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| {% include breadcrumbs.html %} | ||
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| # Communication with the outside world | ||
| <div class="header_line"><br/></div> | ||
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| This page is about the hexagonal architecture. | ||
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| The above picture has been taken from | ||
| [Growing Object-Oriented Software Guided by Tests](http://www.growing-object-oriented-software.com/figures.html) | ||
| by Steve Freeman and Nat Pryce ([Creative Commons License](http://creativecommons.org/licenses/by-sa/4.0/)). | ||
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| The central idea here is to move the dependencies to external systems | ||
| to the boundaries of your domain model. By defining ports (realized as interfaces) | ||
| and adapters (implementations of these interfaces that connect to the | ||
| external system), we | ||
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| - Prevent externalities leaking into our system, such as relational data models, | ||
| JSON and/or XML message structures, and so forth. | ||
| - Make it easy to plug in fakes and stubs into our ports, which in turn keeps | ||
| our tests independent of the external systems and guarantees that they remain fast! | ||
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| Since some of the coding katas in this repository specifically address | ||
| this topic, this page discusses the required concepts that are common | ||
| to all these katas. These concepts all fall under the umbrella of what's | ||
| generally known as the | ||
| [Hexagonal Architecture](http://alistair.cockburn.us/Hexagonal+architecture) | ||
| (a.k.a. Ports and Adapters). Uncle Bob elaborated on this concept and coined it the | ||
| [clean architecture](https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html). | ||
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| ## TDD and communication with external systems | ||
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| A question that is often raised is: How do I practice TDD when I | ||
| have to deal with the dependencies of my system to the outside world? | ||
| Examples of such dependencies are databases, file systems, networks, etc. | ||
| So isn't it inevitable then that I eventually have to include | ||
| tests involving these external systems in my tests and as | ||
| a consequence my tests (gradually) become slow? | ||
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| Many people are inclined to introduce mocks into their tests at | ||
| this point, but apparently these people haven't heard of the | ||
| [don't mock what you don't own](http://jmock.org/oopsla2004.pdf) | ||
| principle. A good but somewhat long read about this topic is also | ||
| Martin Fowler's well-known | ||
| [mocks aren't stubs](https://martinfowler.com/articles/mocksArentStubs.html). | ||
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| So what do we do then when mocks are not meant to be used for this purpose? | ||
| The answer is to apply dependency | ||
| inversion and to use so-called ports and adapters. | ||
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|  | ||
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| - [Dependency Inversion](https://stackify.com/dependency-inversion-principle/) | ||
| is the last of the five well-known | ||
| [SOLID](http://blog.cleancoder.com/uncle-bob/2020/10/18/Solid-Relevance.html) | ||
| principles. In a nutshell, it boils down to dictating to the outside world | ||
| what the data should look like, as opposed to letting the outside world | ||
| determine your (object) model. The idea is to think in terms of the | ||
| domain model first and to make the | ||
| [primary and secondary actors](https://codesoapbox.dev/ports-adapters-aka-hexagonal-architecture-explained/) | ||
| comply with the ubiquitous language established by our domain model. | ||
| This implies e.g. that no field names or data models from external systems | ||
| should be able to "leak" into our domain model! | ||
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| - [Ports and adapters](https://alistair.cockburn.us/hexagonal-architecture/): | ||
| as is explained in | ||
| [this article](https://codesoapbox.dev/ports-adapters-aka-hexagonal-architecture-explained/), | ||
| "The main principle of the Ports & Adapters architecture is to have | ||
| inputs and outputs on the edges of technology-agnostic code". | ||
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| ## References | ||
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| - A good read is the | ||
| [Ports and adapters as they should be](https://medium.com/wearewaes/ports-and-adapters-as-they-should-be-6aa5da8893b) | ||
| post by Felipe Flores. | ||
| By realizing the ports & adapters by using | ||
| [the adapter pattern](https://refactoring.guru/design-patterns/adapter) | ||
| (which is polymorphic by definition), we keep our system under | ||
| development testable all the time. | ||
| - Another good website with project structure and code examples is [Hexagonal Me](https://jmgarridopaz.github.io/). | ||
| - Nicolas Carlo on separating infrastructure (tests) from domain (tests): [If you mock, are you even testing?](https://understandlegacycode.com/blog/if-you-mock-are-you-even-testing/) | ||
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| {% include share_buttons.html %} |