Threaded Applications

1
Threaded Applications

• Introducing additional threads in a Delphi
  application is ltRelativelygt easy

2
Threaded Applications

• A Thread is?
• Why would you introduce additional threads?
• Basic Thread Code
• Variables and Thread Safe Code
• Some Thread Models - Communication

3
What is a Thread ?

• Simple Program Sequential Steps
• Waits to read data
• Single Thread
• Windows OS runs programs at the same time
• Time allocation, System Waits
• The OS also allows sections of a program to run
  independently in the same way.
• Each TThread Object introduces an additional
  independent set of processor steps (thread)
  within the application.

4
Why Introduce Multiple Threads?

• Users are no longer prepared to wait
• If more than (?)lt1-2 Secondsgt then Thread
• More efficient
• Actions happen in other Thread wait cycles
• Many things lend themselves to Background
  running
• Large database reports
• Internet access
• Can make things simpler
• Less need for state engines

5
Basic Thread Code

• Inherit from TThread Object
• Override the Execute Routine.
• Handle exceptions within Execute
• Keep Execute Thread safe
• Must free TThread Object or the Application will
  not close
• Must allow Execute to terminate

6
Thread Safety

• Code is unsafe when a variable can be changed by
  separate threads in an uncontrolled manner and
  the change affects the program flow.
• Procedure Local Variables are safe
• Object Private and Protected variables and
  objects can be managed
• Thread Local gtgt threadvar X Integer
• Global Variable Equivalent ???

7
Synchronization Objects

• Used to manage access to variables
• TCriticalSection
• Protect blocks of code
• Acquire
• Release
• TSimpleEvent
• One Thread initiates action then waits until
  other thread has set the event

8
Synchronize Routine

• Only works with VCL thread
• You get to pass a procedure of object without
  parameters
• The procedure needs to be able to establish its
  context to access variables
• The calling thread is suspended until the
  procedure returns

9
Thread Communication Models

• Separate (console) process
• Single Run
• Recover Results in OnTerminate
• Server with Post Messages
• Post results home
• Server with Synchronized callback
• Call VCL Thread with results
• Multiplexed Server
• Wait for thread to complete action

10
Threads to Multiplex
Listen on Port
Spawn session
Handle Queries In order
Database Access
IP Session
Sql Query
Quey And Wait
Sql Results
11

• Slides and demonstration code
• http//www.innovasolutions.com.au/delphistuf/ADUGJ
  anuary2006.htm