Using offline locking patterns - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 493

not update the row and instead alerts the user. To see how this pattern works, let's
look at an example.

13.2.1 Applying the Optimistic Offline Lock pattern

The scenario shown in figure 13.2 illustrates how the Optimistic Offline Lock pat-
tern can be used to handle concurrent updates for the Acknowledge Order use
case. In this scenario, user A is attempting to acknowledge the order while user B
cancels the order.
The sequence of events is as follows:

User A: Acknowledge Order User B: Cancel Order

Transaction A.1:

BEGIN TRANSACTION

SELECT VERSION, ...
FROM PLACED_ORDER
WHERE ORDER_ID = ?

COMMIT

Transaction B.1:

BEGIN TRANSACTION

SELECT VERSION, ...
FROM PLACED_ORDER
WHERE ORDER_ID = ?
...
UPDATE
PLACED_ORDER
SET
STATUS='CANCELLED',
VERSION = VERSION + 1
WHERE ORDER_ID = ? AND
VERSION = ?

Transaction A.2:

COMMIT

BEGIN TRANSACTION

UPDATE
PLACED_ORDER
SET VERSION = VERSION + 1
...
WHERE ORDER_ID = ?
AND VERSION = ?

Figure 13.2
An example of how the Optimistic Offline
Lock pattern handles concurrent updates

...FAIL...

Time