How Do Concurrent Transactions Affect Data Consistency in PDO MySQL?

Understanding Transactions in PDO MySQL

Transactions provide four fundamental features known as ACID (Atomicity, Consistency, Isolation, and Durability). They ensure that database operations, even if executed in stages, are applied safely and without conflicts from other connections when committed.

Concurrent Transactions

The question posed relates to the potential interference of two simultaneously running PHP scripts, both executing transactions. To understand this, let's consider the following hypothetical scenario:

The Employees Table

id	name	salary
1	ana	10000

PHP Scripts and Transaction Sequence

Two scripts, script1.php and script2.php, with identical transaction logic, are executed concurrently. The transaction sequence is as follows:

1. Both scripts select data.
2. Both scripts update data.
3. script1.php commits its transaction.
4. script2.php commits its transaction.

Possible Outcomes

Depending on database isolation level settings, the resulting salary for Ana can be either 11000 or 12000.

• 11000: In isolation levels other than SERIALIZABLE, both transactions can overlap since data was obtained before either commit. The final salary reflects the combined update amounts.
• 12000: In SERIALIZABLE isolation level, transactions are forced to execute individually. The final salary will be the total of both updates regardless of the order they were executed.

Conclusion

The behavior of concurrent transactions depends on the chosen isolation level and blocking/non-blocking reads. SERIALIZABLE isolation with autocommit turned off guarantees independent transaction execution, resulting in a final salary of 12000. Other isolation levels and SERIALIZABLE with autocommit enabled allow transactions to overlap, leading to a final salary of 11000. Understanding these concepts is crucial for designing databases that support multiple concurrent connections efficiently.

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