I'm sorry, this is a very general question but I will try to narrow it down.
I'm new to this whole transaction thing in MySQL/PHP but it seems pretty simple. I'm just using mysql not mysqli or PDO. I have a script that seems to be rolling back some queries but not others. This is uncharted territory for me so I have no idea what is going on.
I start the transaction with mysql_query('START TRANSACTION;'), which I understand disables autocommit at the same time. Then I have a lot of complex code and whenever I do a query it is something like this mysql_query($sql) or $error = "Oh noes!". Then periodically I have a function called error_check() which checks if $error is not empty and if it isn't I do mysql_query('ROLLBACK;') and die($error). Later on in the code I have mysql_query('COMMIT;'). But if I do two queries and then purposely throw an error, I mean just set $error = something, it looks like the first query rolls back but the second one doesn't.
What could be going wrong? Are there some gotchas with transactions I don't know about? I don't have a good understanding of how these transactions start and stop especially when you mix PHP into it...
EDIT:
My example was overly simplified I actually have at least two transactions doing INSERT, UPDATE or DELETE on separate tables. But before I execute each of those statements I backup the rows in corresponding "history" tables to allow undoing. It looks like the manipulation of the main tables gets rolled back but entries in the history tables remain.
EDIT2:
Doh! As I finished typing the previous edit it dawned on me...there must be something wrong with those particular tables...for some reason they were all set as MyISAM.
Note to self: Make sure all the tables use transaction-supporting engines. Dummy.
I'd recommend using the mysqli or PDO functions rather than mysql, as they offer some worthwhile improvements—especially the use of prepared statements.
Without seeing your code, it is difficult to determine where the problem lies. Given that you say your code is complex, it is likely that the problem lies with your code rather than MySQL transactions.
Have you tried creating some standalone test scripts? Perhaps you could isolate the SQL statements from your application, and create a simple script which simply runs them in series. If that works, you have narrowed down the source of the problem. You can echo the SQL statements from your application to get the running order.
You could also try testing the same sequence of SQL statements from the MySQL client, or through PHPMyAdmin.
Are your history tables in the same database?
Mysql transactions only work using the mysqli API (not the classic methods). I have been using transactions. All I do is deactivate autocommit and run my SQL statements.
$mysqli->autocommit(FALSE);
SELECT, INSERT, DELETE all are supported. as long as Im using the same mysqli handle to call these statements, they are within the transaction wrapper. nobody outside (not using the same mysqli handle) will see any data that you write/delete using INSERT/DELETE as long as the transaction is still open. So its critical you make sure every SQL statement is fired with that handle. Once the transaction is committed, data is made available to other db connections.
$mysqli->commit();
Related
This is odd. I'm running a query with just a single INSERT, preceded by a SET statement. The query looks something like this:
SET #discount:=(SELECT discount * :applyDiscount FROM fra_cus WHERE customerID=:customerID AND franchiseID=:franchiseID);
INSERT INTO discounts_applied (unitID, franchiseID, customerID, amount)
VALUES(:unitID, :franchiseID, :customerID, #discount * :price);
It appears that if I prepare these as two separate PDO queries, lastInsertID() works fine... but if I prepare them and execute them in the same statement, lastInsertID() returns nothing.
It's not the end of the world, but it's annoying. Anyone know why this would be the case? For the record, there's a reason I need to define #discount as a variable (pertains to triggers on one of the tables). Also this is all taking place within a larger transaction.
First of all, I would strongly recommend to run every query in a distinct API call. This is how an Application Programming Interface is intended to work.
It won't only prevent situations like this but also will make your code a multitude times more readable and maintainable.
And it will make your code much safer too. You can run multiple statements in a single call only at the expense of the native prepared statements. However virtual this vulnerability is, why taking chances at all?
Why not to make a regular SELECT query instead of SET, get the resulting value into a PHP variable and then use it among other variables, just through a placeholder? I don't see any reason why there should be such a complex way to deal with simple data.
In case I failed to convince you, the reason is simple. You are running two queries, and the first one doesn't trigger any insert ids. And obviously, you need this query's metadata (errors, affected rows, whatever), not the other one's first. So you get it. And to get the second'query's metadata you have to ask a database for it. The process is explained in my article: Treating PHP delusions - The only proper PDO tutorial: Running multiple queries with PDO. Basically PDOStatement::nextRowset() is what you need.
