Ejb Transactions: Session 5

EJB Transactions Session 5

Transactions

Grey Matter, Sella Synergy India

Think Spot

Is your transaction safe?

How will you handle this situation? Let’s say you have two different transactions running, hitting the same database. You don’t want the effect of one transaction to corrupt the state of another transaction.

Transactions

Grey Matter, Sella Synergy India

Road Map

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What is Transaction ?

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ACID Properties of Transaction

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EJB Transactions

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Container Managed Transaction Setup

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Forcing Rollback

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Transactions

How Transaction React to System/Application Exceptions

Grey Matter, Sella Synergy India

2min

What is Transaction?

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Transaction

A transaction is a mechanism to handle groups of operations as though they were one In a transaction, either all operations occur or none at all. Operations involved in a transaction may rely on many different servers and databases.

Grey Matter, Sella Synergy India

2min

ACID Properties of Transaction

A transaction is formally defined by the set of properties known by the acronym ACID

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The acronym of ACID stands for:

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Transaction

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Atomic

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Consistent

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Isloated

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Durable

Grey Matter, Sella Synergy India

2min

EJB Transactions

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A transactions can be determined: By a client around one or more method invocations

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By the container around each method invocations

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By the bean in a single method

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Clien t tx1.begin(); EJB1.method(); EJB2.method(); tx1.commit();

Transactions

This is an example of a client demarcated transaction. Notice that the transaction can span one or more EJBs, one or more methods, and one or more databases. Grey Matter, Sella Synergy India

2min

The Tag

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The tag: Specifies whether a bean uses container or bean managed transactions −

Used only with <session> or <message-driven>

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(entity EJBs always use container managed transactions) Can be either Bean or Container

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Examples: <enterprise-beans> <sessions> <ejb-name>JspSession ... Container Transactions

Grey Matter, Sella Synergy India

2min

Container-Managed Transactions 

CMT Are started and stopped by the container before and after a method invocations 

Is configured in the deployment descriptor

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Is mandatory for entity EJBs.

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This is a scenario where the container invoke() starts a new transaction 1 From for a single Client method call.

Transactions

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Pre - invocation 2 Start TX

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Check Security

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DB Load

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Etc. invoke()

Container

Server

3 Component

Post - invocation4 ●

DB Store

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Commit / Rollback TX

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Etc.

Grey Matter, Sella Synergy India

2min

The Tag

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The tag declares how container-managed transactions should work for one or more methods. Example: ...

<method> <ejb-name>JspSession <method-name>* Required

. . . Transactions

Grey Matter, Sella Synergy India

2min

The Six Values of

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The tag can have six values Never

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Mandatory

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NotSupported

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Supports

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Required

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RequiresNew

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Transactions

This is the default, when transaction attributes are not specified

Grey Matter, Sella Synergy India

2min

Never

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Never specifies that the EJB must never be invoked within the context of a transactions

Scenario 1 (Client TX): Client

1 – begin 2 – method

Contai ner

Active Transaction No Active Transaction

EJ B

Remote Exception Scenario 2 (No Client TX): Client 1 – method

Transactions

Contai ner

Grey Matter, Sella Synergy India

1 – method

EJ B

2min

Mandatory

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Mandatory transactions require the client to pass a global transaction to the EJB

Active Transaction No Active Transaction method

Scenario 1 (Client TX): Client

1 – begin 2 – method 3– Commit

Scenario 2 (No Client TX): Client method

Transactions

Contai ner

Contai ner

Grey Matter, Sella Synergy India

Transaction Required Exception

EJ B

EJ B

2min

NotSupported

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NotSupported cause client transaction to be suspended during the EJB's execution.

Scenario 1 (Client TX): Client

1 – begin 2 – method 3– Commit

Scenario 2 (No Client TX): Client method

Transactions

Contai ner

Contai ner

Grey Matter, Sella Synergy India

Active Transaction No Active Transaction method

method

EJ B

EJ B

2min

Supports

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Supports implies that the EJB will be included in any clientstarted transactions.

Scenario 1 (Client TX): Client

1 – begin 2 – method 3– Commit

Scenario 2 (No Client TX): Client method

Transactions

Contai ner

Contai ner

Grey Matter, Sella Synergy India

Active Transaction No Active Transaction method

method

EJ B

EJ B

2min

Required

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Required transactions force the EJB to be executed within a transaction started either by the client or the container

Scenario 1 (Client TX): Client

1 – begin 2 – method 3– Commit

Scenario 2 (No Client TX): Client method

Transactions

Contai ner

Contai ner

Grey Matter, Sella Synergy India

Active Transaction No Active Transaction method

1 – begin 2 – method 3– Commit

EJ B

EJ B

2min

RequiresNew

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RequiresNew transactions force the EJB to execute within a new transaction that can only be started the container

