These issues aren't necessarily caused by the framework itself, but your choice of technology certainly guides the outcome of your codebase and I've found that many of these web frameworks often point you in the wrong direction right out of the gate. The hexagonal architecture, or ports and adapters architecture, is an architectural pattern used in software design.It aims at creating loosely coupled application components that can be easily connected to their software environment by means of ports and adapters.This makes components exchangeable at any level and facilitates test automation. To ensure the clean division of the different modules, the domain layer doesn’t import from any of the other parts of the application. They all just have their own quirks. Good luck. For this example, I chose to use Flask and SqlAlchemy Core (just the DB API, not the ORM). Hexagonal Architecture in Python using Flask and SqlAlchemy The source code for the project described in this post is available on Github . There's not many resources on approaching DDD in Python, so I thought I'd write a series of articles on how we might apply the SOLID principles and the DDD tactical patterns to modern Python. Independence from databases. Actually, this isn’t a trait of DDD, it’s baked into the foundation of the SOLID principles (especially the L and the D) and thus also in hexagonal architecture. The original intent of Hexagonal Architecture is: Allow an application to equally be driven by users, programs, automated test or batch scripts, and to be developed and tested in isolation from its eventual run-time devices and databases. Because of the way the application is laid out, I didn't end up using any of the connector libraries (like flask-sqlalchemy) that help combine these libraries into a full-featured framework. But that's kinda the point of the whole exercise - maybe the part of your application that handles web requests shouldn't know the intimate details of where your data is being stored! To be able to do this we needed to implement the __hash__() function as ignored earlier in this article. One that comes to mind is overriding ORM methods and hooks in Django. The concept for this library is ambitious and its core team has not as much spare time as it would like to dedicate, so don't expect rapid development here. However, I think there are some good takeaways regardless. Although, having a proper REST interface we could drop our original main entry point, with which we’re back to “just” three hexagons. You can get it working, but it’s not a first-class citizen and it’s just not Pythonic. In other words, the approach can’t be fetched in a generic framework — therefore it also doesn’t exist or I just couldn’t find it — but it can be ported to any other language, as I did myself, back and forth, with Kotlin. First of all, an ideal project would have a clean codebase that is simple to read. The library is still in a planning phase, so expect much refactorization and many changes to its API. Flask is significantly more lightweight but is often paired with other libraries (notably the SqlAlchemy ORM) to provide a similar experience. The configuration conftest.py is necessary for Pytest to know where to find, for example, fixtures. A common pattern here is dependency injection. Asombroso Ddd ⭐ 39 Una lista cuidadosamente curada de recursos sobre Domain Driven Design, Eventos, Event Sourcing, Command Query Responsibility Segregation (CQRS). What we’ve did with the REST interface, we could also do to the repository adapter. Right now we have three hexagons: The code we’ve discussed can be found on GitHub: https://github.com/douwevandermeij/voting-system/tree/initial. In the public interface you can make a vote, get a list of all votes and the total amount of votes. About the fixture(s), we have one and that’s meant to be able to test the REST interface. Now this is where interfaces come in handy. Last, but not least – technical debt should be kept at bay to not pose a threat of lowering a team’s velocity. The preceding figure shows what a hexagonal architecture might look like. Let’s directly define a test for the adapter: I think this is pretty self-explanatory code. The new files are bold: For the tests I’ve added two API tests, a configuration file and a fixture. I think this shows that Python in a way is trying to keep up with its competition, for example with TypeScript. This both ensures you’re testing what you actually mean to as well as speeds up your tests dramatically. DDD, CQRS and Hexagonal Architecture example using inject package. Though, with “newer” versions of Python it does come with abstract base classes and methods and also (even) typing (hinting). The tests are pretty straightforward, though there’s a bit of magic in Pytest with the way these fixtures are injected. The main downside to hexagonal architecture is the amount of boilerplate code you end up writing, especially for simple programs. This is because in Python there is no such thing as an interface, like languages as Java do have. You don’t know what you’ve voted for but for this example this is not relevant. Today, I'd like to challenge some common practices in the Python community and propose an alternative. I bet you … If you’re building an MVP and time to market is the most important factor in the success of your business, then maybe this approach isn’t for you. Maintainability is at the heart of good software design. By nature, DDD and TDD fit very well together. After experiencing real confidence that your changes are working as expected and not breaking any existing functionality, you'll never want to go back! These are pure core business logic services. By the way, the necessity of comments in code is considered a “smell”, a.k.a. In general, taking an approach that isn't commonly adopted in the community can make it difficult to get help if you get stuck. Documented in 2005 by Alistair Cockburn, Hexagonal Architecture is a software architecture that has many advantages and has seen renewed interest since 2015.. Or plain old MVC? I'm also available for consulting, code/architecture review or hire :). We might, I think all these terms boil down to the same thing of targeting a clean architecture (credits to Uncle Bob). Many Python developers are now taking an interest in high-level software architecture patterns such as hexagonal/clean architecture, event-driven architecture, and strategic patterns prescribed by domain-driven design (DDD). This is a great feature of FastAPI but doesn’t add anything to this article. It's a basic JSON API that could power a blog, storing posts and associated metadata in a Postgres database. In the past, I’ve heard that term applied to pretty much any kind of backend code, but now when I look at the domain layer it reads more like something that I could show to a non-programmer and they would understand what’s going on. Bob Gregory wrote Ports and Adapter with Command Handler pattern in Python. This could be happening