I'm using spring-data-jpa interface CrudRepository to save large data sets in a database on a daily batch import.
#Bean
public ItemWriter<MyEntity> jpaItemWriter() {
RepositoryItemWriter<MyEntity> writer = new RepositoryItemWriter<>();
writer.setRepository(repository);
writer.setMethodName("save");
return writer;
}
The default implementation for this interface is SimpleJpaRepository, which offers a saveAndFlush() method. What is that for? Would this method be any help for me, eg regarding performance, if I run this method rather than save()?
One example would be if you were using Optimistic Locking and wanted to explicitly catch an OptimisticLockException and throw it back to the client. If the changes are only flushed to the database on transaction commit (i.e. when your transactional method returns) then you cannot do so. Explicity flushing from within your transactional method allows you to catch and rethrow/handle.
From the JPA Specification:
3.4.5 OptimisticLockException Provider implementations may defer writing to the database until the end of the transaction, when
consistent with the lock mode and flush mode settings in effect. In
this case, an optimistic lock check may not occur until commit time,
and the OptimisticLockException may be thrown in the "before
completion" phase of the commit. If the OptimisticLockException must
be caught or handled by the application, the flush method should be
used by the application to force the database writes to occur. This
will allow the application to catch and handle optimistic lock
exceptions
So, in answer to your question, it is not performance related but there may be cases when you want to explicitly flush to the database from within a transactional method.
According to Spring Data's Javadoc, saveAndFlush:
Saves an entity and flushes changes instantly.
if you using save method, it flushes changes when the underlying transaction commits.
Related
Does a propagation-Required default #Transactional collects all queries and executes them at the end of the method altogether or does it open a db transaction and executes BEGIN, every query as it finds it and when transaction finishes executes COMMIT?
Is this what is referred as Logical vs Physical transactions?
I am wondering that because I am using a #Transactional tests that executes GET endpoint + DELETE endpoont + GET endpoint with READ_UNCOMMITED, behavior manages to work well, but I see no trace of delete queries in the logs, only selects.
I would have expected I see all the queries issued and then a rollback, but I have the feeling that the transaction is just modifying the managed entities of the persistance context and just tries to save by the end of the test...
If I should be seeing all the delete queries as the repository.removes() are executed then it might be that for some reason hibernate is only logging queries out of a readonly=false transaction
Maybe this answer helps you: JPA flush vs commit
If there is an active transaction, JPA/Hibernate will execute the flush method when transaction is committed. Meanwhile, all changes applied to entities are collected in the Unit of Work.
In flush() the changes to the data are reflected in database after encountering flush, but it is still in transaction.flush() MUST be enclosed in a transaction context and you don't have to do it explicitly unless needed (in rare cases), when EntityTransaction.commit() does that for you.
You can change this behavior changing the flush strategy.
Is it good practice to call org.hibernate.Session.flush() separately?
As said in org.hibernate.Session docs,
Must be called at the end of a unit of work, before commiting the transaction and closing the session (depending on flush-mode, Transaction.commit() calls this method).
Could you explain the purpose of calling flush() explicitely if org.hibernate.Transaction.commit() will do it already?
In the Hibernate Manual you can see this example
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
for (int i = 0; i < 100000; i++) {
Customer customer = new Customer(...);
session.save(customer);
if (i % 20 == 0) { // 20, same as the JDBC batch size
// flush a batch of inserts and release memory:
session.flush();
session.clear();
}
}
tx.commit();
session.close();
Without the call to the flush method, your first-level cache would throw an OutOfMemoryException
Also you can look at this post about flushing
flush() will synchronize your database with the current state of object/objects held in the memory but it does not commit the transaction. So, if you get any exception after flush() is called, then the transaction will be rolled back.
You can synchronize your database with small chunks of data using flush() instead of committing a large data at once using commit() and face the risk of getting an OutOfMemoryException.
commit() will make data stored in the database permanent. There is no way you can rollback your transaction once the commit() succeeds.
One common case for explicitly flushing is when you create a new persistent entity and you want it to have an artificial primary key generated and assigned to it, so that you can use it later on in the same transaction. In that case calling flush would result in your entity being given an id.
Another case is if there are a lot of things in the 1st-level cache and you'd like to clear it out periodically (in order to reduce the amount of memory used by the cache) but you still want to commit the whole thing together. This is the case that Aleksei's answer covers.
flush(); Flushing is the process of synchronizing the underlying persistent store with persistable state held in memory. It will update or insert into your tables in the running transaction, but it may not commit those changes.
You need to flush in batch processing otherwise it may give
OutOfMemoryException.
Commit(); Commit will make the database commit. When you have a persisted object and you change a value on it, it becomes dirty and hibernate needs to flush these changes to your persistence layer. So, you should commit but it also ends the unit of work (transaction.commit()).
It is usually not recommended to call flush explicitly unless it is necessary. Hibernate usually auto calls Flush at the end of the transaction and we should let it do it's work. Now, there are some cases where you might need to explicitly call flush where a second task depends upon the result of the first Persistence task, both being inside the same transaction.
