Does it only affect whatever commands were after the relevant BEGIN transaction?
For example:
BEGIN TRAN
UPDATE orders SET orderdate = '01-08-2013' WHERE orderno > '999'
Now, assume someone else performs a data import that inserts 10,000 new records into another table.
If I subsequently issue a ROLLBACK command, do those records get discarded or is it just the command above that gets rolled back?
Sorry if this a stupid question, I'm only just starting to use COMMIT and ROLLBACK.
Any transaction is confined to the connection it was opened on.
One of the four ACID properties of any relational database management system is Isolation. That means your actions are isolated from other connections and vice versa until you commit. Any change you do is invisible to other connections and if you roll it back they will never know it happened. That means in turn that changes that happened somewhere else are invisible to you until they are committed. Particularly that means that you can't ROLLBACK anyone else's changes.
The Isolation is achieved in one of two ways. One way is to "lock" the resource (e.g. the row). If that happens any other connection trying to read from that row has to wait until you finish your transaction.
The other way is to create a copy of the row that contains the old values. In this case all other connections will see the old version until you commit your transaction.
SQL Server can use both isolation methods. Which one is used depends on the isolation level you choose. The two Snapshot Isolation Levels provide the "copy method" the other four use the "lock method". The default isolation level of "read committed" is one of the "lock method" isolation levels.
Be aware however that the isolation level "READ UNCOMMITTED" basically circumvents these mechanisms and allows you to read changes that others started and have not yet committed. This is a special isolation level that can be helpful when diagnosing a problem but should be avoided in production code.
Related
There are many situations where I don't know what kind of isolation I should use in MySQL to avoid using corrupt data.
For example, let's say an application to make reservations for a theater. A user selects a seat and, within a transaction, it is checked that the selected seat is free and, if it is, the reservation is saved in a second step.
It may happen that while is verified that the seat is free and, before saving the reservation, another user reserve it first.
For situations like this and working with MySQL, what isolation level should I use?
Shadow's right about the need for application level support for this kind of operation. In a web app in particular, each page view or AJAX request is a separate operation and may use its own connection, so you can't hold a transaction in an uncommitted state for multiple pageviews.
That being said, the default REPEATABLE READ isolation level should serve you well if your transactions do SELECT ... FOR UPDATE ; UPDATE ; COMMIT sequences.
I am having trouble finding an answer to this using google or Stack Overflow, so perhaps people familiar with Percona XtraDB can help answer this. I fully understand how unexpected deadlocks can occur as outlined in this article, and the solution is to make sure you wrap your transactions with retry logic so you can restart them if they fail. We already do that.
https://www.percona.com/blog/2012/08/17/percona-xtradb-cluster-multi-node-writing-and-unexpected-deadlocks/
My questions is about normal updates that occur outside of a transaction in auto commit mode. Normally if you are writing only to a single SQL DB and perform an update, you get a last in wins scenario so whoever executes the statement last, is golden. Any other data is lost so if two updates occur at the same time, one of them will take hold and the others data is essentially lost.
Now what happens in a multi master environment with the same thing? The difference in cluster mode with multi master is that the deadlock can occur at the point where the commit happens as opposed to when the lock is first taken on the table. So in auto commit mode, the data will get written to the DB but then it could fail when it tries to commit that to the other nodes in the cluster if something else modified the exact same record at the same time. Clearly the simply solution is to re-execute the update again and it would seem to me that the database itself should be able to handle this, since it is a single statement in auto commit mode?
So is that what happens in this scenario, or do I need to start wrapping all my update code in retry handling as well and retry it myself when this fails?
Autocommit is still a transaction; a single statement transaction. Your single statement is just wrapped up in BEGIN/COMMIT for you. I believe your logic is inverted. In PXC, the rule is "commit first wins". If you start a manual transaction on node1 (ie: autocommit=0; BEGIN;) and UPDATE id=1 and don't commit then on node2 you autocommit an update to the same row, that will succeed on node2 and succeed on node1. When you commit the manual UPDATE, you will get a deadlock error. This is correct behavior.
It doesn't matter if autocommit or not; whichever commits first wins and the other transaction must re-try. This is the reason why we don't recommend writing to multiple nodes in PXC.
Yes, if you want to write to multiple nodes, you need to adjust your code to "try-catch-retry" handle this error case.
I'm starting out with MySQL trnsactions and I have a doubt:
In the documentation it says:
Beginning a transaction causes any pending transaction to be
committed. See Section 13.3.3, “Statements That Cause an Implicit
Commit”, for more information.
