Can MySQL Transactions with PDO Run Concurrently Without Interference?

Exploring the Concurrency of MySQL Transactions with PDO

The topic of database transactions is often accompanied by the acronym ACID, representing Atomicity, Consistency, Isolation, and Durability. In simpler terms, transactions ensure that database operations are performed reliably and are not affected by other concurrent connections.

However, a question arises: can multiple PHP scripts run transactions simultaneously without interference? To understand this, consider the following scenario:

Assume a table named "employees" with two fields: "id" and "salary." Two scripts, script1.php and script2.php, execute the following code simultaneously:

<code class="php">$conn->beginTransaction();

$stmt = $conn->prepare("SELECT * FROM employees WHERE name = ?");
$stmt->execute(['ana']);
$row = $stmt->fetch(PDO::FETCH_ASSOC);

$salary = $row['salary'];
$salary = $salary + 1000;

$stmt = $conn->prepare("UPDATE employees SET salary = {$salary} WHERE name = ?");
$stmt->execute(['ana']);

$conn->commit();</code>

What would be the resulting salary for "ana" in this scenario?

The answer depends on the isolation level configured for the MySQL InnoDB table engine. InnoDB supports four isolation levels, as specified by the SQL standard:

• Read Uncommitted
• Read Committed
• Repeatable Read
• Serializable

By default, MySQL uses the "Read Committed" isolation level. In this level, the result would be 11000. This is because script1.php and script2.php read the same data before either of them commits.

If the "Serializable" isolation level is used, the result would be 12000. This is because Serializable ensures that transactions are executed sequentially and prevents other transactions from interfering with locked data.

In the given example, the sequence of events is:

1. script1.php selects data
2. script2.php selects data
3. script1.php updates data
4. script2.php updates data
5. script1.php commits
6. script2.php commits

With "Read Committed" isolation, both scripts read the same data before any of them commits. Therefore, the increase in salary is not isolated and is applied twice, resulting in an increase of 2000.

With "Serializable" isolation, the first transaction (script1.php) locks the row for "ana" when it selects the data. When script2.php tries to select the same data, it will wait until the first transaction commits. This ensures that the data is not modified by another transaction while it is being read, and the increase in salary is only applied once, resulting in an increase of 1000.

Therefore, the interference between simultaneous transactions depends on the isolation level and the specific sequence of operations. Understanding the implications of different isolation levels is crucial for ensuring that transactions behave as intended in concurrent environments.

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