What Are Decision Tables? Simplifying Complex Scenarios in System Design

Think of preconditions as the rules you need to follow before you can even start a process. In system design, a precondition is something that must be true or a specific state that the system and its actors must be in before a use case can begin. They set the stage, making sure everything is in place for a successful interaction.

A Simple Analogy: Getting Ready to Cook

Before you start cooking a meal, you need to check if you have all the ingredients. In this analogy, having the ingredients is the precondition. If you don’t have them, you can’t start cooking. Similarly, a system needs to meet certain conditions before a user can perform an action.

Examples from Everyday Life

Preconditions are all around us. Here are a few examples:

Boarding a Bus

Use Case: Board the bus.

Preconditions:

• The bus has arrived at the stop.
• You have a valid ticket or pass.

Watching a Movie Online

Use Case: Stream a movie.

Preconditions:

• You are logged into your streaming account.
• Your internet connection is active.

Why Preconditions Are So Important

By defining preconditions clearly, you can:

• Prevent Errors: They ensure a use case never starts from a bad or unexpected state, avoiding a range of potential issues.
• Improve Clarity: They provide a simple way to communicate the exact starting requirements to both developers and testers.
• Simplify Testing: Preconditions make it clear exactly what a test needs to set up before running a test case.

A Simple Checklist for Writing Preconditions

When you write a precondition, make sure it is:

• Specific: Don’t write “The user is ready.” Instead, write “The user is logged in.”
• Testable: Can a tester confirm the condition is true? “The account has a positive balance” is testable.
• A “Yes/No” Question: The precondition should be something that is either true or false.