Watch two transactions quietly corrupt each other.

A failure you can hold in your head

A hospital rule says at least one doctor must stay on call. Two are on call right now. Both decide to go home at the same moment.

1. ●Dr ADr A checks how many doctors are on call. The answer is two.
2. ■Dr BDr B checks at the same instant. Also two.
3. ●Dr AA sees that one other doctor remains, so A takes themselves off call.
4. ■Dr BB sees that one other doctor remains, so B takes themselves off call.

Nobody is on call. Neither transaction touched the other’s row, nothing was locked, no error was raised, and both committed successfully. This is write skew, and most databases permit it at the level you are probably running right now.

One run, all the way through

Here is the whole thing, step by step, with the real numbers — produced by running this schedule through the same simulator the rest of the site uses. Nothing here needs you to press anything. Watch for the moment where nothing at all goes wrong: every statement below succeeds.

PostgreSQL 16 · REPEATABLE READ

Key 1

Dr A

Key 2

Dr B

= 0

off call

= 1

on call

1. 0b1begin
2. 1b2begin
3. 2r1[P:1..2]range read over keys 1..2→ {1=1, 2=1}

   part of the table before the run started

4. 3r2[P:1..2]range read over keys 1..2→ {1=1, 2=1}

   part of the table before the run started

5. 4w1[1=0]write key 1 = 0
6. 5w2[2=0]write key 2 = 0

   T1 wrote key 1 at step 4 and T2 wrote key 2 at step 5. Each had already read the row the other wrote, neither saw the other's write, and no row was written twice — so nothing conflicted and both committed.

7. 6c1commit
8. 7c2commit

The table is left as 1=0 2=0

Neither transaction saw uncommitted data, neither wrote a row the other wrote, no lock was contended and no error was raised. Each read was correct as of the moment its snapshot was taken. The two snapshots were taken before either write, and that is the whole bug.

How to use this

1. 1

   Pick a failure

   Each scenario is a real, classic failure written out as two or three transactions with their statements interleaved in a specific order.

2. 2

   Step through it

   Press Next to advance one statement at a time. The score shows who did what and when; the panels below show the state of the engine at that exact moment.

3. 3

   Change one thing

   Swap the engine or raise the isolation level and the same steps re-run instantly. What changes — and what stubbornly does not — is the whole lesson.