IV: Strategic Design
As systems grow too complex to know completely at the level of individual objects, we need techniques for manipulating and comprehending large models. This part of the book presents principles that enable the modeling process to scale up to very complicated domains. Most such decisions must be made at team level or even negotiated between teams. These are the decisions where design and politics often intersect.
随着系统的增长，它会变得越来越复杂，当我们无法通过分析对象来理解系统的时候，就需要掌握一些操纵和理解大模型的技术了。本书的这一部分将介绍一些原则。遵循这些原则，就可以对非常复杂的领域进行建模。大部分这样的决策都需要由团队来制定，甚至需要多个团队共同协商制定。这些决策往往是把设计和策略综合到一起的结果。
The goal of the most ambitious enterprise system is a tightly integrated system spanning the entire business. Yet the entire business model for almost any such organization is too large and complex to manage or even understand as a single unit. The system must be broken into smaller parts, in both concept and implementation. The challenge is to accomplish this modularity without losing the benefits of integration, allowing different parts of the system to interoperate to support the coordination of various business operations. A monolithic, all-encompassing domain model will be unwieldy and loaded with subtle duplications and contradictions. A set of small, distinct subsystems glued together with ad hoc interfaces will lack the power to solve enterprise-wide problems and allows consistency problems to arise at every integration point. The pitfalls of both extremes can be avoided with a systematic, evolving design strategy.
最负雄心的企业系统意欲实现一个涵盖所有业务、紧密集成的系统。然而在几乎所有这种规模的组织中，整体业务模型太大也太复杂了，因此难以管理，甚至很难把它作为一个整体来理解。我们必须在概念和实现上把系统分解为较小的部分。但问题在于，如何保证实现这种模块化的同时，不失去集成所具备的好处；从而使系统的不同部分能够进行互操作，以便协调各种业务操作。如果设计一个把所有概念都涵盖进来的单一领域模型，它将会非常笨拙，而且将会出现大量难以察觉的重复和矛盾。而如果用临时拼凑的接口把一组小的、各自不同的子系统集成到一起，又不具备解决企业级问题的能力，并且在每个集成点上都有可能出现不一致。通过采用系统的、不断演变的设计策略，就可以避免这两种极端问题。
Even at this scale, domain-driven design does not produce models unconnected to the implementation. Every decision must have a direct impact on system development, or else it is irrelevant. Strategic design principles must guide design decisions to reduce the interdependence of parts and improve clarity without losing critical interoperability and synergy. They must focus the model to capture the conceptual core of the system, the “vision” of the system. And they must do all this without bogging the project down. To help accomplish these goals, Part IV explores three broad themes: context, distillation, and large-scale structure.
即使在这种规模的系统中采用领域驱动设计方法，也不要脱离实现去开发模型。每个决策都必须对系统开发产生直接的影响，否则它就是无关的决策。战略设计原则必须指导设计决策，以便减少各个部分之间的互相依赖，在使设计意图更为清晰的同时而又不失去关键的互操作性和协同性。战略设计原则必须把模型的重点放在捕获系统的概念核心，也就是系统的“远景”上。而且在完成这些目标的同时又不能为项目带来麻烦。为了帮助实现这些目标，这一部分探索了 3 个大的主题：上下文、精炼和大型结构。
Context, the least obvious of the principles, is actually the most fundamental. A successful model, large or small, has to be logically consistent throughout, without contradictory or overlapping definitions. Enterprise systems sometimes integrate subsystems with varying origins or have applications so distinct that very little in the domain is viewed in the same light. It may be asking too much to unify the models implicit in these disparate parts. By explicitly defining a BOUNDED CONTEXT within which a model applies and then, when necessary, defining its relationship with other contexts, the modeler can avoid bastardizing the model.
