Thanks much for the article I learned quite a lot from it . Well, we could say that the consumer is a primary adapter, and the producer is a secondary adapter, but: We would separate them, putting them in 2 different root folders (primary/presentation vs secondary/infrastructure), but I feel this is gonna break the package cohesion principles, as we would be separating two pieces of code that are very strongly related, to the point that I consider them part of the same module. Agile .NET developer focused on Clean architecture. I posted the actual link with Russian translation in the comment above. In most languages and in its most simple form, this specification, the Port, will be an Interface, but it might actually be composed of several Interfaces and DTOs. Thanks for the great article. Check my answer here: https://herbertograca.com/2017/11/16/explicit-architecture-01-ddd-hexagonal-onion-clean-cqrs-how-i-put-it-all-together/comment-page-2/#comment-18281. The other satellite applications that need to communicate with it, should have their own port&adapter, so that if in the future you need to change it, it will be easier to do. Further inwards, we have the Domain Layer. Thanks for this post, it’s a reference I return to often! “I tend to see it as a secondary adapter because its activities are not triggered by direct user action, but its debatable.”. Both should depend on abstractions. First things first. Details should depend on abstractions. Thank u for taking the time to reason about the question, keep up the great work! What it does, is to act on one of the tools of the system, a persistence mechanism like a DB: a queue. It’s important to note that the Ports (Interfaces) belong inside the business logic, while the adapters belong outside. Let me know when u publish it and i will put a link on my blog. This rule says that source code dependencies can only point inwards. It’s a good practice to have different packages for each Adapter implementation. So if we put the UI in each component, that means we will be coupling 2 components explicitly on the UI layer. Do we already have the Application Services in place and are now adding a Command Bus? │ │ ├───Application Hi, tkx for letting me know. We can quote Uncle Bob’s Plugin Architecture, about the relationship between Visual Studio and Resharper. Use Cases interacting with the Domain and independent of external services. Furthermore, Ports & Adapters architecture explicitly identifies three fundamental blocks of code in a system: The application core is what we should really care about. A port is nothing more than a specification of how the tool can use the application core, or how it is used by the Application Core. Every details has abstractions protecting the Business Rules to be coupled to tech stuff. So far we have been segregating the code based on layers, but that is the fine-grained code segregation. As you can imagine, the typical application flow goes from the code in the user interface, through the application core to the infrastructure code, back to the application core and finally deliver a response to the user interface. Externally the user, other systems and services. Sure, let me know when you do and I’ll add it at the bottom. DDD, Hexagonal, Onion, Clean, CQRS, … How I put it all together, series of posts about Software Architecture, Connecting the tools and delivery mechanisms to the Application Core, Package by component and architecturally-aligned testing, https://github.com/katzien/go-structure-examples, https://github.com/dotnet-architecture/eShopOnContainers, https://github.com/hgraca/explicit-architecture-php, DDD, Hexagonal, Onion, Clean, CQRS – Phien Le Blogger, On Hexagonal architecture : Why? Another external layer is the Infrastructure Layer that implements Data Access, Dependency Injection Framework (DI) and other frameworks specifics. – the core implements the use cases (application services or command handlers), which are the ports From that point of view I was wondering what you’re opinion is on building for brownfield landscapes, especially those where there are parts of the landscape one cannot change. When a component needs to use data that belongs to another component, let’s say a billing component needs to use the client name which belongs to the accounts component, the billing component will contain a query object that will query the data storage for that data. As explained before, this a fundamental rule of Ports & Adapters Architecture, Onion Architecture and Clean Architecture. I recommend to design the infrastructure in a way you can split it when necessary, particularly when you have distinct adapters with overlapping concerns. ( Log Out /  Plans are worthless, but planning is everything. But our business logic only depends on the port (interface), which is designed to fit the business logic needs, so it doesn’t depend on a specific adapter or tool. Build status. Moving on, there are design principles that you should understand before implementing the Hexagonal Architecture style. Hexagonal Architecture (aka Ports and Adapters) is one strategy to decouple the use cases from the external details. According to Simon brown talk about modular monolith, in vertical slice architecture that each feature split separately with its on internal layering, for communicating between feature sets controllers exposes for communication between components. Hope is well for you sir, thank you for this article, would you mind sharing the code with this kind of implmented