I have a method that has the propagation = Propagation.REQUIRES_NEW transactional property:
#Transactional(propagation = Propagation.REQUIRES_NEW)
public void createUser(final UserBean userBean) {
//Some logic here that requires modification in DB
}
This method can be called multiple times simultaneously, and for every transaction if an error occurs than it's rolled back (independently from the other transactions).
The problem is that this might force Spring to create multiple transactions, even if another one is available, and may cause some performance problems.
Java doc of propagation = Propagation.REQUIRED says: Support a current transaction, create a new one if none exists.
This seems to solve the performance problem, doesn't it?
What about the rollback issue ? What if a new method call rolls back while using an existing transaction ? won't that rollback the whole transaction even the previous calls ?
[EDIT]
I guess my question wasn't clear enough:
We have hundreds of clients connected to our server.
For each client we naturally need to send a feedback about the transaction (OK or exception -> rollback).
My question is: if I use REQUIRED, does it mean only one transaction is used, and if the 100th client encounters a problem the 1st client's transaction will rollback as well ?
Using REQUIRES_NEW is only relevant when the method is invoked from a transactional context; when the method is invoked from a non-transactional context, it will behave exactly as REQUIRED - it will create a new transaction.
That does not mean that there will only be one single transaction for all your clients - each client will start from a non-transactional context, and as soon as the the request processing will hit a #Transactional, it will create a new transaction.
So, with that in mind, if using REQUIRES_NEW makes sense for the semantics of that operation - than I wouldn't worry about performance - this would textbook premature optimization - I would rather stress correctness and data integrity and worry about performance once performance metrics have been collected, and not before.
On rollback - using REQUIRES_NEW will force the start of a new transaction, and so an exception will rollback that transaction. If there is also another transaction that was executing as well - that will or will not be rolled back depending on if the exception bubbles up the stack or is caught - your choice, based on the specifics of the operations.
Also, for a more in-depth discussion on transactional strategies and rollback, I would recommend: «Transaction strategies: Understanding transaction pitfalls», Mark Richards.
If you really need to do it in separate transaction you need to use REQUIRES_NEW and live with the performance overhead. Watch out for dead locks.
I'd rather do it the other way:
Validate data on Java side.
Run everyting in one transaction.
If anything goes wrong on DB side -> it's a major error of DB or validation design. Rollback everything and throw critical top level error.
Write good unit tests.
Related
I have 2 questions related to each other
Q1 What exactly is a transaction boundary in hibernate/Spring Data JPA.
I am new to JPA , so please give a very basic example so I can understand as I tried to read multiple blogs but still not very clear.
Q2 And on top of it, what does this mean-
In hibernate, persist() method guarantees that it will not execute an INSERT statement if it is called outside of transaction boundaries, save() method does not guarantee the same.
What is outside and inside of a transaction boundary and how executions are performed outside boundaries?
A transaction is a unit of work that is either executed completely or not at all.
Transactions are fairly simple to use in a typical relational database.
You start a transaction by modifying some data. Every modification starts a transaction, you typically can't avoid it. You end the transaction by executing a commit or rollback.
Before your transaction is finished your changes can't be seen in other transactions (there are exceptions, variations and details). If you rollback your transaction all your changes in the database are undone.
If you commit your changes your changes become visible to other transactions, i.e. for other users connected to the same database. Implementations vary among many other things if changes become visible only for new transactions or also for already running transactions.
A transaction in JPA is a database transaction plus additional stuff.
You can start and end a transaction by getting a Transaction object and calling methods on it. But nobody does that anymore since it is error prone. Instead you annotate methods with #Transaction and entering the method will start a transaction and exiting the method will end the transaction.
The details are taken care of by Spring.
The tricky part with JPAs transactions is that within a JPA transaction JPA might (and will) choose to delay or even avoid read and write operations as long as possible. For example when you load an entity, and load it again in the same JPA transaction, JPA won't load it from the database but return the same instance it returned during the first load operation. If you want to learn more about this I recommend looking into JPAs first level cache.
A transaction boundary it's where the transaction starts or is committed/rollbacked.
When a transaction is started, the transaction context is bound to the current thread. So regardless of how many endpoints and channels you have in your Message flow your transaction context will be preserved as long as you are ensuring that the flow continues on the same thread. As soon as you break it by introducing a Pollable Channel or Executor Channel or initiate a new thread manually in some service, the Transactional boundary will be broken as well.
some other people ask about it - look it up.
If you do not understand something write to me again more accurately and I will explain.
I really hope I helped!
Why do I need Transaction in Hibernate for read-only operations?
Does the following transaction put a lock in the DB?
Example code to fetch from DB:
Transaction tx = HibernateUtil.getCurrentSession().beginTransaction(); // why begin transaction?