其中上下文是最不易引起注意的原则，但实际上它却是最根本的。无论大小，成功的模型必须在逻辑上一致，不能有互相矛盾或重叠的定义。有时，企业系统会集成各种不同来源的子系统，或包含诸多完全不同的应用程序，以至于无法从同一个角度来看待领域。要把这些不同部分中隐含的模型统一起来可能是要求过高了。通过为每个模型显式地定义一个 BOUNDED CONTEXT，然后在必要的情况下定义它与其他上下文的关系，建模人员就可以避免模型变得缠杂不清。
Distillation reduces the clutter and focuses attention appropriately. Often a great deal of effort is spent on peripheral issues in the domain. The overall domain model needs to make prominent the most value-adding and special aspects of your system and be structured to give that part as much power as possible. While some supporting components are critical, they must be put into their proper perspective. This focus not only helps to direct efforts toward vital parts of the system, but it keeps the vision of the system from being lost. Strategic distillation can bring clarity to a large model. And with a clearer view, the design of the CORE DOMAIN can be made more useful.
通过精炼可以减少混乱，并且把注意力集中到正确的地方。人们通常在领域的一些次要问题上花费了太多的精力。整体领域模型需要突出系统中最有价值和最特殊的那些方面，而且在构造领域模型时应该尽可能把注意力集中在这些部分上。虽然一些支持组件也很关键，但绝不能把它们和领域核心一视同仁。把注意力集中到正确的地方不仅有助于把精力投入到关键部分上，而且还可以使系统不会偏离预期方向。战略精炼可以使大的模型保持清晰。有了更清晰的视图后，CORE DOMAIN 的设计就会发挥更大的作用。
Large-scale structure completes the picture. In a very complex model, you may not see the forest for the trees. Distillation helps, by focusing the attention on the core and presenting the other elements in their supporting roles, but the relationships can still be too confusing without an overarching theme, applying some system-wide design elements and patterns. I’ll give an overview of a few approaches to large-scale structure and then go into depth on one such pattern, RESPONSIBILITY LAYERS, to explore the implications of using such a structure. The specific structures discussed are only examples; they are not a comprehensive catalog. New ones should be invented as needed, or these should be modified, through a process of EVOLVING ORDER. Some such structure can bring a uniformity to the design that accelerates development and improves integration.
大型结构是用来描述整个系统的。在非常复杂的模型中，人们可能会“只见树木，不见森林”。精炼确实有帮助，它使人们能够把注意力集中到核心元素上，并把其他元素表示为支持作用，但如果不贯彻某个主旨来应用一些系统级的设计元素和模式的话，关系仍然可能非常混乱。我将概要介绍几种大型结构方法，然后详细讨论其中一种模式——RESPONSIBILITY LAYER（职责层），通过这个示例来探索使用大型结构的含义。我们所讨论的特殊结构只是一些例子，它们并不是大型结构的全部。当需要的时候，应该创造新的结构，抑或修改这些结构，但均需遵循演化顺序（EVOLVING ORDER）的过程来进行。一些大型结构能够使设计保持一致性，从而加速开发，并提高集成度。
These three principles, useful separately but particularly powerful taken together, help to produce good designs—even in a sprawling system that no one completely understands. Large-scale structure brings consistency to disparate parts to help those parts mesh. Structure and distillation make the complex relationships between parts comprehensible while keeping the big picture in view. BOUNDED CONTEXTS allow work to proceed in different parts without corrupting the model or unintentionally fragmenting it. Adding these concepts to the team’s UBIQUITOUS LANGUAGE helps developers work out their own solutions.
这 3 种原则各有各的用处，但结合起来使用将发挥更大的力量，遵守这些原则就可以创建出好的设计，即使是对一个非常庞大的没有人能够完全理解的系统也是如此。大型结构能够保持各个不同部分之间的一致性，从而有助于这些部分的集成。结构和精炼能够帮助我们理解各个部分之间的复杂关系，同时保持整体视图的清晰。BOUNDED CONTEXT 使我们能够在不同的部分中进行工作，而不会破坏模型或是无意间导致模型的分裂。把这些概念加进团队的 UBIQUITOUS LANGUAGE 中，可以帮助开发人员找出他们自己的解决方案。