T R A N S A C T I O N S  

Chapter  

7 

7. Transactions 

A Unit of Work in Action 

An EJB container must provide some service to the applications deployed on and the user 

using the applications. The transaction service allows an application and / or the user of an 

application to specify the unit of work that is defined by four different aspects called ACID 

for short: 

Atomic: A transaction is performed completely or not at all meaning that any changes 

made during the transaction must be reversed if the transaction is not successfully 

completed. 

Consistency: The transaction must ensure that the system (data store) is in a 

consistent state after the transaction has ended. Of course the system must have been 

in a consistent state before the transaction was started otherwise this does not apply. 

Note that the application developer must still make his/her part to ensure 

consistency.  

Isolation: The transaction must run isolated from any other processes or transactions 

until transaction is finished. The two extremes are that either all other sees the 

changes during a transaction or that only the process in the transaction sees the 

changes and the other see the old data. As you maybe can imagine the isolation comes 

with a performance hit. Therefore in J2EE are several types of isolations defined. 

Durable: The changes in a transaction must be made persistent when a transaction 

finishes.  

Transactions are pretty important and most people take it for given but a good 

implementation needs a good design from the beginning. Imagine that you want to order a 

flight on line. The airline would not be very happy when you could reserve the seats but 

never pay for the flight. On the other hand you want to get the selected seats after you paid 

for the tickets. These requirements mean that the seats must be locked when they are 

selected until the customer pays and if not the seats must be released. 

52