because in this file nothing is being used from the import itself. Turtle is a Python feature like a drawing board, which let us command a turtle to draw all over it! I've been a software engineer for about 5 years now at a few tech companies of various sizes. Now, when you’re testing your API code (i.e. Figure 2.4: A hexagonal architecture is also called a "ports-and-adapters" architecture since the application core provides specific ports for each adapter to interact with. It works best for web applications but I’m sure you could adapt the same principles for any type of program. This is one of the many forms of DDD (Domain Driven Design Architecture). You understand the pain to comprehend simple logic written in complex blocks of code. Even if you don't adopt this exact architecture for your next project, think hard about the separation of all the different functions of your application. The clean separation helped redefine (for me) what “business logic” actually is: it’s the part of the application that dictates what your program should actually do. The vote_repository knows about a vote, so both are part of the domain. The actual bulk of your application is called the domain layer, and it’s where your business logic lives. The application core is represented as a hexagon, giving this architecture style its name. This article tries to give a hands-on example of how to implement hexagonal architecture in Python, among other architectural patterns and design principles. But the "hexagonal architecture" name stuck, and that's the name many people know it by today. python flask sqlalchemy hexagonal-architecture blog-post Updated Dec 29, 2020; Python; bjudson / topsy Star 41 Code Issues Pull requests Demo app exploring ports and adapters architecture … The save(...) function of the Vote will store itself to the Vote-repository, we’ll talk about this later on in this article. In another article I’ll talk about this specific topic. Knowing the domain, we can add the adapter package and the main entry point to the application (and tests). I also used the typing library available in Python 3.5+ (and earlier versions through type comments) as well as mypy for static type analysis. For example our framework will "adapt" a SQL "port" to any number of different SQL servers for our application to use. Note that in the code example we need to do some strange ifand add a comment to explain what is happening. The domain itself is then clean of dependencies and specific implementation, but does contain the business logic of what the service is about — why it has reason for existence in the first place. When running a setup like this on a microservice in production, our entire test suite (a few hundred tests - it was a small service) ran in around 10-20ms. Hexagonal architecture was proposed by Alistair Cockburn in 2005. Most people are using the ORM, so the vast majority of information that's available caters to questions about that. Hey, I'm Alex. It's a great feeling working on a codebase that's written in a way that's easy to test. How do I draw a hexagon using the turtle module of python? But the “hexagonal architecture” name stuck, and that’s the name many people know it by today. Cockburn explainsthat t… Turtle comes in the turtle library.The turtle module can be used in both object-oriented and procedure-oriented ways. One criticism of this approach is that it might be seen as not Pythonic, but rather better fit for a language like Java or Go. I leveraged dataclasses to model the data stored in the database. The main challenge I faced getting the project finished was working with the typing library. Cockburn had his "eureka moment" about ports and adapters after reading Object Design: Roles, Responsibilities, and Collaborations by Rebecca Wirfs-Brock and Alan McKean. This means that you define which implementation to be used outside of the application itself and make it part of the configuration, for example with environment variables. You might think right now, don’t we just have a layered architecture? Hexagonal Architecture, a layered architecture, is also called the Ports and Adapters architecture. Secondly, there should be high test coverage to ensure that the project works as expected. Furthermore import * is considered a “smell” and should be prevented. But first I’ll show you the full application structure including the REST interface and tests. Let’s just say it’s a feature of the language. It's a basic JSON API that could power a blog, storing posts and associated metadata in a Postgres database. We can add multiple different adapters each responsible for persisting a Vote all in their own way while respecting the interface. The main goal of this architecture is to avoid knows structural pitfalls in software design. Further, introducing an enhancement or a new feature gives nightmares to developers. For many applications, a hexagonal architecture will be structured identically to a layered architecture, with a few specific details: There is increased discipline in defining interfaces between each layer (the ports), rather than calling impl to impl. Il ne se veut en aucun cas une retranscription de cette dernière mais un partage de connaissance quant à son usage intensif depuis 2 ans sur l’un des projets où nous intervenons. Notice the set() and the set literal with {} . In this case it’s deliberate to tell Pytest where to find the fixtures. It takes a little bit more time and effort up front, but it more than pays off in the long run. for dependency injection. Python is a language that's known for several good backend web frameworks - Django and Flask being the most popular among them. Best practices can slow your application down. We’ll leave out the initial requirement of a list of all votes. It is a clean architecture project template which is based on hexagonal-architecture principles built with .Net core. The goal was to find a way to solve or otherwise mitigate general caveats introduced by object oriented programming. "Hexagonal architecture" was actually the working name for the "ports and adapters pattern," which is the term Cockburn settled on in the end. An example backend implementing Hexagonal Architecture in Python using Flask and SqlAlchemy. from app.adapter.inmemory_vote_repository import InMemoryVoteRepository, https://github.com/douwevandermeij/voting-system/tree/initial, https://github.com/douwevandermeij/voting-system/tree/rest, The Lockdown Coder: Getting to Grips with Flutter, Deciphering Google’s Mysterious ‘batchexecute’ System. This is because it has the concept of different ports, which can be adapted for any given layer. Any help will be appreciated. The term “Hexagonal Architecture” has been around for a long time. What does this look like in practice? I think that the confusion you're facing comes from the fact that you are trying to approach an already existing Three-tier application from an Hexagonal Architecture point of view.
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