Getting Started with DB2 Express-C

218 Getting Started with DB2 Express-C
Figure 13.4 – Lost Update
In Figure 13.4 there are two applications attempting to update the same row. The one on
the left is application App1, and the one on the right is application App2. The sequence of
events is then:
1. App1 updates a row
2. App2 updates the same row
3. App1 commits
4. App2 commits
App1's update is lost when App2 make its update, hence the term “Lost Update”.
13.3.2 Uncommitted read
An uncommitted read, or “dirty read” allows for an application to read information that has
not been committed, and therefore is not necessarily accurate.
Figure 13.5 – Uncommitted Read
Figure 13.5 follows this sequence of events:
1. App1 updates a row
2. App2 reads the new value from that row
3. App1 rolls back its changes to that row
App2 is reading uncommitted data, and hence invalid data, which is why this problem is
called an “uncommitted read”