Transactions

1
Transactions

• Source
• Enterprise JavaBeans, 3rd Edition, Richard
  Monson-Haefel

2
Transaction Definition

• Unit of work that accesses one or more shared
  resources (usually databases)
• set of one or more activities related to each
  other
• must be completed together or not at all
• cohesion of unit is normally mission critical
• examples
• ATM
• withdraw from one source, deposit to another
• Order System
• locate item, charge account, schedule shipment
• Medical System
• identify medical state, dispense prescription

3
Transaction Properties

• Atomic
• Transaction must execute completely or not at all
• Consistent
• Data in database is always in a consistent state
  (makes sense)
• constraints (primary keys, referential integrity,
  etc.)
• Isolated
• Transaction executes without interference
• Durable
• changes are not lost if the system crashes

4
EJB Transaction Support

• Declarative
• transaction management controlled through
  deployment descriptor
• Programmatic
• direct programming calls to Java Transaction
  Service (JTS) API
• transaction management code mixed with business
  code
• change in transactional behavior requires change
  in business code

5
Transaction Scope

• (one or more tasks that operate as a unit of
  work succeed or be rolled back together)
• Tasks
• EJB methods
• Unit of Work
• Each bean visited by the methods during a thread
  of execution depending on bean transaction
  attributes
• ends when thread of execution
• completes normally
• an exception is thrown
• transaction is rolled back

6
Transaction Attributes

• Supports Transactions
• Supports_at_
• Required
• RequiresNew
• Mandatory
• Recommended for CMP 2.0 Entity Beans support
  required
• Transactions not Supported
• NotSupported_at_
• Never_at_
• _at_ support optional for CMP 2.0 Entity Beans

7
Setting the Declarative Transaction Attribute

• ltejb-jargt
• ...
• ltassembly-descriptorgt
• ltcontainer-transactiongt
• ltmethodgt
• ltejb-namegtAccountlt/ejb-namegt
• ltmethod-namegtlt/method-namegt
• lt/methodgt
• lttrans-attributegtSupportslt/trans-attribut
  egt
• lt/container-transactiongt
• lt/assembly-descriptorgt
• lt/ejb-jargt

8
Container and Beans
Home Interface
Container
Pool
Home
Home Stub
create
Client
Remote Object Stub
EJBObject
Account Bean
transfer
Remote Interface
9
Not Supported
Client
Invoked EJB
Thread of Execution
Clients Transaction Context
No Transaction Context

• Transaction is suspended during the method of the
  Invoked EJB resumes when method complete
• Transaction scope is not propagated to Invoked
  EJB or anything it invokes

10
Supports
Client
Invoked EJB
(A)
(B)
Thread of Execution
Clients Transaction Context
No Transaction Context

• Joins the transaction context if invoked as part
  of a transaction (A)
• Does not require a transaction can be invoked
  outside of a transaction context (B)

11
Required
Client
Invoked EJB
(A)
(B)
Thread of Execution
Clients Transaction Context

• Joins the transaction context if invoked as part
  of a transaction (A)
• Initiates its own transaction context of invoked
  outside of a transaction (B)

12
RequiresNew
Client
Invoked EJB
(A)
(B)
Thread of Execution
Clients Transaction Context

• Initiates its own transaction context whether
  called within am existing transaction context (A)
  or outside of a transaction context (B)
• Initiated transaction completes prior to
  returning to caller

13
Mandatory
Client
Invoked EJB
(A)
(B)
Transaction Exception
Thread of Execution
Clients Transaction Context

• Joins the transaction context if invoked as part
  of a transaction (A)
• Throws a Transaction Exception if not called
  within a transaction context (B)
  (TransactionRequiredException for Remote Clients
  TransactionRequiredLocalException for Local
  Clients)

14
Never
Client
Invoked EJB
(A)
Transaction Exception
(B)
Thread of Execution
Clients Transaction Context

• Throws a Transaction Exception if called within a
  transaction context (A) (RemoteException to
  Remote Clients EJBException to Local Clients)
• Must be invoked outside of a transaction context
  (B)

15
Isolation and Database Locking

• What happens when two or more transactions
  attempt to access the same data
• Dirty Reads
• first transaction reads uncommitted changes from
  a second transaction that may eventually be
  rolled back
• Repeatable Reads
• data guaranteed to be the same if read multiple
  times during the same transaction implemented
  with locks or snapshots
• Phantom Reads
• first transaction sees new rows added by second
  transaction after beginning of the first
  transaction

