Drupal 7 Module Development

Database Access
Advanced subjects
While the five basic types of queries cover the vast majority of our database needs,
there are two other advanced subjects we should cover: Transactions and master/
slave replication. We will just touch on each briefly.
Transactions
A transaction in a database is a way to wrap two or more queries together and
declare that they should be atomic. That is, either all succeed or none succeed. That
can be very useful when, say, saving a node; we don't want only some fields to get
written and then an error to break the process halfway through. We saw an example
of that in Chapter 6. In a nutshell, in Drupal we start a transaction by creating a
transaction object. Everything we do to the database is then part of the transaction
until that object is destroyed, at which point the entire query is committed at once. In
most cases, we let PHP destroy the transaction object for us when a function ends.
function my_function() {
$transaction = db_transaction();
try {
$id = db_insert('example')
->fields(array(
'field1' => 'mystring',
'field2' => 5,
))
->execute();
my_other_function($id);
return $id;
}
catch (Exception $e) {
$transaction->rollback();
watchdog_exception('type', $e);
}
}
In most cases we don't need a transaction, and they have no effect on Select queries.
They're most useful when we're going to be running a lot of data modification
queries (Insert, Update, Delete, and Merge) together, and the system will be broken
if only some of them run. Imports, rebuilds of lookup tables, and other modules
allowed to run queries via hook in the middle of our process are good candidates.
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