For example, you might need to persist a new Entity and then use the Id of that Entity to do some other task inside the same transaction, on that case it's required to explicitly flush the entity first.
#Transactional
void someServiceMethod(Entity entity){
em.persist(entity);
em.flush() //need to explicitly flush in order to use id in next statement
doSomeThingElse(entity.getId());
}
Also Note that, explicitly flushing does not cause a database commit, a database commit is done only at the end of a transaction, so if any Runtime error occurs after calling flush the changes would still Rollback.
By default flush mode is AUTO which means that: "The Session is sometimes flushed before query execution in order to ensure that queries never return stale state", but most of the time session is flushed when you commit your changes. Manual calling of the flush method is usefull when you use FlushMode=MANUAL or you want to do some kind of optimization. But I have never done this so I can't give you practical advice.
session.flush() is synchronise method means to insert data in to database sequentially.if we use this method data will not store in database but it will store in cache,if any exception will rise in middle we can handle it.
But commit() it will store data in database,if we are storing more amount of data then ,there may be chance to get out Of Memory Exception,As like in JDBC program in Save point topic
I am looking at EntityManager API, and I am trying to understand an order in which I would do a record lock. Basically when a user decides to Edit a record, my code is:
entityManager.getTransaction().begin();
r = entityManager.find(Route.class, r.getPrimaryKey());
r.setRoute(txtRoute.getText());
entityManager.persist(r);
entityManager.getTransaction().commit();
From my trial and error, it appears I need to set WWEntityManager.entityManager.lock(r, LockModeType.PESSIMISTIC_READ); after the .begin().
I naturally assumed that I would use WWEntityManager.entityManager.lock(r, LockModeType.NONE); after the commit, but it gave me this:
Exception Description: No transaction is currently active
I haven't tried putting it before the commit yet, but wouldn't that defeat the purpose of locking the record, since my goal is to avoid colliding records in case 50 users try to commit a change at once?
Any help as to how to I can lock the record for the duration of the edit, is greatly appreciated!
Thank You!
Performing locking inside transaction makes perfectly sense. Lock is automatically released in the end of the transaction (commit / rollback). Locking outside of transaction (in context of JPA) does not make sense, because releasing lock is tied to end of the transaction. Also otherwise locking after changes are performed and transaction is committed does not make too much sense.
It can be that you are using pessimistic locking to purpose other than what they are really for. If my assumption is wrong, then you can ignore end of the answer. When your transaction holds pessimistic read lock on entity (row), following is guaranteed:
No dirty reads: other transactions cannot see results of operations you performed to locked rows.
Repeatable reads: no modifications from other transactions
If your transaction modifies locked entity, PESSIMISTIC_READ is upgraded to PESSIMISTIC_WRITE or transaction fails if lock cannot be upgraded.
Following coarsely describes scenario with obtaining locking in the beginning of transaction:
entityManager.getTransaction().begin();
r = entityManager.find(Route.class, r.getPrimaryKey(),
LockModeType.PESSIMISTIC_READ);
//from this moment on we can safely read r again expect no changes
r.setRoute(txtRoute.getText());
entityManager.persist(r);
//When changes are flushed to database, provider must convert lock to
//PESSIMISTIC_WRITE, which can fail if concurrent update
entityManager.getTransaction().commit();
Often databases do not have separate support for pessimistic read, so you are actually holding lock to row since PESSIMISTIC_READ. Also using PESSIMISTIC_READ makes sense only if no changes to the locked row are expected. In case above changes are done always, so using PESSIMISTIC_WRITE from the beginning on is reasonable, because it saves you from the risk of concurrent update.
In many cases it also makes sense to use optimistic instead of pessimistic locking. Good examples and some comments about choosing between locking strategies can be found from: Locking and Concurrency in Java Persistence 2.0
Great work attempting to be safe in write locking your changing data. :) But you might be going overboard / doing it the long way.
First a minor point. The call to persist() isn't needed. For update, just modify the attributes of the entity returned from find(). The entityManager automatically knows about the changes and writes them to the db during commit. Persist is only needed when you create a new object & write it to the db for the first time (or add a new child object to a parent relation and which to cascade the persist via cascade=PERSIST).
Most applications have a low probability of 'clashing' concurrent updates to the same data by different threads which have their own separate transactions and separate persistent contexts. If this is true for you and you would like to maximise scalability, then use an optimistic write lock, rather than a pessimistic read or write lock. This is the case for the vast majority of web applications. It gives exactly the same data integrity, much better performance/scalability, but you must (infrequently) handle an OptimisticLockException.
optimistic write locking is built-in automatically by simply having a short/integer/long/TimeStamp attribute in the db and entity and annotating it in the entity with #Version, you do not need to call entityManager.lock() in that case
If you were satisfied with the above, and you added a #Version attribute to your entity, your code would be:
try {
entityManager.getTransaction().begin();
r = entityManager.find(Route.class, r.getPrimaryKey());
r.setRoute(txtRoute.getText());
entityManager.getTransaction().commit();
} catch (OptimisticLockException e) {
// Logging and (maybe) some error handling here.
