Databases Reference
In-Depth Information
6
Locking and Concurrency
WHAT'S IN THIS CHAPTER?
➤
ACID: the properties of transactions worth protecting
➤
Avoiding concurrency dangers with locks
➤
Lock resources and modes
➤
A look at how lock escalation works
➤
A brief description of deadlocks
➤
Understanding how isolation levels af ect locking behavior
WROX.COM CODE DOWNLOADS FOR THIS CHAPTER
The
wrox.com
code downloads for this chapter are found at
www.wrox.com/remtitle
.cgi?isbn=1118177657
on the Download Code tab. The code is in the Chapter 6 download
and individually named according to the names within the chapter.
OVERVIEW
Tommy Cooper, the late great comic magician, did a trick in which he put two handkerchiefs,
one white and one blue, into a bag. He said a magic word, pulled them out again, and then
stated that the white one had turned blue, and the blue one had turned white. It's an excellent
trick, though perhaps misunderstood, because the audience gets the impression that no change
has occurred at all, and that he is simply pretending that the colors have swapped.
All joking aside, when you put something into a database, you have a certain level of expecta-
tion. You want to be assured that any data that has been entered can be retrieved in the same
state, notwithstanding another process coming along and explicitly changing or deleting it.
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