Can Two PHP Scripts Run Transactions Simultaneously Without Interfering?

Understanding PDO MySQL Transactions: Impact on Concurrent Script Execution

Background

In the PHP documentation, transactions are described as mechanisms that ensure the integrity and isolation of database operations. They guarantee that any changes made within a transaction are applied to the database safely and without interference from other connections.

Question

This raises the question of whether two separate PHP scripts can run transactions simultaneously without interfering with each other.

Explanation

The potential for interference results from the sequence of events between the two scripts. Consider the following example:

script1.php and script2.php

<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;//increasing salary

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

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

Sequence of Events

Imagine this scenario:

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.

Possible Outcomes

Depending on MySQL's isolation level and the use of locking reads, the resulting salary of 'ana' can be either:

• 11000: Transactions overlap due to a lower isolation level or non-locking reads.
• 12000: Transactions are executed individually due to a higher isolation level or locking reads.

Conclusion

The outcome of concurrent transactions depends on the isolation level and the use of locking reads in MySQL. Understanding these concepts is essential for preventing interference between transactions executed by multiple scripts.

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