// In your case you are lucky - you could simply rerun the whole method.
// Although often automatic recovery is difficult and possibly dangerous/undesirable
// in which case we need to report the error back to the user for manual recovery
}
i.e. no explicit locking at all - the entity manager handles it automagically.
IF you had a strong need to avoid concurrent data update "clashes", and are happy to have your code with limited scalability then serialise data access via pessimistic write locking:
try {
entityManager.getTransaction().begin();
r = entityManager.find(Route.class, r.getPrimaryKey(), LockModeType.PESSIMISTIC_WRITE);
r.setRoute(txtRoute.getText());
entityManager.getTransaction().commit();
} catch (PessimisticLockException e) {
// log & rethrow
}
In both cases, a successful commit or an exception with automatic rollback means that any locking carried out is automatically cleared.
Cheers.
I have the following piece of code that inserts or updates a bean in the database.
I have a static function in the HibernateUtil that returns a singleton instance from the Hibernate session.
hibSession = HibernateUtil.currentSession();
hibSession.saveOrUpdate(bean);
hibSession.flush();
This is existing code, I am wondering if there is any reason that make the programmer use flush instead of simply committing and what flush does exactly.
The flush() method synchronizes modifications bound to the current persistence context with the underlying DB. The flush() method does not end the running transaction.
One concrete usage of the flush() method is to force database triggers or generator logic (generated ID, for instance) to execute.
flush is re-syncs the DB with Hibernate. It is useful if you have a trigger set on a table. The trigger will run on flush and does not need the transaction to commit().
flush synchronises the Hibernate session with the database, commit ends the database transaction.
How can I configure Hibernate to apply all saves, updates, and deletes to the database server immediately after the session executes each operation? By default, Hibernate enqueues all save, update, and delete operations and submits them to the database server only after a flush() operation, committing the transaction, or the closing of the session in which these operations occur.
One benefit of immediately flushing database "write" operations is that a program can catch and handle any database exceptions (such as a ConstraintViolationException) in the code block in which they occur. With late or auto-flushing, these exceptions may occur long after the corresponding Hibernate operation that caused the SQL operation.
Update:
According to the Hibernate API documentation for interface Session, the benefit of catching and handling a database exception before the session ends may be of no benefit at all: "If the Session throws an exception, the transaction must be rolled back and the session discarded. The internal state of the Session might not be consistent with the database after the exception occurs."
Perhaps, then, the benefit of surrounding an "immediate" Hibernate session write operation with a try-catch block is to catch and log the exception as soon as it occurs. Does immediate flushing of these operations have any other benefits?
How can I configure Hibernate to apply all saves, updates, and deletes to the database server immediately after the session executes each operation?
To my knowledge, Hibernate doesn't offer any facility for that. However, it looks like Spring does and you can have some data access operations FLUSH_EAGER by turning their HibernateTemplate respectively HibernateInterceptor to that flush mode (source).
But I warmly suggest to read the javadoc carefully (I'll come back on this).
By default, Hibernate enqueues all save, update, and delete operations and submits them to the database server only after a flush() operation, committing the transaction, or the closing of the session in which these operations occur.
Closing the session doesn't flush.
One benefit of immediately flushing database "write" operations is that a program can catch and handle any database exceptions (such as a ConstraintViolationException) in the code block in which they occur. With late or auto-flushing, these exceptions may occur long after the corresponding Hibernate operation that caused the SQL operation
First, DBMSs vary as to whether a constraint violation comes back on the insert (or update ) or on the subsequent commit (this is known as immediate or deferred constraints). So there is no guarantee and your DBA might even not want immediate constraints (which should be the default behavior though).
Second, I personally see more drawbacks with immediate flushing than benefits, as explained black in white in the javadoc of FLUSH_EAGER:
Eager flushing leads to immediate
synchronization with the database,
even if in a transaction. This causes
inconsistencies to show up and throw a
respective exception immediately, and
JDBC access code that participates in
the same transaction will see the
changes as the database is already
aware of them then. But the drawbacks
are:
additional communication roundtrips with the database, instead of a single
batch at transaction commit;
the fact that an actual database rollback is needed if the Hibernate
transaction rolls back (due to already
submitted SQL statements).
And believe me, increasing the database roundtrips and loosing the batching of statements can cause major performance degradation.
Also keep in mind that once you get an exception, there is not much you can do apart from throwing your session away.
To sum up, I'm very happy that Hibernate enqueues the various actions and I would certainly not use this EAGER_FLUSH flushMode as a general setting (but maybe only for the specific operations that actually require eager, if any).
Look in to autocommit though it is not recommended. If your work includes more than one update or insert SQL statement, you autocommit some of the work, and then a statement fails, you have a potentially arduous task of undoing the first part of the action. It gets really fun when the 'undo' operation fails.
Anyway, here's a link that shows how to do it.