Using an exposed domain model - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 298

The sequence of events is as follows:

The web container invokes the connection filter ( OpenSessionInViewFil-
ter or OpenPersistenceManagerInViewFilter ).

1

The connection filter, which manages connections, calls the persistence
framework to open the connection.

2

The connection filter calls the transaction management filter.

3

The transaction management filter begins a transaction. 4

The transaction management filter calls the servlet.

5

The servlet invokes the PlaceOrderService . 6

The PlaceOrderService is invoked and updates the pending order.

7

The servlet forwards the request to the JSP page. 8

The JSP uses the domain objects such as the PendingOrder to generate the
response.

9

The transaction management filter commits a transaction.

10

The connection filter calls the persistence framework to close the connec-
tion.

11

This design is extremely simple. The filters manage connections and ensure that
the servlet and JSP pages execute within a transaction. In addition, filters can be
used with both local transactions and JTA transactions. But one problem with
managing transactions in the presentation tier is that handling transaction roll-
backs and retries is quite tricky.

Handling transaction retries
In chapter 12, we will see that an application must sometimes roll back and retry a
transaction if, for example, a deadlock or an optimistic locking error occurs. The
problem with trying to roll back a transaction in the presentation tier is that it is
not always possible to undo the side effects of the servlets and JSP pages that were
executed as part of the transaction. For example, to roll back a transaction the
presentation tier must reset the HttpServletResponse and clear any output gener-
ated by any JSP pages. To guarantee that this can be done, the application must
buffer the output of the JSP pages to ensure that none of it is sent back to the
browser, which increases the application's memory usage.
You must also solve the problem of servlet API s that can only be called once.
For example, the input stream of a ServletRequest can be read only once, which