So, let's say I'm using two drivers at the same time (in the specific mysql and sqlite3)
I have a set of changes that must be commit()ted on both connections only if both dbms didn't fail, or rollBack()ed if one or the another did fail:
<?php
interface DBList
{
function addPDO(PDO $connection);
// calls ->rollBack() on all the pdo instances
function rollBack();
// calls ->commit() on all the pdo instances
function commit();
// calls ->beginTransaction() on all the pdo instances
function beginTransaction();
}
Question is: will it actually work? Does it make sense?
"Why not use just mysql?" you would say! I'm not a masochist! I need mysql for the classic fruition via my application, but I also need to keep a copy of a table that is always synchronized and that is also downloadable and portable!
Thank you a lot in advance!
I suspect you put the cart before the horses! If
two databases are in sync
a transaction commits successfully on one DB
No OS-level error occures
then the transaction will also commit successully on the second DB.
So what you would want to do is:
- Start the transaction on MySQL
- Record all data-changing SQL (see later)
- Commit the transaction on MySQL
- If the commit works, run the recorded SQL against SQlite
- if not, roll back MySQL
Caveat: The assumption above is only valid, if the sequence of transactions is identical on both DBs. So you would want to record the SQL into a MySQL table, which is subject to the same transaction logic as the rest. This does the serialization work for you.
You mistake PDO with a database server. PDO is just an interface, pretty much like the database console. It doesn't perform any data operations of its own. It cannot insert or select data. It cannot perform data locks or transactions. All it can do is to send your command to database server and bring back results if any. It's just an interface. It doesn't have transactions on it's own.
So, instead of such fictional trans-driver transactions you can use regular ones.
Start two, one for each driver, and then rollback them accordingly. By the way, with PDO one don't have to rollback manually. Just set PDO in exception mode, write your queries and add commit at the end. In case one of queries failed, all started transactions will be rolled back automatically due to script termination.
A long time ago I wrote a php class that handles postgresql db connections.
I've added transactions to my insert/update functions and it works just fine for me. But recently I found out about the "pg_prepare" function.
I'm a bit confused about what that function does and if it'll be better to switch to it.
Currently whenever I do an insert/update my sql looks like this:
$transactionSql = "PREPARE TRANSACTION ".md5(time()).";"
.$theUpdateOrDeleteSQL.";".
."COMMIT;";
This will return something like:
PREPARE TRANSACTION '4601a2e4b4aa2632167d3cc62b516e6d';
INSERT INTO users (username,g_id,email,password)
VALUES('test', '1', 'test','1234');
COMMIT;
I've structured my database with relations and I'm using (when it's possible):
ON DELETE CASCADE
ON UPDATE CASCADE
But I want to be 100% sure things are clean in the database and there are no leftovers if/when there is a failure upon updating/deleting or inserting.
It would be nice if you can share your opinion/experience about pg_prepare, do I really need the "prepare transaction" and any other addition things that might help me? :)
No, you don't need 2 phase commit !...
For safe PHP database handling, do not use pg_query directly, rather wrap it in a special function which does the following :
opens the database connection on your first query
if using persistent connections, ensure the connection is in a known state
register_shutdown_function to a function that issues a ROLLBACK
make sure autocommit is off, or simply issue a BEGIN before the first query
log database error and slow queries
only uses pg_query_params() which takes care of sql injections nicely
That way, if your script crashes or whatever, a rollback is issued automatically. You can only commit by explicitly comitting.
If you use persistent connections beware : php's handling of pg_pconnect is a little ... buggy.
No you don't need prepare transaction (that is intended for distributed transactions across different servers - as Milen has already pointed out.
I'm not sure how the PHP interface handles that, but as long as you can make sure you are not running in auto commit mode, things should be fine.
If you can't control the auto commit mode, simply put your statements into a BEGIN ... COMMIT block
Can I insert something into a MySQL database using PHP and then immediately make a call to access that, or is the insert asynchronous (in which case the possibility exists that the database has not finished inserting the value before I query it)?
What I think the OP is asking is this:
<?
$id = $db->insert(..);
// in this case, $row will always have the data you just inserted!
$row = $db->select(...where id=$id...)
?>
In this case, if you do a insert, you will always be able to access the last inserted row with a select. That doesn't change even if a transaction is used here.
If the value is inserted in a transaction, it won't be accessible to any other transaction until your original transaction is committed. Other than that it ought to be accessible at least "very soon" after the time you commit it.
There are normally two ways of using MySQL (and most other SQL databases, for that matter):
Transactional. You start a transaction (either implicitly or by issuing something like 'BEGIN'), issue commands, and then either explicitly commit the transaction, or roll it back (failing to take any action before cutting off the database connection will result in automatic rollback).
