Referring to this, http://docs.oracle.com/cd/B19306_01/server.102/b14220/transact.htm
How the transactions are managed into the MySQL database.
My concern is, I have an application that is writing a few queries similar to the given link for completing a full transaction. I want to ensure that it always write correct & full transaction for final save & doesn't write incomplete transaction into SQL in case of power failure or some other circumstances to ensure the correctness of the transaction.
Just want to know how it can be implemented into MySQL db.
First, make sure you are using a transactional storage engine, such an InnoDB.
Next, make sure you understand the statements that cause implicit commit.
Read documentation on transactions
Related
I have a quick question that I can't seem to find online, not sure I'm using the right wording or not.
Do MySql database automatically synchronize queries or coming in at around the same time? For example, if I send a query to insert something to a database at the same time another connection sends a query to select something from a database, does MySQL automatically lock the database while the insert is happening, and then unlock when it's done allowing the select query to access it?
Thanks
Do MySql databases automatically synchronize queries coming in at around the same time?
Yes.
Think of it this way: there's no such thing as simultaneous queries. MySQL always carries out one of them first, then the second one. (This isn't exactly true; the server is far more complex than that. But it robustly provides the illusion of sequential queries to us users.)
If, from one connection you issue a single INSERT query or a single UPDATE query, and from another connection you issue a SELECT, your SELECT will get consistent results. Those results will reflect the state of data either before or after the change, depending on which query went first.
You can even do stuff like this (read-modify-write operations) and maintain consistency.
UPDATE table
SET update_count = update_count + 1,
update_time = NOW()
WHERE id = something
If you must do several INSERT or UPDATE operations as if they were one, you'll need to use the InnoDB engine, and you'll need to use transactions. The transaction will block SELECT operations while it is in progress. Teaching you to use transactions is beyond the scope of a Stack Overflow answer.
The key to understanding how a modern database engine like InnoDB works is Multi-Version Concurrency Control or MVCC. This is how simultaneous operations can run in parallel and then get reconciled into a consistent "view" of the database when fully committed.
If you've ever used Git you know how you can have several updates to the same base happening in parallel but so long as they can all cleanly merge together there's no conflict. The database works like that as well, where you can begin a transaction, apply a bunch of operations, and commit it. Should those apply without conflict the commit is successful. If there's trouble the transaction is rolled back as if it never happened.
This ability to juggle multiple operations simultaneously is what makes a transaction-capable database engine really powerful. It's an important component necessary to meet the ACID standard.
MyISAM, the original engine from MySQL 3.0, doesn't have any of these features and locks the whole database on any INSERT operation to avoid conflict. It works like you thought it did.
When creating a database in MySQL you have your choice of engine, but using InnoDB should be your default. There's really no reason at all to use MyISAM as any of the interesting features of that engine (e.g. full-text indexes) have been ported over to InnoDB.
It could be a dumb question, and tried to search for it and found nothing.
I been using mysql for years(not that to long) but i never had tried mysql transactions.
Now my question is, what would happen if i issue an insert or delete statement from multiple clients using transactions? does it would lock the table and prevent other client to perform there query?
what would happen if other client issue a transaction query while the other client still have unfinished transaction?
I appreciate for any help will come.
P.S. most likely i will use insert using a file or csv it could be a big chunk of data or just a small one.
MySQL automatically performs locking for single SQL statements to keep clients from interfering with each other, but this is not always sufficient to guarantee that a database operation achieves its intended result, because some operations are performed over the course of several statements. In this case, different clients might interfere with each other.
Source: http://www.informit.com/articles/article.aspx?p=2036581&seqNum=12
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 have a server which sends up to 20 UPDATE statements to a separate MySQL server every 3-5 seconds for a game. My question is, is it faster to concat them together(UPDATE;UPDATE;UPDATE). Is it faster to do them in a transaction then commit the transaction? Is it faster to just do each UPDATE individually?
Any insight would be appreciated!
It sort of depends on how the server connects. If the connection between the servers is persistent, you probably won't see a great deal of difference between concatenated statements or multiple separate statements.
However, if the execution involves establishing the connection, executing the SQL statement, then tearing down the connection, you will save a lot of resources on the database server by executing multiple statements at a time. The process of establishing the connection tends to be an expensive and time-consuming one, and has the added overhead of DNS resolution since the machines are separate.
It makes the most logical sense to me to establish the connection, begin a transaction, execute the statements individually, commit the transaction and disconnect from the database server. Whether you send all the UPDATE statements as a single concatenation or multiple individual statements is probably not going to make a big difference in this scenario, especially if this just involves regular communication between these two servers and you need not expect it to scale up with user load, for example.
The use of the transaction assumes that your 3-5 second periodic bursts of UPDATE statements are logically related somehow. If they are not interdependent, then you could skip the transaction saving some resources.
As with any question regarding performance, the best answer is if your current system is meeting your performance and scaling needs, you ought not pay too much attention to micro-optimizing it just yet.
It is always faster to wrap these UPDATEs into single transaction block.
Price for this is that if anything fails inside that block it would be that nothing happened at all - you will have to repeat your work again.
Aslo, keep in mind that transactions in MySQL only work when using InnoDB engine.
I'm running mySql server that being updated every 4 hours.
In the mean while data can be retrieved.
Does mySql handle this scenario where the DB is being updated
and a query from the user is received?
or I should handle this scenario?
Is it possible to create a snapshot of the DB just before the update takes place
and query this DB?
Thanks
If you are worried about how concurrent writes are handled, that's one thing, but you can safely read from a MySQL database that is being updated by another thread without worrying about corrupted data.
Of course, if the writing thread is performing multiple writes that need to be atomic (ie all or none), make sure he is using a transaction.
You can use transactions and/or locking to make sure that every user sees a consistent view (i.e. completely updated or not updated at all). The database engine itself is very well suited to the scenario. If you are not worried about what readers see at the exact time the updates are running, you don't need to do anything.
Considering my scenario I think that no further actions are required.
If the write comes before the read the user will get the most updated data
otherwise it does not, and he will wait for next query.
This fits my requirements.
Thank you all