Heuristics in Software Design: A Practical Guide

in software architecture, making the right design decisions often involves navigating complex trade-offs. Heuristics, or rules of thumb, help us streamline these decisions by focusing on practical approaches that work most of the time. This guide explores key heuristics for bounded contexts, business logic implementation, architectural patterns, and testing strategies.

What Is a Heuristic?

A heuristic is an experience-based technique that helps solve problems quickly without guaranteeing a perfect solution. In software design, heuristics allow us to focus on the essential features of a system while ignoring the complexities that don’t immediately affect our goals. By using heuristics, we can make faster decisions that are “good enough” while reducing the cognitive load of overthinking.

Example:

When deciding how to break up a large monolithic system into microservices, a heuristic might suggest starting with broad service boundaries and then iterating over time. This rule of thumb helps prevent over-engineering by avoiding premature optimization.

Heuristics for Designing Bounded Contexts

Bounded contexts are fundamental in Domain-Driven Design (DDD) and represent areas of the software where a consistent model and language are applied. A common heuristic is to avoid making bounded contexts as small as possible right from the start. Instead, let the domain model guide the size of the bounded context.

Here are some helpful heuristics for bounded context design:

1. Start Broad and Refine as Needed

When you’re unfamiliar with the business domain or its complexities, it’s a good idea to start with broader boundaries. Wide boundaries offer flexibility, allowing you to adjust as you learn more about the domain. Over time, you can decompose the context into smaller ones as needed.

2. Minimize Changes Across Contexts

Software changes that affect multiple bounded contexts are costly and error-prone, especially when different teams manage them. If frequent changes span several contexts, it could indicate poor boundary design. Effective design ensures that most changes remain within a single context, reducing the need for cross-team coordination.

3. Logical vs. Physical Boundaries

It’s easier to refactor logical boundaries than physical boundaries (i.e., services or deployments). Therefore, it’s better to encapsulate closely related subdomains within the same bounded context early on. If necessary, split them into separate contexts later, once you have a clearer understanding.’

Business Logic Implementation Patterns

Choosing the right business logic implementation pattern depends on the complexity of the subdomain. Here’s a heuristic decision tree to guide your choices:

1. Transaction Script: Use for Simple Business Logic

• Best for subdomains with straightforward data operations.
• Ideal for supporting subdomains or generic subdomains where third-party integrations are common.

2. Active Record: Use for Complex Data Structures

• Suitable when data structures are complex but the business logic is still simple.
• This pattern encapsulates how data is mapped to the database, making it great for subdomains with a lot of database interactions.

3. Domain Model: Use for Complex Business Logic

• Best for core subdomains where business logic involves complicated rules, algorithms, and invariants.
• Offers flexibility in adapting to future changes.

4. Event-Sourced Domain Model: Use for Auditable, High-Complexity Logic

• Ideal for domains involving monetary transactions or where deep analytics are required.
• Event sourcing helps trace changes over time, making it indispensable for auditing.

Heuristics for Architectural Patterns

Now that you’ve chosen the right business logic implementation pattern, selecting the appropriate architectural pattern becomes straightforward.

1. CQRS for Event-Sourced Domain Models

• Event sourcing requires CQRS (Command Query Responsibility Segregation) to handle the separation of read and write models. Without CQRS, you’d be limited to querying data by ID only.

2. Ports & Adapters for Domain Models

• When implementing a domain model, the ports & adapters architecture (also known as hexagonal architecture) works best. It keeps the domain layer free from persistence concerns, unlike layered architecture.

3. Layered Architecture for Active Record

• The active record pattern pairs well with a layered architecture that includes a service layer to handle logic beyond data persistence.

4. Minimal Layered Architecture for Transaction Script

• The transaction script pattern can function with a minimal layered architecture, usually consisting of only three layers: presentation, application, and data.

CQRS can also be beneficial for other business logic patterns if the subdomain requires multiple persistent models. For example, if you need to represent the same data in different ways, CQRS offers flexibility.

Testing Strategies Based on Heuristics

Once you’ve chosen the business logic and architectural patterns, you can derive your testing strategy. Let’s look at three common strategies.

1. Testing Pyramid for Domain Models

• The testing pyramid emphasizes unit tests, with fewer integration and end-to-end tests.
• This strategy is ideal for domain models, as aggregates and value objects can be tested effectively in isolation.

2. Testing Diamond for Active Record

• The testing diamond focuses on integration tests. This is because active record patterns typically have business logic spread across both the service and data layers.
• It’s crucial to test how these layers integrate.

3. Reversed Testing Pyramid for Transaction Script

• The reversed testing pyramid prioritizes end-to-end tests, ensuring the entire workflow from start to finish works correctly.
• Since the transaction script pattern usually involves minimal layers, end-to-end tests can effectively cover the full application flow.

Tactical Design Decision Tree

The decision tree below summarizes how you can select business logic patterns, architectural styles, and testing strategies based on heuristics:

1. Identify Subdomain Type: Core, supporting, or generic.
2. Choose Business Logic Pattern: Use the decision tree to decide between transaction script, active record, domain model, or event-sourced domain model.
3. Select Architectural Pattern: Use layered architecture, ports & adapters, or CQRS based on the chosen business logic.
4. Pick Testing Strategy: Based on the complexity of your business logic, select between the testing pyramid, diamond, or reversed pyramid.

While these heuristics provide a structured framework for making design decisions, remember that they are guidelines, not hard rules. Every software system is unique, and it’s important to adapt your approach based on your team’s experience and the specific requirements of the project.

Conclusion

Heuristics simplify complex decisions, making them invaluable tools in software design. By starting with broad bounded contexts, selecting business logic patterns based on subdomain complexity, and aligning your architecture and testing strategy, you can create flexible, scalable systems without over-complicating your design.

Use these heuristics as guiding principles, but don’t be afraid to adapt them as your understanding of the business domain grows. Ultimately, the best design decisions come from a blend of experience, domain knowledge, and critical thinking.