Sorry, but i has to bring this up again. Need some clear answers to calm
down our customers :)
A scenario:
The EJB Supermarket has a newly opened store where they has specialized
in selling EJB related software. The have a super modern line of 30 cash
registers all running a new sale system that of course is developed
using EJB.
Ok, the mega seller for the last couple of weeks is a box with the jBoss
EJB server, bundled with some utility programs for only $49, and that
includes 1 year of support! As you all understand there are more or less
always a queue of peoples with a jBoss box in their hand.
Now what happens in this new system when a cashier is scanning an item?
There is a call to a session bean called MySession. This bean will in
turn do a findByPrimaryKey to the entity bean Item and when he gots the
remote interface he will query the bean of its price using method
getPrice and its description using method getDescription.
The Item bean is configured to use Commit Option A, since there is no
others interacting with the database. But the consultants that has
developed the system has now been argueing for a while what it means
that the ejb server is using pessimistic concurrency? They have read a
couple of threads on the mailing lists how this works during
transactions: an entity bean will serialize calls from multiple clients
that calls from different transactions. But in this case all calls are
made without transactions. What does pessimistic concurrenct mean in
that situation?
Say that cash register 1 and 2 at the same time scans a jBoss box,
instanciates a MySession which will call the findByPrimaryKey for the
jBoss item to get price/description. Will there be 1 or 2 instances of
the jBoss item? If one: how will that instance serv the calling clients?
Will they be handled one after the other? What about the readonly flag
that you may set on a bean, how will that change the situation?
Thanks,
Lennart
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