Scenario 1 (Client TX): Client

1 – begin 2 – method 3– Commit

Scenario 2 (No Client TX): Client method

Transactions

Contai ner

Contai ner

Grey Matter, Sella Synergy India

Active Transaction No Active Transaction New EJ Transaction B method

1 – begin 2 – method 3– Commit

EJ B

2min

A CMT Example ejb-jar.xml Snippet: <enterprise-beans> <session> <ejb-name> AnEJB ... Containers ... <method> <ejb-name>AnEJB <method-name>* Required TRansactioins

Grey Matter, Sella Synergy India

2min

Puzzle Corner

Which transaction attributes can cause an in-progress transaction to be suspended?

NotSupported and RequiresNew

Transactions

Grey Matter, Sella Synergy India

2min

Transaction isolation level

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The transaction isolation level controls the behavior of a transaction, by balancing: The integrity of the data vs

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The concurrent access speed

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Transactions

Grey Matter, Sella Synergy India

2min

Integrity Problem Scenarios

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A Dirty Read happens when reading a row that was modified by someone else (and that person did not commit yet).

A Non-repeatable Read happens when reading a new committed value while in the middle of a transaction (i.e. The data change during processing)

A Phantom Row happens when reading a row that was deleted by someone else ( and that person did not commit yet)

Transactions

Grey Matter, Sella Synergy India

2min

Per-Method Isolation Level •

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For CMT, transaction isolation levels can be set at the method level. Four levels are available: TRANSACTION_READ_UNCOMMITTED, TRANSACTION_READ_COMMITTED, TRANSACTION_REPEATABLE_READ, TRANSACTION_SERIALIZABLE

To Set Specific Isolation Level to a Method in weblogic-ejb-jar.xml: TRANSACTION_READ_UNCOMMITTED <method> <ejb-name>dogEJB <method-name>*

Transactions

Grey Matter, Sella Synergy India

2min

TRANSACTION_READ_UNCOMMITTED

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TRANSACTION_READ_UNCOMMITTED Enables reading in modified data even if that data was not committed

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yet Is the most permissive transaction isloation level

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Is the fastest transaction isolation level

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Allows dirty reads, phantom rows and non-repeatable reads.

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Transactions

Grey Matter, Sella Synergy India

2min

TRANSACTION_READ_COMMITTED

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TRANSACTION_READ_COMMITTED Enables reading in modified data if it has been committed

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Does NOT allow dirty reads

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Allows phantom rows and non-repeatable reads

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Transactions

Grey Matter, Sella Synergy India

2min

TRANSACTION_REPEATABLE_READ

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TRANSACTION_REPEATABLE_READ Enables reading in modified data if it has been committed, but with the

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guarantee that the data will not change again Does NOT allow dirty reads and non-repeatable reads

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Allows phantom rows

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Transactions

Grey Matter, Sella Synergy India

2min

TRANSACTION_SERIALIZABLE

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TRANSACTION_SERIALIZABLE Locks data so that only 1 transaction at a time can access it: transactions

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are done in 'series' Is the most restrictive transaction isolation level, resulting in the highest

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integrity Is the slowest transaction isolation level

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Does NOT allow dirty reads, phantom rows and non-repeatable reads.

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Transactions

Grey Matter, Sella Synergy India

2min

Best Practices – Design Considerations

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If possible, always use a CMT

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Think about transaction granularity when designing EJBs: Transaction configuration can have a big impact on

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performance. Too many fine-grain transactions generate excessive traffic

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to transaction and resource managers. Too many coarse – grain transactions may block other

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clients accessing the same resources.

Transactions

Grey Matter, Sella Synergy India

2min

Best Practices – Design Considerations

Try to limit a web request to a single transaction

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Do not have transactions span user input

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Try creating a facade session bean that wraps multiple EJB

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method invocations to avoid creating multiple transactions Use Required on methods that update, create, delete or

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modify databases Use Supports or Not Supported on methods that perform get

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or read-only operations.

Transactions

Grey Matter, Sella Synergy India

2min

Recap

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Mark a method with one of six transaction attributes : Required

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RequiresNew

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Mandatory

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Supports

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Notsupported

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Never

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Transactions

Grey Matter, Sella Synergy India

2min

Recap

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When the method is called, the container uses the attributes to do one of

the five things: ●

Run the method in the caller’s transaction. OR

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Suspends the caller’s transaction and Start a new transaction. OR

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Suspend the caller’s transaction and run the method without a transaction. OR

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Throw an exception because the caller does not have a transaction.

Transactions

Grey Matter, Sella Synergy India

2min

Q&A

Handling JavaException Collections Framework

Grey Matter, Sella Synergy India

2min