Thursday, 8 November 2012

DDD Notes Part III

Part III Refactoring Toward Deeper Insight

8 - Breakthrough

- Breakthroughs cannot be forced, the usually occur after many modest refactorings.
- Concentrate on knowledge crunching and cultivating robust Ubiquitous Language.
- Watch for opportunities to gain deeper insight and do not hesitate to make even minor improvements.
- Breakthroughs can lead to a cascade of more breakthroughs.

9 - Making Implicit Concepts Explicit

- Developers need to sensitize to the hints revealing implicit concepts and sometimes search them out.
- Listen to the language the domain experts use. It is a warning when the experts use vocabulary that is not present in the design.
- Watch for puzzled expressions on domain experts faces when you mention particular phrases.
- When the missing concept is not on the surface of conversations with domain experts, try digging around the most awkward part of the design - i.e. the place that is hardest to explain.
- Actively engage domain experts in the search, play with ideas or use them for validation.
- Usually contradictions from opposing domain experts can reveal deeper insight to the domain.
- Business books on the domain are a great source of information when the domain expert is not available.
- Explicit Constraints, such as invariants, should be extracted in the design so that their intention is more obvious by using descriptive method names within the object.
- Constraints do not belong in the object if they require data that doesn't fit in the object's definition. Or if requirements conversation revolves around the constraint but they are hidden in procedural code within the object.
- The Specification Pattern provides a way of expressing conditional rules explicitly in the model (predicates) whilst keeping the logic in the domain layer.
- Specifications determine if an object satisfies a condition, the logic having probably been extracted from an attribute of the property.
- Specifications work well with repositories by specifying criteria to select against, esp with ORMs.
- Specs defined in terms of interfaces decouple designs and help clear logjams by using prototypes.

10 - Supple Design

- Relieve the client from having to know about the implementation by using Intention Revealing Interfaces.
- Describe purpose and effect but not how it is achieved and always with Ubiquitous language.
- Functions do not affect the state of an object and are therefore 'side-effect-free'.
- Function should be able to be called repeatedly and deeply nested. Functions are predictable and safe.
- Commands cause change of state and should be kept separate from queries (CQRS). Commands should not return domain data - leave that to functions.
- Where possible, return Value Objects from functions so you don't have to maintain its life-cycle. Furthermore, all Value Object operations should be functions as they are immutable and have no state.
- Move complex queries into Value Objects to reduce side effects.
- Make invariant conditions explicit by unit testing and making assertions.
- Search for meaningful units of functionality to make designs flexible and easier to understand.
- Use your intuition about the domain to decompose design elements (classes/aggregates) into cohesive units.
- Changes are more easily applied to Conceptual Contours and they are easier to combine and reuse.
- Standalone classes with zero dependencies other than primitives are easier to understand and reduces mental overload by having extremely low coupling.
- Every dependency is suspect until proven to be fundamental to the concept of the object.
- Prioritise the most complex computations into standalone classes (value objects?) and allow other classes to depend on them.
- Operations who's argument and return value are of the same type are said to be closed under the set of instances of that type. Closed operations do not introduce further dependencies.
- Closed Operations can operate under abstract types.
- Declarative design can be burdened by frameworks and DSLs. Use only lightweight plugins that solve a particular mundane problem well and that leave you to design more accurately.
- The specification pattern can be very useful for declarative design because individual specifications can be combined and chained to produce meaningful sentences using Ubiquitous Language.
- When trying to achieve Supple Design start with identifiable subdomains (conceptual contours?) and tackle problems step by step. It is better to make a big impact on one area, making some part of the design supple, rather than to spread efforts thinly.
- Draw on established formalisms of the domain when you can.

11 - Applying Analysis Patterns

- Analysis Patterns are not technical solutions but guides to help you work out the model.
- Analysis patterns can carry tried and tested experience of implementation and maintenance from a mature project and bring model insights, direction & discussion.
- Analysis Patterns provide cleanly abstracted vocabulary.
- Development can be stimulated by using analysis patterns and results often resemble adapted the form adapted to the circumstances, but sometimes can inspire the development in other directions.
- It is wise not to adjust the basic concept of an analysis pattern so as to keep the well understand terms and phrases consistent in your project. If model definitions change then keep class/method names up to date.
- Analysis patterns focus on the most critical decisions and offer choices. They anticipate downstream consequences that are expensive to discover yourself.

12 - Relating Design Patterns to the Model

- Design patterns present design elements that have solved problems before in purely technical terms. They can be applied to the model because they correspond to general objects that emerge in domains.
- The conventional focus of the strategy pattern focuses on substituting different algorithms. However, its use as a domain pattern focuses on its ability to express a process or policy rule as a concept.
- When a technical design pattern is used in the model we need another motivational layer of meaning to correlate actual business strategy to make the pattern more than just a useful implementation technique.
- A consequence of the strategy pattern is that it can increase the number of objects in the design and requires clients to be aware of the different strategies. This can be mitigated by implementing strategies as stateless objects that are contexts can share.
- The Composite pattern offers the same behaviour at every structural level of an inheritance hierarchy.
- The Flyweight pattern has no correspondence to the domain model, its only value is at the design level when implementing value objects. The Composite pattern differs as it is for conceptual objects composed of conceptual objects of the same type.
- This is what makes Composite pattern a domain, as the pattern applies to both model and implementation.

13 - Refactoring Toward Deeper Insight

- Is a multifaceted process: Live in the Domain > Keep looking at things differently > Maintain an unbroken dialogue with Domain experts.
- Seeking insight into the domain creates a broader context for the process of refactoring.
- Refactoring can begin in many ways - usually as a result of some awkward realisation or difficulty implementing a reasonable requirement, divergence of language, or any source of dissatisfaction.
- Exploration teams can be tasked with thinking about how to improve the model. These teams need to be self organised and small in members meeting for short frequent periods. The meetings should be spaced out over a few days to allow time to digest the new model. Brainstorming sessions should always exercise the Ubiquitous language. The end result is a refinement of the language to be formalised in code.
- Knowledge should be sought from other sources in the domain to gain insight. It is always possible that the domain has already got well defined concepts and useful abstractions.
- Refactoring toward deeper insight both leads to and benefits from a supple design. A supple design communicates it intent and limit mental overload by reducing dependencies and side effects and being fine grained only where it is most critical to users.
- Continuous refactoring has to be considered a best practice and part of the ongoing exploration of subject matter, education of developers and meeting of minds of developers and domain experts. Take any opportunity to refactor that benefits the integrity of the design and consolidates the teams's understanding. 
- Development suddenly comes to the brink of a breakthrough and plunges through to a deep model-then again begins a phase of steady refinement.

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