I have more or less 5 users on the same web application ( It is a local application for testing ) and all of them share the same MySQL user to interact with the database.
My question is: If I use transactions in the code and two of them start a transaction ( because of inserting, updating or something ) Could it be that the transactions interfere with each other?
I see in the statements that cause an implicit commit Includes starting a transaction. Being a local application It's fast and hard to tell if there is something wrong going on there, every query turns out as expected but I still have the doubt.
The implicit commit occurs within a session.
So for instance you start a transaction, do some updates and then forget to close the transaction and start a new one. Then the first transaction will implicitely committed.
However, other connections to the database will not be affected by that; they have their own transactions.
You say that 5 users use the same db user. That is okay. But in order to have them perform separate operations they should not use the same connection/session.
With MySQl by default each connection has autocommit turned on. That is, each connection will commit each query immediately. For an InnoDb table each transaction is therefore atomic - it completes entirely and without interference.
For updates that require several operations you can use a transaction by using a START TRANSACTION query. Any outstanding transactions will be committed, but this won't be a problem because mostly they will have been committed anyway.
All the updates performed until a COMMIT query is received are guaranteed to be completed entirely and without interference or, in the case of a ROLLBACK, none are applied.
Other transations from other connections see a consistent view of the database while this is going on.
This property is ACID compliance (Atomicity, Consistency, Isolation, Durability) You should be fine with an InnoDB table.
Other table types may implement different levels of ACID compliance. If you have a need to use one you should check it carefully.
This is a much simplified veiw of transaction handling. There is more detail on the MySQL web site here and you can read about ACID compliance here
I am working with MySQL 5.0 from python using the MySQLdb module.
Consider a simple function to load and return the contents of an entire database table:
def load_items(connection):
cursor = connection.cursor()
cursor.execute("SELECT * FROM MyTable")
return cursor.fetchall()
This query is intended to be a simple data load and not have any transactional behaviour beyond that single SELECT statement.
After this query is run, it may be some time before the same connection is used again to perform other tasks, though other connections can still be operating on the database in the mean time.
Should I be calling connection.commit() soon after the cursor.execute(...) call to ensure that the operation hasn't left an unfinished transaction on the connection?
There are thwo things you need to take into account:
the isolation level in effect
what kind of state you want to "see" in your transaction
The default isolation level in MySQL is REPEATABLE READ which means that if you run a SELECT twice inside a transaction you will see exactly the same data even if other transactions have committed changes.
Most of the time people expect to see committed changes when running the second select statement - which is the behaviour of the READ COMMITTED isolation level.
If you did not change the default level in MySQL and you do expect to see changes in the database if you run a SELECT twice in the same transaction - then you can't do it in the "same" transaction and you need to commit your first SELECT statement.
If you actually want to see a consistent state of the data in your transaction then you should not commit apparently.
then after several minutes, the first process carries out an operation which is transactional and attempts to commit. Would this commit fail?
That totally depends on your definition of "is transactional". Anything you do in a relational database "is transactional" (That's not entirely true for MySQL actually, but for the sake of argumentation you can assume this if you are only using InnoDB as your storage engine).
If that "first process" only selects data (i.e. a "read only transaction"), then of course the commit will work. If it tried to modify data that another transaction has already committed and you are running with REPEATABLE READ you probably get an error (after waiting until any locks have been released). I'm not 100% about MySQL's behaviour in that case.
You should really try this manually with two different sessions using your favorite SQL client to understand the behaviour. Do change your isolation level as well to see the effects of the different levels too.
I'm a bit confused by the documentation here. I have a transaction, which
start transaction
does some updates
does some selects
does some more updates
commit
I want my selects at step 3 to see the results of updates in step 2 but I want to be able to roll back the whole thing.
read committed seems to imply that selects only show data that's been committed, and repeatable read seems to imply that all subsequent selects will see the same data as existed at the time of the 1st select - thus ignoring my updates. read uncommitted seems to do the right thing, but: "but a possible earlier version of a row might be used" -- this is also not acceptable, as my selects MUST see the result of my updates.
is serializable really my only hope here?
I'm working off the documentation here
Transaction isolation levels describe only the interaction between concurrent transactions. With any isolation level, stuff that you have updated within the same transaction will be updated when you re-select them from that transaction.
The right isolation level in your case seems to be read commited, so you can rollback at any point and uncommited data is not visible in other transactions.