16
Database Locks

• How do you prevent overlapping transactions from
  viewing the others data
• Read Lock
• data is prevented from changing while transaction
  in progress
• Write Lock
• prevents other transactions from modifying the
  data
• permits dirty reads by other transactions
• Exclusive Write Lock
• prevents other transactions from reading and
  modifying data
• prevents dirty reads
• Snapshots - frozen view of data

17
Transaction Isolation Levels

• Read Uncommitted
• can read data being modified by another
  transaction
• allows dirty, non-repeatable, and phantom reads
• Read Committed
• cannot read data being modified by another
  transaction
• allows non-repeatable and phantom reads prevents
  dirty reads
• Repeatable Read
• cannot change data being read by another
  transaction
• allows phantom reads prevents dirty and
  non-repeatable reads
• Serializable
• transactions can neither read or write same data

18
Specifying Isolation Level

• ltweblogic-ejb-jargt
• lttransaction-isolationgt
• lt!-- TRANSACTION_SERIALIZABLE
• TRANSACTION_READ_COMMITTED
• TRANSACTION_READ_UNCOMMITTED
• TRANSACTION_REPEATABLE_READ
• --gt
• ltisolation-levelgtTRANSACTION_READ_COMMITTEDlt
  /isolation-levelgt
• ltmethodgt
• ltejb-namegtAccountlt/ejb-namegt
• ltmethod-namegtlt/method-namegt
• ltmethodgt
• lt/transaction-isolationgt

19
Message Driven Bean Transactions

• Valid Values
• Not Supported
• Required
• Client transaction context not propagated to MDB
• Supports, RequiresNew, Mandatory, and Never are
  relative to client

20
Exceptions within Transactions

• System Exceptions (ex. NullPointerException,
  EJBException)
• container automatically rolls back transaction
• bean instance is discarded
• ApplicationException (ex. AccountOverdrawException
  )
• container does not automatically rollback
  transaction
• exception delivered to client
• client optionally signals rollback
• EJBContext.setRollbackOnly()

21
CMR and Transactions

• Collections obtained through CMR must be accessed
  within the same transaction
• public void processAccounts(CustomerLocal
  customer)
• Collection accounts customer.getAccounts()
• for (Iterator iaccounts.iterator()
  i.hasNext() )
• processAccounts must have a transaction context
  active or will receive a java.lang.IllegalStateExc
  eption when accessing accounts

22
Transaction/Bank Example

• Account
• Entity Bean
• Deployments (NotSupported, Supports, RequiresNew)
• reports Application Exception (on overdraw)
• Teller
• Stateless Session Bean
• Deployments (Required)
• calls EJBContext.setRollbackOnly() on exception
• TxClient
• instantiates Accounts with varying transaction
  support
• uses Teller to perform actions on Accounts

23
AccountEJB

• public abstract class AccountEJB implements
  EntityBean
• public void reset() setBalance(0.0F)
• public void deposit(float amount) throws
  AccountException
• setBalance(getBalance() amount)
• public void withdraw(float amount) throws
  AccountException
• float balance getBalance()
• if ((balance - amount) gt 0)
  setBalance(balance - amount)
• else
• setBalance(balance - 1.0F) //subtract
  fee
• throw new AccountException("overdraw")
• ...

24
TellerEJB

• public class TellerEJB implements SessionBean
• public void transfer(
• AccountRemote fromAccount, AccountRemote
  toAccount, float amount)
• throws AccountException
• try
• toAccount.deposit(amount)
• fromAccount.withdraw(amount)
• catch (AccountException ex)
  ctx_.setRollbackOnly() throw ex
• public void reset(AccountRemote account)
  //similar to transfer

25
TxClient Initialization

• fromAccounts_ new Hashtable(LAST_TYPE)
• toAccounts_ new Hashtable(LAST_TYPE)
• for(int iFIRST_TYPE iltLAST_TYPE i)
• String id uidGenerator_.createUID().toString()
  
• AccountRemote account createAccount(TX_TYPEi
  , id)
• toAccounts_.put(TX_TYPEi, account)
• id uidGenerator_.createUID().toString()
• account createAccount(TX_TYPEi, id)
• fromAccounts_.put(TX_TYPEi, account)

26
TxClient Transfer

• void transfer(int fromType, int toType) throws
  Exception
• AccountRemote from ...
• AccountRemote to
• teller_.reset(from)
  teller_.reset(to)
• float amount 5.00F
• try
• teller_.transfer(from, to, amount)
• catch (AccountException ex)
  System.out.print("transfer failed " ex)
• finally
• System.out.println("from("
  from.getId() ")" from.getBalance()
• ", to(" to.getId()
  ")" to.getBalance())