//readonly operation here
tx.commit() // why tx.commit? I don't want to write anything
Can I use session.close() instead of tx.commit()?
Transactions for reading might look indeed strange and often people don't mark methods for transactions in this case. But JDBC will create transaction anyway, it's just it will be working in autocommit=true if different option wasn't set explicitly. But there are practical reasons to mark transactions read-only:
Impact on databases
Read-only flag may let DBMS optimize such transactions or those running in parallel.
Having a transaction that spans multiple SELECT statements guarantees proper Isolation for levels starting from Repeatable Read or Snapshot (e.g. see PostgreSQL's Repeatable Read). Otherwise 2 SELECT statements could see inconsistent picture if another transaction commits in parallel. This isn't relevant when using Read Committed.
Impact on ORM
ORM may cause unpredictable results if you don't begin/finish transactions explicitly. E.g. Hibernate will open transaction before the 1st statement, but it won't finish it. So connection will be returned to the Connection Pool with an unfinished transaction. What happens then? JDBC keeps silence, thus this is implementation specific: MySQL, PostgreSQL drivers roll back such transaction, Oracle commits it. Note that this can also be configured on Connection Pool level, e.g. C3P0 gives you such an option, rollback by default.
Spring sets the FlushMode=MANUAL in case of read-only transactions, which leads to other optimizations like no need for dirty checks. This could lead to huge performance gain depending on how many objects you loaded.
Impact on architecture & clean code
There is no guarantee that your method doesn't write into the database. If you mark method as #Transactional(readonly=true), you'll dictate whether it's actually possible to write into DB in scope of this transaction. If your architecture is cumbersome and some team members may choose to put modification query where it's not expected, this flag will point you to the problematic place.
All database statements are executed within the context of a physical transaction, even when we don’t explicitly declare transaction boundaries (e.g., BEGIN, COMMIT, ROLLBACK).
If you don't declare transaction boundaries explicitly, then each statement will have to be executed in a separate transaction (autocommit mode). This may even lead to opening and closing one connection per statement unless your environment can deal with connection-per-thread binding.
Declaring a service as #Transactional will give you one connection for the whole transaction duration, and all statements will use that single isolation connection. This is way better than not using explicit transactions in the first place.
On large applications, you may have many concurrent requests, and reducing database connection acquisition request rate will definitely improve your overall application performance.
JPA doesn't enforce transactions on read operations. Only writes end up throwing a TransactionRequiredException in case you forget to start a transactional context. Nevertheless, it's always better to declare transaction boundaries even for read-only transactions (in Spring #Transactional allows you to mark read-only transactions, which has a great performance benefit).
Transactions indeed put locks on the database — good database engines handle concurrent locks in a sensible way — and are useful with read-only use to ensure that no other transaction adds data that makes your view inconsistent. You always want a transaction (though sometimes it is reasonable to tune the isolation level, it's best not to do that to start out with); if you never write to the DB during your transaction, both committing and rolling back the transaction work out to be the same (and very cheap).
Now, if you're lucky and your queries against the DB are such that the ORM always maps them to single SQL queries, you can get away without explicit transactions, relying on the DB's built-in autocommit behavior, but ORMs are relatively complex systems so it isn't at all safe to rely on such behavior unless you go to a lot more work checking what the implementation actually does. Writing the explicit transaction boundaries in is far easier to get right (especially if you can do it with AOP or some similar ORM-driven technique; from Java 7 onwards try-with-resources could be used too I suppose).
It doesn't matter whether you only read or not - the database must still keep track of your resultset, because other database clients may want to write data that would change your resultset.
I have seen faulty programs to kill huge database systems, because they just read data, but never commit, forcing the transaction log to grow, because the DB can't release the transaction data before a COMMIT or ROLLBACK, even if the client did nothing for hours.
Is there any risk to call beginTransaction on one session multiple times? I mean for example:
session.beginTransaction();
session.saveOrUpdate(object1);
// .... some works
session.beginTransaction();
session.delete(object2);
// ... some other works
session.getTransaction.commit();
I did this and it seems there is no problem (any exception or warning). In fact I want to know what happens when I use transaction in such a way.
The javadocs give an explanation
beginTransaction() Begin a unit of work and return the associated Transaction object. If a new underlying transaction is required, begin the transaction. Otherwise continue the new work in the context of the existing underlying transaction. The class of the returned Transaction object is determined by the property hibernate.transaction_factory.
So - it has no effect, it continues using the existing transaction. So don't do it, as it might confuse readers (including you). You can have multiple transactions if you commit the first and then start the second.
Yes. There wont be any compile/run time exceptions thrown. However there are undesired results that might come up, resulting in the partly unsaved data. I have faced this issues sometime back.
