I am confused about the working of LockModeTypes in JPA:
LockModeType.Optimistic
it increments the version while committing.
Question here is : If I have version column in my entity and if I don't specify this lock mode then also it works similarly then what is the use of it?
LockModeType.OPTIMISTIC_FORCE_INCREMENT
Here it increments the version column even though the entity is not updated.
but what is the use of it if any other process updated the same row before this transaction is committed? this transaction is anyways going to fail. so what is the use of this LockModeType.
LockModeType.PESSIMISTIC_READ
This lock mode issues a select for update nowait(if no hint timeout specified)..
so basically this means that no other transaction can update this row until this transaction is committed, then its basically a write lock, why its named a Read lock?
LockModeType.PESSIMISTIC_WRITE
This lock mode also issues a select for update nowait (if no hint timeout specified).
Question here is what is the difference between this lock mode and LockModeType.PESSIMISTIC_READ as I see both fires same queries?
LockModeType.PESSIMISTIC_FORCE_INCREMENT
this does select for update nowait (if no hint timeout specified) and also increments the version number.
I totally didn't get the use of it.
why a version increment is required if for update no wait is there?
I would first differentiate between optimistic and pessimistic locks, because they are different in their underlying mechanism.
Optimistic locking is fully controlled by JPA and only requires additional version column in DB tables. It is completely independent of underlying DB engine used to store relational data.
On the other hand, pessimistic locking uses locking mechanism provided by underlying database to lock existing records in tables. JPA needs to know how to trigger these locks and some databases do not support them or only partially.
Now to the list of lock types:
LockModeType.Optimistic
If entities specify a version field, this is the default. For entities without a version column, using this type of lock isn't guaranteed to work on any JPA implementation. This mode is usually ignored as stated by ObjectDB. In my opinion it only exists so that you may compute lock mode dynamically and pass it further even if the lock would be OPTIMISTIC in the end. Not very probable usecase though, but it is always good API design to provide an option to reference even the default value.
Example:
`LockModeType lockMode = resolveLockMode();
A a = em.find(A.class, 1, lockMode);`
LockModeType.OPTIMISTIC_FORCE_INCREMENT
This is a rarely used option. But it could be reasonable, if you want to lock referencing this entity by another entity. In other words you want to lock working with an entity even if it is not modified, but other entities may be modified in relation to this entity.
Example: We have entity Book and Shelf. It is possible to add Book to Shelf, but book does not have any reference to its shelf. It is reasonable to lock the action of moving a book to a shelf, so that a book does not end up in another shelf (due to another transaction) before end of this transaction. To lock this action, it is not sufficient to lock current book shelf entity, as the book does not have to be on a shelf yet. It also does not make sense to lock all target bookshelves, as they would be probably different in different transactions. The only thing that makes sense is to lock the book entity itself, even if in our case it does not get changed (it does not hold reference to its bookshelf).
LockModeType.PESSIMISTIC_READ
this mode is similar to LockModeType.PESSIMISTIC_WRITE, but different in one thing: until write lock is in place on the same entity by some transaction, it should not block reading the entity. It also allows other transactions to lock using LockModeType.PESSIMISTIC_READ. The differences between WRITE and READ locks are well explained here (ObjectDB) and here (OpenJPA). If an entity is already locked by another transaction, any attempt to lock it will throw an exception. This behavior can be modified to waiting for some time for the lock to be released before throwing an exception and roll back the transaction. In order to do that, specify the javax.persistence.lock.timeout hint with the number of milliseconds to wait before throwing the exception. There are multiple ways to do this on multiple levels, as described in the Java EE tutorial.
LockModeType.PESSIMISTIC_WRITE
this is a stronger version of LockModeType.PESSIMISTIC_READ. When WRITE lock is in place, JPA with the help of the database will prevent any other transaction to read the entity, not only to write as with READ lock.
The way how this is implemented in a JPA provider in cooperation with underlying DB is not prescribed. In your case with Oracle, I would say that Oracle does not provide something close to a READ lock. SELECT...FOR UPDATE is really rather a WRITE lock. It may be a bug in hibernate or just a decision that, instead of implementing custom "softer" READ lock, the "harder" WRITE lock is used instead. This mostly does not break consistency, but does not hold all rules with READ locks. You could run some simple tests with READ locks and long running transactions to find out if more transactions are able to acquire READ locks on the same entity. This should be possible, whereas not with WRITE locks.
LockModeType.PESSIMISTIC_FORCE_INCREMENT
this is another rarely used lock mode. However, it is an option where you need to combine PESSIMISTIC and OPTIMISTIC mechanisms. Using plain PESSIMISTIC_WRITE would fail in following scenario:
transaction A uses optimistic locking and reads entity E
transaction B acquires WRITE lock on entity E
transaction B commits and releases lock of E
transaction A updates E and commits
in step 4, if version column is not incremented by transaction B, nothing prevents A from overwriting changes of B. Lock mode LockModeType.PESSIMISTIC_FORCE_INCREMENT will force transaction B to update version number and causing transaction A to fail with OptimisticLockException, even though B was using pessimistic locking.