27
TxClient Execution

• private static void execute(final Context jndi)
  throws Exception
• TxClient client new TxClient(jndi)
• for(int iFIRST_TYPE iltLAST_TYPE i)
• for(int jFIRST_TYPE jltLAST_TYPE j)
  
• System.out.println(TX_TYPEi "-gt"
  TX_TYPEj)
• client.transfer(i,j)

28
TxClient Output

• NotSupported-gtNotSupported
• transfer failed overdrawfrom(19197 )-1.0,
  to(19196 )5.0
• NotSupported-gtSupports
• transfer failed overdrawfrom(19197 )-1.0,
  to(19198 )0.0
• NotSupported-gtRequiresNew
• transfer failed overdrawfrom(19197 )-1.0,
  to(19200 )5.0
• Supports-gtNotSupported
• transfer failed overdrawfrom(19199 )0.0,
  to(19196 )5.0
• Supports-gtSupports
• transfer failed overdrawfrom(19199 )0.0,
  to(19198 )0.0
• Supports-gtRequiresNew
• transfer failed overdrawfrom(19199 )0.0,
  to(19200 )5.0

29
TxClient Output

• RequiresNew-gtNotSupported
• transfer failed overdrawfrom(19201 )-1.0,
  to(19196 )5.0
• RequiresNew-gtSupports
• transfer failed overdrawfrom(19201 )-1.0,
  to(19198 )0.0
• RequiresNew-gtRequiresNew
• transfer failed overdrawfrom(19201 )-1.0,
  to(19200 )5.0
• ltAdditional Outputgt
• DEFAULT-gtDEFAULT
• transfer failed overdrawfrom(19203 )0.0,
  to(19202 )0.0

30
Managing the transfer() Transaction Context
5 register toAccountEJB
Supports
TxClient
toAccountEJB
4 deposit
Required
TellerEJB
Supports
7 deposit
1 transfer()
fromAccountEJB
2 register TellerEJB
10 verify all will work
8 register fromAccountEJB
Transaction Manager
11 commit or rollback all updates
6 add toAccountEJB

• Transaction Context
• TellerEJB
• toAccountEJB
• fromAccountEJB

3 create tx context add TellerEJB
9 add fromAccountEJB
31
Distributed Tx ArchitectureTwo Phase Commit
(2-PC/TPC)
Transaction Coordinator
Resource Manager
1. Prepare to Commit
TX Manager
2. Return
4. commit
Resource Manager
TX Manager
5. return
3. Log Result
32
Resource Managers

• Examples
• JDBC Drivers
• JMS Provider

33
Adding Another Resource Manager (JMS) to Teller

• public void transfer(
• AccountRemote fromAccount, AccountRemote
  toAccount, float amount)
• throws AccountException
• try
• msg_.setString("type", "transfer")
• //...
• publisher_.publish(msg_)
• toAccount.deposit(amount)
• fromAccount.withdraw(amount)
• catch (AccountException ex)
  ctx_.setRollbackOnly() throw ex
• //...
• //reset() is similar to transfer

34
Checking commited Results in TxClient

• try
• teller_.reset(from)
• teller_.reset(to)
• finally
• if (subscriber_.receive(1000 1)
  null)
• System.out.println("didn't get first
  reset message")
• if (subscriber_.receive(1000 1)
  null)
• System.out.println("didn't get second
  reset message")

35
Checking rollback Results in TxClient

• try teller_.transfer(from, to, amount)
• catch (AccountException ex)
• System.out.print("transfer failed "
  ex.getMessage())
• finally
• System.out.print("from(" from.getId()
  ")" from.getBalance()
• ", to(" to.getId()
  ")" to.getBalance())
• if (subscriber_.receive(1000 1)
  null)
• System.out.println("no message")

36
TxClient Output with Two Resource Managers

• //note that resets received expected messages
• NotSupported-gtNotSupported
• transfer failed overdrawfrom(19205 )-1.0,
  to(19204 )5.0no message
• NotSupported-gtSupports
• transfer failed overdrawfrom(19205 )-1.0,
  to(19206 )0.0no message
• NotSupported-gtRequiresNew
• transfer failed overdrawfrom(19205 )-1.0,
  to(19208 )5.0no message
• Supports-gtNotSupported
• transfer failed overdrawfrom(19207 )0.0,
  to(19204 )5.0no message
• Supports-gtSupports
• transfer failed overdrawfrom(19207 )0.0,
  to(19206 )0.0no message
• Supports-gtRequiresNew
• transfer failed overdrawfrom(19207 )0.0,
  to(19208 )5.0no message

