How can you update old code with domain-driven design?

Domain-driven design (DDD) is a way of organizing software development around the core business problems and solutions of a domain. It helps you create a shared language, model, and architecture that reflect the essence and value of your software. But what if you have a legacy codebase that does not follow DDD principles? How can you update old code with domain-driven design and benefit from its advantages? In this article, we will explore some practical steps and patterns that can help you transform your old code into a more domain-oriented and maintainable system.

1 Assess your current situation

The first step is to understand the current state of your codebase and identify the pain points and opportunities for improvement. You can use tools like code analysis, metrics, tests, and documentation to evaluate the quality, complexity, and cohesion of your code. You can also talk to your stakeholders, users, and developers to gather feedback and insights about the business domain and the software functionality. This will help you define the scope and goals of your refactoring process and prioritize the most critical and valuable areas to update.

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Amlendu Kumar

Architect • Digital Transformation • Microservices • SOA • Methodologist • Practitioner
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IMHO this is the most important step to decide that should proceed or not. Must get answer Why (Motivation ZAF) column of Contextual perspective (ultimately budget will be required to proceed) and Physical perspective as complexity will be increased definitely. Since this will affect only QoS (Quality of Services or NFR) so business layer will not be affect but “Motivation” must be business driven. Physical model will be required to get broad picture of complexity. Don’t biased to “To Be” architecture and prepare λ and present to stakeholders.

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2 Extract your domain model

The next step is to extract your domain model from your old code and separate it from the technical and infrastructural concerns. A domain model is a representation of the core concepts, rules, and behaviors of your business domain. It should be expressed in a ubiquitous language that is understood by both developers and domain experts. You can use techniques like domain storytelling, event storming, and example mapping to discover and refine your domain model. You can also use patterns like entity, value object, aggregate, and domain service to design and implement your domain model in code.

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Maximilian Beck

Solving complex business problems with software.
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Make sure to restructure in small steps to continuously collect feedback on the changes that are being made. The previous step should have produced (in collaboration with the stakeholders of the product) a documentation of the domain model including bounded contexts and their aggregates, entities value objects, etc. Divide the restructuring and tackle (depending on the size of the system) one bounded context at a time.

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3 Apply strategic design

The third step is to apply strategic design to your domain model and define the boundaries and relationships between different parts of your system. Strategic design helps you deal with the complexity and diversity of large-scale domains by dividing them into smaller and more manageable subdomains. You can use patterns like bounded context, context map, and subdomain to identify and isolate the subdomains of your system and specify how they communicate and integrate with each other. This will help you reduce coupling, increase cohesion, and align your code with your business strategy.

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Sufyan Mesameh

Network Observability/Tools Architect
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Alongside strategic design, consider applying tactical patterns within a single bounded context. Utilizing patterns like Entities & Aggregates construct and enrich your domain, leading to enhanced business clarity.

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4 Introduce domain events

The fourth step is to introduce domain events to your system and enable a more reactive and event-driven approach to your domain logic. A domain event is a record of something significant that happened in your domain. It captures the state change, the context, and the data of the event. You can use patterns like event sourcing, event notification, and event processing to store, publish, and consume domain events in your system. This will help you decouple your subdomains, improve your scalability, and enhance your traceability.

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5 Refactor your application layer

The fifth step is to refactor your application layer and adapt it to your new domain model and events. Your application layer is responsible for coordinating the interactions between your user interface, your domain model, and your infrastructure. It should be thin, simple, and consistent. You can use patterns like command, query, mediator, and saga to design and implement your application layer in code. This will help you separate your read and write operations, simplify your dependencies, and handle complex transactions.

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Sufyan Mesameh

Network Observability/Tools Architect
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Code refactoring offers numerous advantages, including improved maintainability, easier bug fixes, facilitates adherence to security standards, performance optimization, and enhanced code reusability. However, it is important to be mindful of compatibility concerns that may arise after the refactoring process. This may necessitate coordinated testing both internally and externally to address potential issues. As a result, investing in automated testing and keeping it up-to-date becomes crucial to ensure successful refactoring results.

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6 Migrate your data and infrastructure

The final step is to migrate your data and infrastructure and align them with your domain model and events. Your data and infrastructure are the external resources that your system depends on, such as databases, message brokers, APIs, etc. They should be abstracted, optimized, and secured. You can use patterns like repository, unit of work, adapter, and anti-corruption layer to access and manipulate your data and infrastructure in code. This will help you hide the implementation details, improve your performance, and protect your system from external changes.

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Gautam Kumar

Java 17 | spring boot | Microservice | System Design | Multi-Cloud | DSA | Full stack | spring cloud | Azure | NOSQL | JPA | AWS | Angular | architect | Web-flux | Cosmos db | Oauth2 | SSO | Kafka | Ex-Vodafone | Ex-UHG
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Domain Analysis: Begin by thoroughly analyzing the existing codebase to identify core domains, subdomains, and relationships. Understand how the application's business logic is structured. Bounded Contexts: Divide the application into bounded contexts, each focusing on a specific domain. This helps isolate concerns and prevents conflicts when updating or adding. Aggregates and Entities: Identify aggregates and entities within each bounded context. Refactor or redesign these components to better model the domain, ensuring data consistency and encapsulation. Strategic Design: Apply domain-driven design patterns like repositories, value objects, and services to the updated codebase. This enhances maintainability, scalability, and flexibility

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How can you update old code with domain-driven design?

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1 Assess your current situation

2 Extract your domain model

3 Apply strategic design

4 Introduce domain events

5 Refactor your application layer

6 Migrate your data and infrastructure

7 Here’s what else to consider

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