Rental Eligibility: When to Use Strategies vs Rule Objects

Rental eligibility determines whether a prospective tenant meets a landlord’s minimum requirements, and it is typically evaluated using either a rule object or a strategy pattern. Use a rule object when the eligibility criteria are fixed, few, and unlikely to change; use a strategy pattern when you need to swap between different eligibility algorithms at runtime, such as for different property types or applicant categories. This article explains the difference, when to choose each, and how they are implemented in rental screening systems.

Formula

The calculator above checks two common rental criteria: a minimum credit score and a minimum income-to-rent ratio. The formula is:

Eligible = (credit_score ≥ 650) AND (income ≥ 3 × rent)

Where:

• credit_score: the applicant’s credit score (range 300–850)
• income: the applicant’s monthly gross income (in USD)
• rent: the monthly rent of the property (in USD)

The result is 1 if both conditions are met, otherwise 0. The income threshold of 3× rent is a common industry rule of thumb, but it varies by market.

Worked Example

Using the defaults: credit score = 700, income = 5000, rent = 1500.

1. Credit check: 700 ≥ 650 → true
2. Income check: 5000 ≥ 3 × 1500 = 4500 → true
3. Both true → eligible = 1

The calculator returns 1, meaning the applicant is eligible under these criteria.

Background: Rule Objects and Strategy Patterns

In software design, a rule object encapsulates a single business rule (e.g., “credit score must be at least 650”) and provides a method to evaluate it against an applicant’s data. A strategy pattern defines a family of algorithms (e.g., “standard screening”, “student screening”, “corporate housing screening”) and makes them interchangeable at runtime. Both are used in rental eligibility systems, but they solve different problems.

How They Work in Practice

Rule objects are typically simple classes with an isEligible(applicant) method. They can be combined with logical operators (AND, OR, NOT) to build a composite rule. For example:

• Rule A: credit score ≥ 650
• Rule B: income ≥ 3× rent
• Composite: A AND B

Strategy pattern, on the other hand, involves a context (the screening process) that holds a reference to a strategy object. The strategy defines the entire eligibility algorithm, which might use multiple rules internally. At runtime, the context can switch strategies based on property type, applicant category, or user preference.

How to Choose Between Them

Choose a rule object when:

• The criteria are static and rarely change.
• You need to evaluate individual conditions separately (e.g., for reporting).
• The logic is simple and can be expressed as a few boolean expressions.

Choose a strategy pattern when:

• You have multiple distinct eligibility algorithms (e.g., different for students, seniors, corporate tenants).
• You need to switch algorithms at runtime without modifying the main screening class.
• The algorithm involves complex logic that goes beyond simple rule composition.

In many real systems, you use both: a strategy pattern to select the overall algorithm, and rule objects inside each strategy to evaluate individual conditions.

Common Problems and Misconceptions

A common misconception is that rule objects and strategy patterns are mutually exclusive. In fact, they are complementary. Another issue is over-engineering: using a strategy pattern for a single, fixed set of criteria adds unnecessary complexity. Conversely, using only rule objects when you need to swap entire algorithms leads to messy conditional code. Also, beware of duplicating logic across strategies—extract shared rules into reusable rule objects.

Comparison: Rule Objects vs. Strategy Pattern

Here is a quick comparison:

• Granularity: Rule objects are fine-grained (single condition); strategy is coarse-grained (whole algorithm).
• Reusability: Rule objects are highly reusable; strategies are more context-specific.
• Flexibility: Strategies allow runtime switching; rule objects are usually static.
• Complexity: Rule objects are simpler; strategies add indirection.
• Use case: Rule objects for fixed criteria; strategy for multiple, interchangeable screening policies.

Costs and Maintenance

Rule objects are cheap to implement and maintain when the number of rules is small. Strategy pattern requires more upfront design and more classes, but it pays off when you have multiple distinct screening policies that change independently. Both require testing to ensure that combinations of rules behave as expected. There is also a cost in terms of code readability—too many tiny rule classes can become hard to navigate.

Alternatives

Alternatives include using a decision table, a rules engine (like Drools), or a simple if-else chain. Decision tables are good when criteria are numerous but static. Rules engines allow externalizing rules in a database or file, but add significant complexity. If-else chains work for small, one-off scripts but become unmaintainable as logic grows. For most rental eligibility systems, a combination of rule objects and strategy pattern is a solid, pragmatic choice.

Frequently Asked Questions

Can I use rule objects without a strategy pattern?

Yes, if you have only one eligibility algorithm, you can simply compose rule objects directly in your screening service. There’s no need for a strategy interface.

When should I refactor from rule objects to strategy pattern?

Refactor when you find yourself adding if-else or switch statements to select between different sets of rules based on property type or applicant category. That’s a sign that the strategy pattern would make the code cleaner.

How do I handle dynamic rules that change over time?

If rules change frequently, consider storing them in a database or configuration file and using a rules engine. Rule objects are best for stable rules.

Is the strategy pattern overkill for a small rental app?

Often yes. For a small app with a single screening policy, simple rule objects are sufficient. Add strategy only when you actually have multiple policies.

Can I combine rule objects and strategy pattern?

Absolutely. A common pattern is to have a strategy for each applicant type, and each strategy internally uses a set of rule objects. This gives you both flexibility and reusability.

What about performance? Does strategy pattern slow down screening?

No, the overhead of a strategy pattern is negligible. The main cost is in code structure, not runtime performance.

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in credit_score | Credit score | - | 700 | 300 | 850
in income | Monthly gross income | USD | 5000 | 0 | 1000000
in rent | Monthly rent | USD | 1500 | 0 | 100000
out eligible | Eligible | - | (credit_score >= 650) * (income >= 3 * rent) | 0