PHP, MySQL, PDO Transactions - Can fetchAll() come before commit()? - php

Question

More transaction questions!
What I have right now is a mess of strung-together queries, that are all manually reversed if any fail:
Code Block 1
$stmt1 = $db->prepare(...); // Update table1, set col=col+1
if($stmt1 = $db->execute(...)){
$stmt2 = $db->prepare(...); // Insert into table2, id=12345
if($stmt2 = $db->execute(...)){
$stmt3 = $db->prepare(...); // Select val from table3
if($stmt3 = $db->execute(...)){
$result = $stmt3->fetchAll();
if($result[0]['val'] == something){
$stmt4 = $db->prepare(...); // Update table4, set status=2
if($stmt4 = $db->execute(...)){
return true;
}else{
$stmt1 = $db->prepare(...); // Update table1, set col=col-1 (opposite of above)
$stmt1 = $db->execute(...);
$stmt2 = $db->prepare(...); // Delete from table2, where id=12345 (opposite of above)
$stmt2 = $db->execute(...);
return false;
}
}
return true;
}else{
$stmt1 = $db->prepare(...); // Update table1, set col=col-1 (opposite of above)
$stmt1 = $db->execute(...);
$stmt2 = $db->prepare(...); // Delete from table2, where id=12345 (opposite of above)
$stmt2 = $db->execute(...);
return false;
}
}else{
$stmt1 = $db->prepare(...); // Update table1, set col=col-1 (opposite of above)
$stmt1 = $db->execute(...);
return false;
}
}
It's a mess, difficult to debug, difficult to add on to, difficult to understand when the queries are large, and won't return all tables back to original state if the connection is lost mid-way through.
This same process is even worse when deleting a row, because everything in it needs to be stored - just in case the operation needs to be undone.
Now, I know most of this will still work when I port it over to a single transaction, but the one part I am unsure of is:
Code Block 2
$result = $stmt3->fetchAll();
if($result[0]['val'] == something){
... continue ...
}else{
... reverse operations ...
return false;
}
Because the results-gathering would take place before the commit() in the transaction. As follows:
Code Block 3
$db->beginTransaction();
try{
$stmt1 = $db->prepare(...);
$stmt1->execute();
$stmt2 = $db->prepare(...);
$stmt2->execute();
$stmt3 = $db->prepare(...);
$stmt3->execute();
$result = $stmt3->fetchAll();
if($result[0]['val'] == something){
$stmt4 = $db->prepare(...);
$stmt4->execute();
}else{
$db->rollBack();
return false;
}
$db->commit();
return true;
}catch(Exception $e){
$db->rollBack();
throw $e;
return false;
}
Will this work? Specifically, can I include the $result = $stmt3->fetchAll(); before the commit(), and then execute the conditional query?
Also, I'm not entirely sure on this, but do I require the $db->rollBack(); within the try block, if the code is exited (return false) before the commit()?

Answer 1

Your first question:
Specifically, can I include the $result = $stmt3->fetchAll(); before the commit(), and then execute the conditional query?
I see no reason why it should not work. A transaction behaves basically the same as operations without transactions - except that changes are only drafts. Any changes you make in the previous statements will be applied to a "working copy" valid for this single session only. For you it will appear completely transparent. However any changes will be rolled back if you do not commit them.
Also worth noting (emphasis mine):
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.
This can cause racing conditions.
Your second question:
Also, I'm not entirely sure on this, but do I require the $db->rollBack(); within the try block, if the code is exited (return false) before the commit()?
From the documentation it says:
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.
Therefore you do not necessarily require to roll back manually as it will be done by the driver itself.
However note the following from the same source as well:
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.
So be sure to check the compatibility beforehand!
A few notes
Do NOT begin a transaction in another transaction. This will commit the first transaction implicitely. See this comment.
Another note from the documentation:
Some databases, including MySQL, automatically issue an implicit COMMIT when a database definition language (DDL) statement such as DROP TABLE or CREATE TABLE is issued within a transaction. The implicit COMMIT will prevent you from rolling back any other changes within the transaction boundary.