Distributed
← Database concurrency & correctness

Lesson 02.5

MVCC & isolation levels

Multi-version concurrency control keeps multiple versions of data so transactions can read consistent snapshots while isolation levels decide which changes they may observe.

A report that disagrees with itself

At closing time, Ava makes the shop's stock report on the computer. The report shows how many cookies are left twice: in the summary at the top and in the total at the bottom. The two numbers should match.

Meanwhile Ben sells one more cookie 🍪, so the number of cookies left changes from 5 to 4. What if that happens while Ava is still making the report?

In this lesson you'll

  1. 1Watch Ava make the report when Ben sells after she's done.
  2. 2Guess what the report says when Ben sells in the middle.
  3. 3Find a way to keep Ava's two numbers matching.