Transaction & Security

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Chuyên đề Lập trình Java & J2EE Chương 16 Biên soạn: Th.S Nguyễn văn Lành

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Transaction & Security Chuyên đề Lập trình Java & J2EE Chương 16 Biên soạn: Th.S Nguyễn văn Lành
Motivation for Transactions  Atomic Operations  Recovery after Network or Machine Failure  Multiple Users Sharing Data  avoid those problems by properly using transactions.
The ACID Properties  Atomicity guarantees that many operations are bundled together and appear as one contiguous unit of work.  Consistency guarantees that a transaction leaves the system’s state to be consistent after a transaction completes.  Isolation protects concurrently executing transactions from seeing each other’s incomplete results.  Durability guarantees that updates to managed resources, such as database records, survive failures.
Transactional Models  Flat transaction is a series of operations that are performed atomically as a single unit of work.  Nested transaction allows to embed atomic units of work within other units of work.
Transsaction demarcation  Programmatic Transactions  you are responsible for issuing a begin statement and either a commit or an rollback statement  BMT: do not alow for Entity bean  Declarative Transactions  Specify transaction attributes in deployment descriptor  Container automatically handle transactional issues  CMT  ClientInitiated Transactions
Transaction attribute  Required  RequiresNew  Supports  Mandatory  Never  NotSupported
Required
Requires New
Mandatory
Never
Bean Managed Transaction
Java Transaction API (JTA)  JTA can be used in client & bean code to programmatically control transactional boundaries.  The interface you use to programmatically control transactions is javax.transaction.UserTransaction.
UserTransaction interface
Code sample
Transactional Isolation  Concurrency Control  locking on the database to prevent the two components from reading data  Four transaction isolation levels:  READ UNCOMMITTED  does not offer any isolation guarantees (dirty read) but offers the highest performance.  READ COMMITTED  REPEATABLE READ  SERIALIZABLE:  Use SERIALIZABLE for missioncritical systems that absolutely must have perfect transactional isolation.  Pessimistic and Optimistic Concurrency Control
Distributed Transactions  Distributed flat transactions allow multiple transaction participants, written by different vendors, to collaborate under one transactional context.  TwoPhase Commit Protocol  Phase One begins by sending a before commit message to all resources involved in the transaction.  Phase Two occurs only if Phase One completed without an abort. At this time, all of the resource managers, which can all be located and controlled separately, perform the actual data updates
2PC protocol
EJB Security  Authentication verifies that the client is who he claims to be.  Authorization checks to see if an already authenticated client is allowed to perform a task.