Auto-commit. Each statement is automatically committed to the database as it's issued.
The default mode may vary, but even if you're in auto-commit mode, you can "switch" to transactional just by issuing a BEGIN.
If you're operating transactionally, any changes you make to the database will be local to your db connection/instance until you issue a commit. Issuing a commit should block until the transaction is fully committed, so once it returns without error, you can assume the data is there.
If you're operating in auto-commit (and your database library isn't doing something really strange), you can rely on data you've just entered to be available as soon as the call that inserts the data returns.
Note that best practice is to always operate transactionally. Even if you're only issuing a single atomic statement, it's good to be in the habit of properly BEGINing and COMMITing a transaction. It also saves you from trouble when a new version of your database library switches to transactional mode by default and suddenly all your one-line SQL statements never get committed. :)
Mostly the answer is yes. You would have to do some special work to force a database call to be asynchronous in the way you describe, and as long as you're doing it all in the same thread, you should be fine.
What is the context in which you're asking the question?
For fun I am replacing the mysqli extension in my app with PDO.
Once in awhile I need to use transactions + table locking.
In these situations, according to the mysql manual, the syntax needs to be a bit different. Instead of calling START TRANSACTION, you do it like so...
SET autocommit=0;
LOCK TABLES t1 WRITE, t2 READ, ...;
... do something with tables t1 and t2 here ...
COMMIT;
UNLOCK TABLES;
(http://dev.mysql.com/doc/refman/5.0/en/lock-tables-and-transactions.html)
My question is, how does this interact with PDO::beginTransaction? Can I use PDO::beginTransaction in this case? Or should I manually send the sql "SET autocommit = 0; ... etc".
Thanks for the advice,
When you call PDO::beginTransaction(), it turns off auto commit.
So you can do:
$db->beginTransaction();
$db->exec('LOCK TABLES t1, t2, ...');
# do something with tables
$db->commit();
$db->exec('UNLOCK TABLES');
After a commit() or rollBack(), the database will be back in auto commit mode.
I have spent a huge amount of time running around this issue, and the PHP documentation in this area is vague at best. A few things I have found, running PHP 7 with a MySQL InnoDB table:
PDO::beginTransaction doesn't just turn off autocommit, having tested the answer provided by Olhovsky with code that fails, rollbacks do not work; there is no transactional behaviour. This means it can't be this simple.
Beginning a transaction may be locking the used tables... I eagerly await for someone to tell me I'm wrong with this, but here are the reasons it could be: This comment, which shows a table being inaccessible when a transaction has started, without being locked. This PHP documentation page, that slips in on the end:
... while the transaction is active, you are guaranteed that no one else can make changes while you are in the middle of your work
To me this behaviour is quite smart, and also provides enough wiggle room for PDO to cope with every database, which is after all the aim. If this is what is going on though, its just massively under documented and should've been called something else to avoid confusion with a true database transaction, which doesn't imply locking.
Charles' answer I think is probably the best if you are after certainty with a workload that will require high concurrency; do it by hand using explicit queries to the database, then you can go by the database's documentation.
Update
I have had a production server up and running using the PDO transaction functions for a while now, recently using AWS's Aurora database (fully compatible with MySQL but built to automatically scale etc). I have proven these two points to myself:
Transactions (purely the ability to commit all database changes together) work using PDO::beginTransaction(). In short, I know many scripts have failed half way through their database select/updates and data integrity has been maintained.
Table locking isn't happening, I've had an index duplication error to prove this.
So, to further my conclusion, looks like the behaviour of these functions seems to change based on database engine (and possibly other factors). As far as I can tell both from experience and the documentation, there is no way to know programmatically what is going on... whoop...
In MySQL, beginning a transaction is different than turning off autocommit, due to how LOCK/UNLOCK TABLES works. In MySQL, LOCK TABLES commits any open transactions, but turning off autocommit isn't actually starting a transaction. MySQL is funny that way.
In PDO, starting a transaction using beginTransaction doesn't actually start a new transaction, it just turns off autocommit. In most databases, this is sane, but it can have side effects with MySQL's mentioned behavior.
You probably should not rely on this behavior and how it interacts with MySQL's quirks. If you're going to deal with MySQL's behavior for table locking and DDL, you should avoid it. If you want autocommit off, turn it off by hand. If you want to open a transaction, open a transaction by hand.
You can freely mix the PDO API methods of working with transactions and SQL commands when not otherwise working with MySQL's oddities.