I am using annotation based declarative approach for spring aop.
sample code
ClassA{
#Transactional(readOnly = false, propagation = Propagation.REQUIRES_NEW)
add()
{
method1();
method2();
method3();
}
}
But I have still doubt on use of propagation.does propagation.Requires_New means that each request will start new transaction.
Second question:
Does failure of any method like method2,method3 will cause the transaction to rollback?
I will be very happy if any can help me to leans transaction propagation.
can someone provide me a real world example where we need a participate in existing transaction.because I visualise that add function that we are using in above example will be independent for all users,or any other function will be independent to each user who is calling. I am not able to find example where other propagation behaviour like PROPAGATION_SUPPORTS ,PROPAGATION_MANDATORY,PROPAGATION_REQUIRES_NEW etc. are used
Answering this comment, not the actual question:
transaction are session specific or
request specific – Vish 3 hours ago
Neither. request and session are both web-specific scopes, while the Spring Transaction abstraction has nothing to do with web technologies.
The scope of #Transactional is per method invocation, as #Transactional is implemented through Spring AOP. The transactional state is kept in ThreadLocal variables which are initialized when the outermost #Transactional method is entered and cleared with commit or rollback when it is left. This whole abstraction works on Java method level, and hence does not require or profit from a web container.
And in response to this Question in the comment below:
thanks #sean,i am stil not able to get
answer where other propagation
behaviour like PROPAGATION_SUPPORTS
,PROPAGATION_MANDATORY,PROPAGATION_REQUIRES_NEW
etc are used. please refer above for
whole question
Here's the list of Propagation values with my comments:
MANDATORY
Support a current transaction, throw an exception if
none exists.
Does not start a new Transaction, just checks whether a transaction is active (must be inside either another #Transactional method call or a programmatically created transaction)
NESTED
Execute within a nested transaction if a current transaction
exists, behave like
PROPAGATION_REQUIRED else.
Start a nested transaction if a transaction exists, start a new transaction otherwise.
NEVER
Execute non-transactionally, throw an exception if a transaction
exists.
Does not start a transaction. Fails if a transaction is present.
NOT_SUPPORTED
Execute non-transactionally, suspend the current transaction if one
exists.
Does not start a transaction. Suspends any existing transaction.
REQUIRED
Support a current transaction, create a new one if none
exists.
If a transaction exists, use that, if not, create a new one. In 95% of cases, this is what you need.
REQUIRES_NEW
Create a new transaction, suspend the current transaction if one
exists.
Always creates a new transaction, no matter if an existing transaction is present. If there is, it will be suspended for the duration of this method execution.
SUPPORTS
Support a current transaction, execute
non-transactionally if none exists.
Can use a transaction if one is present, but doesn't need one (and won't start a new one either)
In most cases, REQUIRED is what you need (hence it's the default in the #Transactional annotation). I have personally never seen any other value but REQUIRED and REQUIRES_NEW in use.
Transaction propagation indicates what should be the behaviour of the given method when it is invoked. REQUIRES_NEW means that a new transaction should always be started, even if there is an ongoing transaction.
If method1(), for example, defines REQUIRES_NEW, than it will execute in a new transaction.
An exception will rollback the current active transaction, yes.
Yes. Requires_New means that each request will start new transaction. and Yes failure in method2,method3 will cause the transaction to rollback, depending on the rollback properties.
check Transactional properties.
Because the TransactionManager has no way to register a XAResource manager, so that it can enlist XAResources in future Transactions.
The only way to solve this problem is to wrap the handle of the interested service interface.
Each method on the interface does the following:
Check if a tx is present
Figure out if the handle for this tx
If not already participating enlist its XAResource using Transaction.enlist(XAResource)
Registers a callback to enable cleanup using Transaction.registerSynchronization(Synchronization).
Does this seem like a reasonable strategy ?
It's a reasonable first draft, but there are some subtleties to watch out for. Getting the corner cases right yourself is hard - you may be better off writing a resource adaptor for your code and having a JCA handle the transaction plumbing, which is what most database and message queue drivers do.
Just because a tx context is present, that does not mean you can enlist with the tx. In particular the spec requires enlistResource to throw RollbackException if the tx is marked rollback only.
Just because the context is present and the tx has a valid state at the time you check it, that does not mean it remains present or valid throughout the lifetime of the method call. You can get race conditions in which the TM calls rollback on the resource whilst your business logic is still running.
The relationship between XAResource object instances, instances of the service interface and transactions is not particularly elegant due to XA's C heritage. In connection oriented APIs like JDBC it is normally one XAResource per Connection, with that XAResource instance potentially managing multiple tx contexts. For connectionless APIs you can use other patterns which may be simpler.
The Synchronization is not necessary. Nor is it necessarily desirable as Synchronizations are volatile so your cleanup won't get called in crash recovery situations. Better to clean up on commit/rollback in non-heuristic cases.