Last updated on Apr 19, 2024
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What are domain services?
2
When to use domain services?
3
How to design domain services?
4
How to test domain services?
5
What are some benefits and challenges of domain services?
6
Here’s what else to consider
Domain-driven design (DDD) is a way of building software that focuses on the core business problems and the domain models that represent them. DDD helps you create a common language, avoid unnecessary complexity, and align your code with your business goals. But how do you design the services that implement the logic and behavior of your domain models? In this article, we'll explore some common patterns and principles for designing domain services in DDD.
Key takeaways from this article
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Apply dependency injection:
Ensure your domain services are flexible by using dependency injection, which allows services to receive the resources they need without being directly coupled to them. This method promotes a design that's easy to maintain and adapt as requirements evolve.
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Encapsulate complex logic:
By moving intricate logic into domain services, you create more readable and maintainable models. Your code becomes cleaner, and changes in one part of the system are less likely to cause issues elsewhere.
This summary is powered by AI and these experts
- Ahmed Mahmoud Albadawy Head Of Technology @GS1 | Solution…
- David Shergilashvili 🧑💻 Engineering Manager | 🏗️ .NET…
1 What are domain services?
Domain services are classes that encapsulate domain logic that doesn't belong to a single entity or value object. They usually operate on multiple domain objects or perform complex calculations or validations. For example, a domain service could be responsible for transferring money between two accounts, checking the availability of a product, or generating a unique identifier for a customer. Domain services are part of the domain layer, which is the heart of your DDD application.
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Domain Service should follow the below pattern based on current trends1. It should be bounded to a specific bounded context. 2. It should have a single, well-defined responsibility.3. It should be designed to be idempotent.4. It should be designed as a single unit of work5. Domain services can be designed to use event sourcing.6. Domain services can be designed to perform certain operations asynchronously,7. Domain services should have robust error-handling mechanisms8. Domain services should be designed with extensibility and reusability in mind.
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2 When to use domain services?
Domain services are not always necessary. In fact, you should try to avoid creating too many domain services, as they can introduce unwanted dependencies and reduce cohesion. A good rule of thumb is to use domain services only when the logic is not a natural responsibility of an entity or value object, and when it is stateless and side-effect free. You should also make sure that the domain service has a clear and meaningful name that reflects its purpose and role in the domain.
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- Ahmed Mahmoud Albadawy Head Of Technology @GS1 | Solution Architect | Digital Transformation | Strategic Planning | Software Director
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Domain service is used when you have a business that is not part of single entity and needs to be implemented. this must be carefully designed and used when needed because on many cases a lot of entities' business moved wrongly to domain services which in turn results on a bad design to be maintained and understood
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3 How to design domain services?
When designing domain services, there are some common patterns and principles that can help you achieve a consistent and effective result. Dependency injection is one of them, allowing you to provide the domain service with the resources it needs, such as repositories, factories, or other services. Interfaces should be used to define the contracts of the domain service and its dependencies, while the facade pattern simplifies interaction between the domain service and its clients. Additionally, the strategy pattern can be used to vary the behavior of the domain service based on different scenarios or rules. All of these techniques will help you create a better domain service.
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- David Shergilashvili 🧑💻 Engineering Manager | 🏗️ .NET Solution Architect | 💻 Software Developer | 🐈 Herding Cats and Microservices
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Name should come from Ubiquitous Language, clearly convey purposeOperate at same level of abstraction as domain - high level vs low levelTake domain objects as input, return domain objects as outputAre "closed against modification, open for extension" - introduce new services vs modifying existingDepend on interfaces, not concrete classes. Enables loose coupling and easier testingExpose coarse-grained, aggregate-oriented interfaces. Fine-grained leads to chattiness and tight couplingReal world example: Fund Transfer service in banking domain orchestrates Accounts, Ledgers, Limits, etc.
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4 How to test domain services?
Testing domain services is essential to guarantee their correctness and reliability. Depending on the level of abstraction and isolation you want to achieve, you can use various types of tests. Unit tests are the most basic and granular, verifying the logic and behavior of a single domain service method. Mocking frameworks can be used to simulate the dependencies of the domain service and isolate it from the rest of the system. Integration tests are more comprehensive, verifying the interaction between the domain service and its dependencies such as repositories, factories, or other services. Real or in-memory implementations of the dependencies can be used to check the state of the system after executing the domain service method. End-to-end tests are the most realistic and complex, verifying the functionality and performance of the whole system from user interface to database. Automation tools can be used to simulate user actions and scenarios and validate expected outcomes and responses.
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- David Shergilashvili 🧑💻 Engineering Manager | 🏗️ .NET Solution Architect | 💻 Software Developer | 🐈 Herding Cats and Microservices
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Unit test domain logic with Test Doubles (mocks/stubs) for dependenciesUse Behavior-Driven Development (BDD) to test expected outcomes given certain scenariosIntegration test with real collaborators to verify end-to-end flowAvoid testing private methods, implementation detailsFocus on observable behaviors and domain invariantsConsider Test Data Builders and Object Mother to reduce test setup duplication
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5 What are some benefits and challenges of domain services?
Domain services can offer both benefits and challenges for your DDD application. On one hand, they can help encapsulate domain logic that is not specific to a single entity or value object, allowing for reusability and testability. Additionally, domain services can separate the domain layer from the infrastructure layer, and thus promote loose coupling and high cohesion. On the other hand, domain services can introduce complexity and overhead for your DDD application, increasing the number of classes and dependencies in your system, as well as making it harder to maintain and understand. Moreover, if they are not aligned with the domain model and the ubiquitous language, domain services can create duplication and inconsistency.
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- David Shergilashvili 🧑💻 Engineering Manager | 🏗️ .NET Solution Architect | 💻 Software Developer | 🐈 Herding Cats and Microservices
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Benefits:Encapsulate complex logic that doesn't fit entitiesAllow coordinating changes that span aggregatesSupport readable models by factoring out logic into descriptive servicesChallenges:Can lead to anemic models if overusedCan turn into "god" services that attract unrelated logicCan be harder to test in isolation given many dependenciesCan introduce coupling between bounded contexts if not carefully designed
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6 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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- David Shergilashvili 🧑💻 Engineering Manager | 🏗️ .NET Solution Architect | 💻 Software Developer | 🐈 Herding Cats and Microservices
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Consider CQRS with domain events to reactively update read models/projectionsUse sagas/process managers to orchestrate long-running, multi-step processesApply ports & adapters architecture to decouple domain from infra concernsLook at Service Layer pattern for non-domain logic like security, txn managementStudy open source DDD samples to see domain services in action
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