Transactions and auto-commit

Now that you're connected via PDO, you must understand how PDO manages transactions before you start issuing queries. If you've never encountered transactions before, they offer 4 major features: Atomicity, Consistency, Isolation and Durability (ACID). In layman's terms, any work carried out in a transaction, even if it is carried out in stages, is guaranteed to be applied to the database safely, and without interference from other connections, when it is committed. Transactional work can also be automatically undone at your request (provided you haven't already committed it), which makes error handling in your scripts easier.

Transactions are typically implemented by "saving-up" your batch of changes to be applied all at once; this has the nice side effect of drastically improving the efficiency of those updates. In other words, transactions can make your scripts faster and potentially more robust (you still need to use them correctly to reap that benefit).

Unfortunately, not every database supports transactions, so PDO needs to run in what is known as "auto-commit" mode when you first open the connection. Auto-commit mode means that every query that you run has its own implicit transaction, if the database supports it, or no transaction if the database doesn't support transactions. If you need a transaction, you must use the PDO::beginTransaction method to initiate one. If the underlying driver does not support transactions, a PDOException will be thrown (regardless of your error handling settings: this is always a serious error condition). Once you are in a transaction, you may use PDO::commit or PDO::rollBack to finish it, depending on the success of the code you run during the transaction.

Warning

PDO only checks for transaction capabilities on driver level. If certain runtime conditions mean that transactions are unavailable, PDO::beginTransaction will still return true without error if the database server accepts the request to start a transaction.

An example of this would be trying to use transactions on MyISAM tables on a MySQL database.

When the script ends or when a connection is about to be closed, if you have an outstanding transaction, PDO will automatically roll it back. This is a safety measure to help avoid inconsistency in the cases where the script terminates unexpectedly--if you didn't explicitly commit the transaction, then it is assumed that something went awry, so the rollback is performed for the safety of your data.

Warning

The automatic rollback only happens if you initiate the transaction via PDO::beginTransaction. If you manually issue a query that begins a transaction PDO has no way of knowing about it and thus cannot roll it back if something bad happens.

Example #1 Executing a batch in a transaction

In the following sample, let's assume that we are creating a set of entries for a new employee, who has been assigned an ID number of 23. In addition to entering the basic data for that person, we also need to record their salary. It's pretty simple to make two separate updates, but by enclosing them within the PDO::beginTransaction and PDO::commit calls, we are guaranteeing that no one else will be able to see those changes until they are complete. If something goes wrong, the catch block rolls back all changes made since the transaction was started, and then prints out an error message.

<?php
try {
  $dbh = new PDO('odbc:SAMPLE', 'db2inst1', 'ibmdb2', 
      array(PDO::ATTR_PERSISTENT => true));
  echo "Connected\n";
} catch (Exception $e) {
  die("Unable to connect: " . $e->getMessage());
}

try {  
  $dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

  $dbh->beginTransaction();
  $dbh->exec("insert into staff (id, first, last) values (23, 'Joe', 'Bloggs')");
  $dbh->exec("insert into salarychange (id, amount, changedate) 
      values (23, 50000, NOW())");
  $dbh->commit();
  
} catch (Exception $e) {
  $dbh->rollBack();
  echo "Failed: " . $e->getMessage();
}
?>

You're not limited to making updates in a transaction; you can also issue complex queries to extract data, and possibly use that information to build up more updates and queries; while the transaction is active, you are guaranteed that no one else can make changes while you are in the middle of your work. For further reading on transactions, refer to the documentation provided by your database server.