Fri Jun 28 11:06:40 PDT 2019
PROPAGATION_REQUIRED = 0; If DataSourceTransactionObject T1 is already started for Method M1.If for another Method M2 Transaction object is required ,no new Transaction object is created .Same object T1 is used for M2
PROPAGATION_MANDATORY = 2; method must run within a transaction. If no existing transaction is in progress, an exception will be thrown
PROPAGATION_REQUIRES_NEW = 3; If DataSourceTransactionObject T1 is already started for Method M1 and it is in progress(executing method M1) .If another method M2 start executing then T1 is suspended for the duration of method M2 with new DataSourceTransactionObject T2 for M2.M2 run within its own transaction context
PROPAGATION_NOT_SUPPORTED = 4; If DataSourceTransactionObject T1 is already started for Method M1.If another method M2 is run concurrently .Then M2 should not run within transaction context. T1 is suspended till M2 is finished.
PROPAGATION_NEVER = 5; None of the methods run in transaction context.
An isolation level: It is about how much a transaction may be impacted by the activities of other concurrent transactions.It a supports consistency leaving the data across many tables in a consistent state. It involves locking rows and/or tables in a database.
The problem with multiple transaction
Scenario 1.If T1 transaction reads data from table A1 that was written by another concurrent transaction T2.If on the way T2 is rollback,the data obtained by T1 is invalid one.E.g a=2 is original data .If T1 read a=1 that was written by T2.If T2 rollback then a=1 will be rollback to a=2 in DB.But,Now ,T1 has a=1 but in DB table it is changed to a=2.
Scenario2.If T1 transaction reads data from table A1.If another concurrent transaction(T2) update data on table A1.Then the data that T1 has read is different from table A1.Because T2 has updated the data on table A1.E.g if T1 read a=1 and T2 updated a=2.Then a!=b.
Scenario 3.If T1 transaction reads data from table A1 with certain number of rows. If another concurrent transaction(T2) inserts more rows on table A1.The number of rows read by T1 is different from rows on table A1
Scenario 1 is called Dirty reads.
Scenario 2 is called Non-repeatable reads.
Scenario 3 is called Phantom reads.
So, isolation level is the extend to which Scenario 1, Scenario 2, Scenario 3 can be prevented. You can obtain complete isolation level by implementing locking.That is preventing concurrent reads and writes to the same data from occurring.But it affects performance .The level of isolation depends upon application to application how much isolation is required.
ISOLATION_READ_UNCOMMITTED :Allows to read changes that haven't yet been committed.It suffer from Scenario 1, Scenario 2, Scenario 3
ISOLATION_READ_COMMITTED:Allows reads from concurrent transactions that have been committed. It may suffer from Scenario 2 and Scenario 3. Because other transactions may be updating the data.
ISOLATION_REPEATABLE_READ:Multiple reads of the same field will yield the same results untill it is changed by itself.It may suffer from Scenario 3.Because other transactions may be inserting the data
ISOLATION_SERIALIZABLE: Scenario 1,Scenario 2,Scenario 3 never happens.It is complete isolation.It involves full locking.It affets performace because of locking.
You can test using
public class TransactionBehaviour { // set is either using xml Or annotation DataSourceTransactionManager manager=new DataSourceTransactionManager(); SimpleTransactionStatus status=new SimpleTransactionStatus(); ; public void beginTransaction() { DefaultTransactionDefinition Def = new DefaultTransactionDefinition(); // overwrite default PROPAGATION_REQUIRED and ISOLATION_DEFAULT // set is either using xml Or annotation manager.setPropagationBehavior(XX); manager.setIsolationLevelName(XX); status = manager.getTransaction(Def); } public void commitTransaction() { if(status.isCompleted()){ manager.commit(status); } } public void rollbackTransaction() { if(!status.isCompleted()){ manager.rollback(status); } } Main method{ beginTransaction() M1(); If error(){ rollbackTransaction() } commitTransaction(); } }
You can debug and see the result with different values for isolation and propagation.
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