1. Lesson Objectives Aims You should be able to:
Identify the inputs and outputs for a given situation
Determine the preconditions for devising a solution to a problem
You should be able to discuss:
The nature, benefits and drawbacks of caching
The need for reusable program components

2. What’s it about
This section is really all about planning and resourcing
Before a project begins, research should be conducted:
What do current systems do
How do they work (exactly)
What is needed
What resources do you need to make a new system?
How will you work efficiently

3. Inputs and outputs A system design should start with the outputs
What is this system actually meant to do?
We can then work backwards to identify the processes required to generate these outputs…
…and the inputs these processes will require!

4. Task
Programs will have a number of inputs and outputs that form some element of the final outcome of the coded solution.
These inputs are combined with processes to create outcomes.

5. Task 1
Consider a simple game of hangman to find a six letter word.
List all the Inputs
List the outputs

6. Now consider the processes and decisions that take place when a letter is selected or entered.
The letter is either correct or incorrect – this can be considered as a process.
The response that is fed back to the player is an output
Create a flowchart diagram showing the inputs, outputs, processes and decisions for the hangman game
Can your design be refined?

7. Caching
Caching is the idea of keeping a local store of data to speed up a process
It may contain:
Frequently used data
Frequently used instructions
Data that a system has predicted it will need next (pre-fetching)

8. A cache may be used in many scenarios:
Caching
A cache may be used in many scenarios:
Storing instructions and data in a CPU
In a database system
On a proxy server in a network
By large website hosts and ISP’s

9. Advantages
Caching will significantly increase system performance as data is available locally, almost immediately
Saves waiting for long read/write operations
Prevents repeated read/write, fetch/store operations
Can reduce bandwidth/network usage

10. Disadvantages Can be very complex to set up, especially in DB systems
Can cause synchronisation issues – is the cache the most up to date or the data store?
Difficult to code prediction routines
If there is a cache “miss” (a branch unexpectedly happens in code or similar) then the whole cache must be flushed and refilled causing a stall in performance

11. This is because if you want to do something, it’s probably:
Reusable Components
The phrase “don’t reinvent the wheel” is constantly reiterated in Computer Science
This is because if you want to do something, it’s probably:
Already been done
Tested
Proven to work well

12. This is where reusable code comes in:
It simply wouldn’t be feasible with the complexity of modern systems to write code that deals with every eventuality
This is where reusable code comes in:
Libraries
Classes
DLL’s

13. By re-using code we can focus on developing new functionality
Existing libraries may cater for:
Drawing and handling the user interface
File load and save
Memory management
Mathematical functions
Input and output operations
Graphics

14. Examples of re-use Component Example Software module or library
Import Random
Entire component reuse
Menu or toolbars being reused across suites of software
Programming language
Reusing the same language across all projects
API
External re-use, allowing 3rd party access to your features and libraries. Can be re-sold
Program Generator
A software tool that will aid in the development of a program using standard routines with little user input

15. Practice Paper

17. Review/Success Criteria
You should know:
The