MySQL Transaction Isolation

How it works

Isolation controls how concurrent transactions observe each other. Under InnoDB REPEATABLE READ, ordinary consistent reads can keep a snapshot across a transaction, while locking reads have a different purpose. The SQL below demonstrates an explicit local transaction boundary; the reviewer adds a second session to test what remains visible after a concurrent commit. That extra session is essential: a one-connection example cannot prove a concurrency claim.

Run in a disposable MySQL lab

Use Oracle MySQL 8.4 with InnoDB, not Bhylu’s SQLite playground. The examples were reviewed on MySQL 8.4.8. XAMPP may provide MariaDB, which has different feature and collation behaviour. Check your actual server before running the script. These lessons do not require a change to Bhylu’s PHP, Apache or normal database service.

1. On a separate lab instance, create the empty bhylu_mysql_lab schema. Stop if it already exists; do not overwrite an existing database.
2. Select that schema and import the complete repair-parts seed once. It sets the documented session mode, utf8mb4 collation, UTC and English date-name locale.
3. Run this lesson in a fresh copy of the seed, not after another lesson’s writes. Use the command-line client or a compatible editor. DELIMITER lines are client instructions for stored-program examples.

Download this complete lesson script. Its setup is the linked seed; review the dataset before changing it. Administrative and destructive examples are restricted to the specifically named disposable objects. Every script repeats the session settings because each new connection has its own session. No MySQL browser runner is currently available.

Example

SET NAMES utf8mb4 COLLATE utf8mb4_0900_ai_ci;
SET SESSION sql_mode = 'STRICT_TRANS_TABLES,ONLY_FULL_GROUP_BY,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION';
SET SESSION time_zone = '+00:00';
SET SESSION lc_time_names = 'en_US';
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;
START TRANSACTION;
SELECT quantity FROM parts WHERE id = 1;
SELECT quantity FROM parts WHERE id = 1;
COMMIT;

Expected result

12
12

In this single-session listing both reads return twelve. In the two-session review, the first transaction keeps its original consistent-read result while a fresh read after commit sees the other session’s update. Isolation is not a substitute for constraints, conditional writes or idempotency. Review anomalies from the business operation instead of choosing an isolation level only by its name.