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full control of the microprocessor。
o To implement tasks that you want to run separately from the main program； you
use threads。
o Each program that is started is a task and has a main thread from which you can start
other threads。
o Threading is not difficult and easily implemented。 What is more difficult is synchroniza
tion between the threads。
o Synchronization is not about the data； but about synchronizing access to code that
modifies data that is shared。 If your data is not shared； you don’t need synchronization。
o In the context of a single application； you will use either the exclusive lock or the Monitor
for synchronization。
o Locking code slows down the code。 You should keep the locks for the shortest time possible。
o To improve throughput； you can take a snapshot of the data。
o Higher…level synchronization abstractions are the reader/writer and producer/consumer
architectures。
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C HA P TE R 1 3 ■ L E AR N IN G AB O U T M U L T IT HR E AD IN G 369
o Reader/writer locks are exclusive locks； but they separate code that reads from code
that writes。 To improve code efficiency； reader/writer locks allow multiple readers； but
only a single writer。 Reader/writer locks are effective only if； for the most part； you are
reading data。
o Producer/consumer locks split the task of producing and consuming data into two sepa
rate steps。 The API uses the producer/consumer concept extensively； examples are
Windows。Forms and IAsyncResult。
o Deadlocks occur because timing changes cause your code to not be deterministic。
o Deadlocks can be partially avoided using Monitors； but the most effective way of avoiding
deadlocks is to use the producer/consumer development technique。 This is because the
producer/consumer architecture takes the approach of handing off data； rather than
sharing data。
o Applications that multitask effectively are applications that have been designed using
logic； rather than development techniques where you think up the code as you go along。
Some Things for You to Do
The following is an exercise to help you apply what you learned in this chapter。
1。 Write a general architecture that generates a series using multiple threads。 For the first
series； generate the square of all numbers between 1 and 100。 For the second series；
generate the Fibonacci series。 That is； after two starting values； each number is the sum
of the two preceding numbers。 The first Fibonacci numbers are 0； 1； 1； 2； 3； 5； 8； 13； 21；
34； 55； 89； 144； 233； 377； 610； 987； 1597； 2584； 4181； 6765； 10946； 17711； 28657； 46368；
75025； 121393； 196418； and 317811。 Outline the limits of multithreading when generating
a series。
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C H A P T E R 1 4
■ ■ ■
Learning About Relational
Database Data
Literally hundreds of techniques； tips； tricks； and libraries are associated with relational data
bases。 Covering relational databases would take an entire a book； and in fact； many books are
devoted to that subject。 This chapter will cover the basics and give you enough knowledge to
read and write to a database。
The focus of this chapter is to demonstrate accessing relational databases with ADO
and the Visual Studio Dataset Designer。 Since we’ll need a database to work with； you’ll also
learn how to create a database with Visual Basic Express。
A relational database stores data； such as the lottery numbers in Chapter 10’s example。
Building on that lottery…prediction example； in this chapter； you’ll see how to create a database
application that reads and writes the lottery numbers； and associates winners with a particular
lottery drawing。
Understanding Relational Databases
As a beginner developer； one of your greatest challenges will be how to manage relational database
data。 A relational database is an old piece of technology。 The relational model on which it is based
was written in about 1969 （according to the relational model entry at http：//en。wikipedia。org/
wiki/Relational_model）。 A relational database is defined as follows （http：//en。wikipedia。org/
wiki/Relational_database）：
Strictly； a relational database is merely a collection of relations （frequently called
tables）。 Other items are frequently considered part of the database； as they help to organize
and structure the data； in addition to forcing the database to conform to a set of
requirements。
Relational Database Tables
A relational database is a collection of tables。 When you were reading and writing a file in
Chapter 10’s example； you were actually reading and writing a table of lottery numbers； which
looked something like this：
371
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372 CH AP T E R 1 4 ■ L E A R N I N G A B OU T R E L A TI O N AL DA TA B AS E D AT A
2000。05。31 5 6 13 23 25 37 43
2000。06。03 7 10 11 18 32 41 5
2000。06。07 15 23 24 28 38 39 45
2000。06。10 1 3 12 23 29 33 27
2000。06。14 2 4 13 19 39 45 26
2000。06。17 3 8 17 19 21 25 35