LockModeType.NONE
this is the default if entities don't provide a version field. It means that no locking is enabled conflicts will be resolved on best effort basis and will not be detected. This is the only lock mode allowed outside of a transaction
Related
I'm looking for a way to save and immediately lock an entity on a DB in order to avoid that other thread access the entity before the thread creator ends.
I'm using Hibernate 4.3.11 and Spring 4.2.5.
Thanks in advance.
Although there is lock mode - LockMode.WRITE - but as the documentation states
A WRITE lock is obtained when an object is updated or inserted. This
lock mode is for internal use only and is not a valid mode for load()
or lock() (both of which throw exceptions if WRITE is specified)..
If it's just that you are only inserting rows then you cannot specifically lock the database rows using hibernate as the rows are not yet committed.
The moment your code (hibernate or without) inserts rows in database and not yet commits - there are transactional locks held which gets released on transaction commit. The nature of locks and the manner in which this internally happens is database specific. However if you are interested in locking some rows (already existing) , then you
can query the data using
session.get(TestEntity.class, 1, LockMode.PESSIMISTIC_WRITE);
This will hold a pessimistic lock (typically by issuing SELECT .... FOR UPDATE) for the duration of transaction and no other thread/transaction can modify the data on which lock has been taken.
A possible way should be increase transaction level to serializable.
This level ensure data is locked until is not used in other transaction.
Hibernate offer's two types of locks Optimistic and Pessimistic. Its straight forward.
1)Optimistic uses versioning where in it will have a version column in the database and check it before it updates or else throw the exception
2)Pessimistic is some thing like a database handles the locking on that row and it will get released after the operation is completed, there are few options are there which is similarly like how you imagine like read lock, write lock
https://docs.jboss.org/hibernate/orm/4.0/devguide/en-US/html/ch05.html
If you are using PostgreSQL I think the below example works:
#Query(value = """with ins_artist as (
insert into artist
values (301, 'Whoever1')
returning *
) select artist_id
from ins_artist
for update""", nativeQuery = true)
#Transactional(propagation = Propagation.REQUIRED)
Long insertArtist(); // returns artist ID
PS: I ran this query on https://postgres.devmountain.com/ . But it would need testing on a Java app.
Dirty Read: The definition states that
dirty reading occurs when a transaction reads data from a row that has been modified by another transaction but not yet committed.
Assuming the definition is correct, I am unable to fathom any such situation.
Due to the principle of Isolation, the transaction A can not see the uncommitted data of the row that has been modified by transaction B. If transaction B has simply not committed, how transaction A can see it in the first place? It is only possible when both operations are performed under same transaction.
Can someone please explain what am I missing here?
"Dirty", or uncommitted reads (UR) are a way to allow non-blocking reads. Reading uncommitted data is not possible in an Oracle database due to the multi-version concurrency control employed by Oracle; instead of trying to read other transactions' data each transaction gets its own snapshot of data as they existed (committed) at the start of the transaction. As a result all reads are essentially non-blocking.
In databases that use lock-based concurrency control, e.g DB2, uncommitted reads are possible. A transaction using the UR isolation level ignores locks placed by other transactions, and thus it is able to access rows that have been modified but not yet committed.
Hibernate, being an abstraction layer on top of a database, offers the UR isolation level support for databases that have the capability.
With Oracle is it possible to (with a single database connection):
lock a single row (row1)
then lock another row (row2)
release the lock on row1 (retaining the lock on row2)
obtain a lock on another row (row3)
release the lock on row2 (retaining the lock on row3)
release the lock on row3
I realize Oracle supports many different kinds of locks (I've found this very helpful: http://broadh2o.net/docs/database/oracle/oracleLocks.html ), so perhaps the answer depends on which kind of locks are chosen. I'm interested in exclusive locks - i.e. locks which prevent other connections from updating the row.
I would like to know if I can perform operations 1-6 using a single database connection. I certainly could use a separate database connection for each row. It seems that locks are released using COMMIT/ROLLBACK, so that would suggest releasing the lock on a single row isn't possible.
You cannot selectively release locks on rows. Once you lock row1, that lock will only be released at the end of your transaction. But the end of your transaction will also release any other locks held by your transaction (i.e. the lock on row2).