37
TxClient Output with Two Resource Managers

• RequiresNew-gtNotSupported
• transfer failed overdrawfrom(19209 )-1.0,
  to(19204 )5.0no message
• RequiresNew-gtSupports
• transfer failed overdrawfrom(19209 )-1.0,
  to(19206 )0.0no message
• RequiresNew-gtRequiresNew
• transfer failed overdrawfrom(19209 )-1.0,
  to(19208 )5.0no message
• //extra output
• DEFAULT-gtDEFAULT
• transfer failed overdrawfrom(19211 )0.0,
  to(19210 )0.0no message

38
Enable JTS Transactions for Connection Pool
39
Enable JTS Transactions in Entity Bean

• ltweblogic-rdbms-jargt
• ltweblogic-rdbms-beangt
• ltejb-namegtAccountlt/ejb-namegt
• ltdata-source-namegtejava.jdbc.txDataSource
• lt/data-source-namegt
• lttable-namegtAccountlt/table-namegt

40
Enable JTS Transactions for JMS
41
Bean Types and Transactions

• Entity
• In EJB 1.1 must be container managed
• may force a rollback with setRollbackOnly()
• Stateless Session
• transactions simple. Abort by throwing
  EJBException
• Stateful Session
• What about the conversational state ?
• Have to respond to transaction aborts to keep
  conversational state consistent

42
Stateful Session Bean and Transactions (Cont)

• Can implement Session Synchronization interface
• afterBegin(), beforeCompletion(),
  afterCompletion(boolean)
• If afterCompletion is false, you can roll back
  the conversational state
• SessionSynchronization can not be implemented
  with bean-managed tx

43
SessionSynchronization Example

• public class SessionSynchronizedTellerEJB
• implements SessionBean, javax.ejb.SessionSynchr
  onization
• public void reset(AccountRemote account)
  throws AccountException ...
• public void transfer(
• AccountRemote fromAccount, AccountRemote
  toAccount, float amount)
• throws AccountException ...
• public void afterBegin()
• public void beforeCompletion()
• public void afterCompletion(boolean committed)
  

44
Programmatic Transactions

• Bean programmer is responsible for issuing
  begin(), commit(), and abort() calls in code
• Discouraged - Why not let the container do it ?
• Use Java Transaction API (JTA)
• Simple layer over Java Transaction Service (JTS)
  which is an implementation of the CORBA Object
  Transaction Service (OTS)

45
User Transaction

• Required EJB Transaction interface
• Provides access to transaction service
• can start and commit
• can mark that transaction must be rolled back
• Client and server components can use
• Container/Server does not have to expose JTS API
  to the bean

46
User Transaction
47
Code Example

• public void transfer(AccountRemote from,
  AccountRemote to, float amount )
• throws AccountException
• UserTransaction tx null
• try
• tx ctx_.getUserTransaction()
• tx.begin()
• // perform JDBC operations and transfer
  the money
• tx.commit()
• catch (Exception ex)
• tx.setRollbackOnly()
• throw new AccountException()

48
Programmatic Transactions in EJB

• Allowed in Session Beans
• ltsessiongt
• ...
• lttransaction-typegtBeanlt/transaction-typegt
• Stateless Session Beans must begin/end in same
  method
• Stateful Session Beans may begin/end in separate
  methods (although not advised)
• EJB 1.1 disallows this for Entity Beans
• Bean controls transactions programmatically

49
Transactional Clients

• End user can also control transactions
• UserTransaction ut (UserTransaction)
  jndiContext.lookup(javax.transaction.UserTransact
  ion)
• ut.begin()
• // perform multiple operations on beans
• account1.deposit( 34 )
• account2.withdraw( 34 )
• ut.commit()

50
Client Issues

• Lots of network traffic required when client
  manages transactions
• More complicated
• Should have asked a session bean to perform the
  set of operations over on the server
• Transactions should be short

51
Performance Implications

• Avoid distributed transactions across multiple
  resource managers and/or multiple transaction
  managers
• Consider JMS synchronization
• Avoid client control of transaction boundaries
• Try to encapsulate transactional business
  operations in session beans

52
EJB Transaction Summary

• Supports EJB goal of removing middleware
  programming burden from bean programmer
• Declare transactional requirements in deployment
  descriptor
• Transactions have performance implications
• limit client control, distributed transactions
• can control frequency of ejbLoad and ejbStore