deadlock-symfony

A PHP app on Symfony and Doctrine that shows how a database deadlock happens.

Setup

sh bin/build.sh

Running the app

docker exec -it varfrog-deadlock-app sh bin/run.sh

Explanation

We have two records of an entity \App\Entity\User in the database with usernames "alice" and "bob" - let's call them users Alice and Bob, respectively.

bin/run.sh launches two commands in parallel:

1. app:lock-alice-and-bob - it first locks Alice, sleeps for some seconds, then tries to lock Bob,
2. app:lock-bob-and-alice - it locks Bob and Alice, without sleeping.

This results in a deadlock in the 1st command.

Output of bin/run.sh:

[error] Got a deadlock in app:lock-alice-and-bob

Command log in a chronological order, lines numbered in sequence:
     1  19:56:39.622337;app:lock-alice-and-bob;Locking Alice and sleeping.
     2  19:56:39.622344;app:lock-bob-and-alice;Locking Bob.
     3  19:56:39.634649;app:lock-bob-and-alice;Locking Alice.
     4  19:56:44.634883;app:lock-alice-and-bob;Locking Bob.
     5  19:56:44.640286;app:lock-alice-and-bob;Got a deadlock.

Explanation of the log:

• Lines 1, 2: the order of these lines does not matter - either of the commands can be started first. Here, app:lock-alice-and-bob locks Alice and sleeps for some seconds, app:lock-bob-and-alice locks Bob.
• Line 3: app:lock-bob-and-alice tries to get a lock on Alice but has to wait since it's locked in line 1 by app:lock-alice-and-bob
• Line 4: app:lock-alice-and-bob tries to get a lock on Bob but it has to wait for app:lock-bob-and-alice to end its database transaction, i.e. for the lock on Bob to be released. But now app:lock-bob-and-alice will never release the lock because it itself is waiting for the same thing - it waits for a lock of Alice to be released by app:lock-alice-and-bob (it's blocking in line 3). This situation cannot be resolved (both commands are infinitely waiting for each other) and thus we get a deadlock (Line 5).