transactions help prevent. Not all applications are susceptible to all those problems: an application with very simple access patterns, such as reading and writing only a single record, can probably manage without transactions. However, for more complex access patterns, transactions can hugely reduce the number of potential error cases you need to think about. Without transactions, various error scenarios (processes crashing, network interruptions, power outages, disk full, unexpected concurrency, etc.) mean that data can become inconsistent in various ways. For example, denormalized data can easily go out of sync with the source data. Without transactions, it becomes very difficult to reason about the effects that complex interacting accesses can have on the database. In this chapter, we went particularly deep into the topic of concurrency control. We discussed several widely used isolation levels, in particular read committed, snapshot isolation (sometimes called repeatable read), and serializable. We characterized those isolation levels by discussing various examples of race conditions: Dirty reads One client reads another client’s w...