Captain Kirk from Star Trek the original series. He’s looking angrily at a computer, which is in the shape of a gray cylinder with a few lights, knobs, and antennae.

Computers are infuriating.

At PyCon 2023 I talked about consistency and isolation in databases, and showed Python implementations of four isolation levels. Here’s the PyCon video and here’s a Talk Python podcast interview with Michael Kennedy (episode page).

The subject goes a bit deeper than I could cover in 30 minutes; here are links for further reading.

First, orient yourself with Kyle “Aphyr” Kingsbury’s map.



Granularity of Locks in a Shared Data Base, Gray et. al. 1975, or the summary in the Morning Paper part 1 and part 2. This is the earliest paper I’ve read about isolation. It’s fundamental.

A Critique of ANSI SQL Isolation Levels, Berenson et. al. 1995, or the Morning Paper summary.


Generalized Isolation Level Definitions, Adya et. al. 2000, Morning Paper.

Optional: Seeing is Believing: A Client-Centric Specification of Database Isolation, Crooks et. al. 2017, Morning Paper.



Linearizability: A Correctness Condition for Concurrent Objects, Herlihy & Wing 1990.

Linearizability versus Serializability on Peter Bailis’s blog.

Visualizing Linearizability, Michael Whittaker’s blog.

We often implement linearizability in a distributed system with a consensus algorithm such as Paxos or Raft.

Other consistency levels

Morning Paper: Distributed Consistency and Session Anomalies.

MongoDB’s “Read Concern” docs.