Depending on the business problem you are trying to solve, potentially you don't really want to lock individual rows. Potentially, you want to use the dbms_lock package to acquire and release some user-defined locks. If you have user-defined locks lock1, lock2, and lock3, then you could acquire and release the three locks just as you've outlined within a single transaction. Setting up user-defined locking, however, can be quite dangerous both because it requires a lot more work from developers who have to protect the right sections of their code with the appropriate locks and because it is possible to request a user-defined lock that will not be released when a transaction commits or is rolled back which makes it possible to really shoot yourself in the foot if you don't handle your exceptions correctly.
First of all i would like to confirm is it the responsibility of developer to follow these properties or responsibilty of transaction Apis like JDBC?
Below is my understanding how we achieve acid properties in JDBC
Atomicity:- as there is one transaction associated with connection, so we do commit or rollback , there are no partial updation.Hence achieved
Consitency:- when some data integrity constraint is voilated (say some check constraint) then sqlexception will be thrown . Then programmer acieve the consistent database by rollbacking the transaction?
one question on above say we do transaction1 and sql excpetion is thrown during transaction 2 as explained above . Now we catch the exception and do the commit will first transaction be commited?
Isolation:- Provided by JDBC Apis.But this leads to the problem of concurrent update . so it has be dealt manually right?
Durability:- Provided by JDBC Apis.
Please let me if above understanding is right?
ACID principles of transactional integrity are implemented by the database not by the API (like JDBC) or by the application. Your application's responsibility is to choose a database and a database configuration that supports whatever transactional integrity you need and to correctly identify the transactional boundaries in your application.
When an exception is thrown, your application has to determine whether it is appropriate to rollback the entire transaction or to proceed with additional processing. It may be appropriate if your application is processing orders from a vendor, for example, to process the 99 orders that succeed and log the 1 order that failed somewhere for users to investigate. On the other hand, you may reject all 100 orders because 1 failed. It depends what your application is doing.
In general, you only have one transaction open at a time (or, more accurately, one transaction per connection). So if you are working in transaction 2, transaction 1 by definition has already completed-- it was either committed or rolled back previously. Exceptions thrown in transaction 2 have no impact on transaction 1.
Depending on the transaction isolation level your application requests (and the transaction isolation levels your database supports) as well as the mechanics of your application, lost updates are something that you may need to be concerned about. If you set your transaction isolation level to read committed, it is possible that you would read a value as 'A' in transaction 1, wait for a user to do something, update the value to 'B', and commit without realizing that transaction 2 updated the value to 'C' between the time you read the data and the time you wrote the data. This may be a problem that you need to deal with or it may be something where it is fine for the last person to update a row to "win".
Your database, on the other hand, should take care of the automatic locking that prevents two transactions from simultaneously updating the same row of the same table. It may do this by locking more than is strictly necessary but it will serialize the updates somehow.
I have a problem understanding read consistency in database (Oracle).
Suppose I am manager of a bank . A customer has got a lock (which I don't know) and is doing some updating. Now after he has got a lock I am viewing their account information and trying to do some thing on it. But because of read consistency I will see the data as it existed before the customer got the lock. So will not that affect inputs I am getting and the decisions that I am going to make during that period?
The point about read consistency is this: suppose the customer rolls back their changes? Or suppose those changes fail because of a constraint violation or some system failure?
Until the customer has successfully committed their changes those changes do not exist. Any decision you might make on the basis of a phantom read or a dirty read would have no more validity than the scenario you describe. Indeed they have less validity, because the changes are incomplete and hence inconsistent. Concrete example: if the customer's changes include making a deposit and making a withdrawal, how valid would your decision be if you had looked at the account when they had made the deposit but not yet made the withdrawal?
Another example: a long running batch process updates the salary of every employee in the organisation. If you run a query against employees' salaries do you really want a report which shows you half the employees with updated salaries and half with their old salaries?
edit
Read consistency is achieved by using the information in the UNDO tablespace (rollback segments in the older implementation). When a session reads data from a table which is being changed by another session, Oracle retrieves the UNDO information which has been generated by that second session and substitutes it for the changed data in the result set presented to the first session.
If the reading session is a long running query it might fail because due to the notorious ORA-1555: snapshot too old. This means the UNDO extent which contained the information necessary to assemble a read consistent view has been overwritten.
Locks have nothing to do with read consistency. In Oracle writes don't block reads. The purpose of locks is to prevent other processes from attempting to change rows we are interested in.
For systems that have large number of users, where users may "hold" the lock for a long time the Optimistic Offline Lock pattern is usually used, i.e. use the version in the UPDATE ... WHERE statement.
You can use a date, version id or something else as the row version. Also the virtual columm ORA_ROWSCN may be used but you need to read up on it first.
When a record is locked due to changes or an explicit lock statement, an entry is made into the header of that block. This is called an ITL (interested transaction list). When you come along to read that block, your session sees this and knows where to go to get the read consistent copy from the rollback segment.