architecture? That’s the reason I am an advocate of feature folders describing the use cases and inside them the necessary classes: At your first look of the solution folders, you can build an idea of the purpose of this software. The preceding figure shows what a hexagonal architecture might look like. This means that the depending class has no knowledge about the concrete class that it is going to use, it has no reference to the fully qualified class name of the classes that it depends on. I just didn’t want to delve into all those details to avoid making my post really long. There is no repository layer, there is an Application Layer, and repositories belong there. I think in your approach that we use the event they act the same. The application is the territory, the reality, the concrete use case where we need to apply our knowledge, and that is what will define what the actual architecture will look like! Having components with their own data storage means each data storage contains: Each component will create a local copy of the data it needs from other components, to be used when needed. A year ago, though, I started reading Clean Code by Robert Martin. Its a way of structuring our code to make it easier to adapt, change. What have you seen until here is Enterprise + Application Business Rules enforced without frameworks dependencies or without database coupling. Independence of delivery mechanisms; 3. │ │ ├───Application Then you will have a worker pulling messages from the bus and sending them to the central application. I still very insistent to put repository interface as part of the domain layer, as domain(s) is possible microservice(s), and each microservice should keep its data as private, and for application service, we should use it as an orchestrator to coordinate domain services. Because the UI usually has complex screens/pages and workflows that require it to know about several backend components. Don’t be naive to assume that the Clean Architecture is the silver bullet. [EDIT – 2017-11-18] I completely missed the DTO I use to return data from the query, so I added it now. I spent a lot of time doing rework, writing untestable code, trying to invent my own (bad) abstractions, and putting all my business logic into anemic services.. Let me introduce the Hexagonal Architecture’s intent: As I said above, the flow of control goes, of course, from the user into the Application Core, over to the infrastructure tools, back to the Application Core and finally back to the user. Let’s see how I applied this principle in the next example: On the left side we found in red an Layered Application where the DepositUseCase depends on the AccountSQLRepository implementation. Adapters providing implementations of frameworks, data access and UI. In event storming sessions there is a policy stickie, which represents business policies like “when a user registers, take money from their bank account”. It was coined by Alistar Cockburn more than 13 years ago, and this received improvements with the Onion and Clean Architectures. You can use any of those approaches or even both at the same time, it depends on the context, it depends on the needs of the project. In one of my followup posts I talk about how I organise the code to reflect all this. In the very centre, depending on nothing outside it, is the Domain Model, which contains the business objects that represent something in the domain. I could easily see them on the same level as Primary Adapters. They are developed by different companies one in Seattle and another in Moscow and still running well together. Which ones depend on which ones? However, this is the theory. Pycharm - mark each package as Sources Root. This way the receiving component has no knowledge about the component sending the event, although they both know about the event and the dispatcher. │ │ │ │ ├───Application The coarse-grained segregation of code is at least as important and it’s about segregating the code according to sub-domains and bounded contexts, following Robert C. Martin ideas expressed in screaming architecture. In this sense the lollipops point to the core for primary ports but point from the core for secondary ports, right? My approach would be that application layer should contain playn language classes and interfaces but i am not sure if my approach is not too puristic , The application layer should contain the use cases and depend only on abstractions. The UI Layer depends on Application Layer and it loads the Infrastructure Layer by indirection. Console - make dev to install each subpackage in editable mode. On the other hand, I see a Kafka Producer as a secondary adapter, since it is “driven”. The repository interface is an abstraction on the persistence engine itself. And other things are almost equal in both. This means that Dependency Injection and Dependency Inversion are not enough to decouple components, we will need some sort of architectural constructs. └───User In DDD, however, i do not recall any reference about it. Nothing in an inner circle can know anything at all about something in an outer circle. These libraries are just details and should be left out of the Application Layer. Collections on the other hand can be specific to the domain layer. My point is that I decide to see query languages as languages, at the same level as PHP, HTML, Javascript or CSS. But I see these as just pieces of big a puzzle. In the diagram below, the Command Handler then uses an Application Service. The solution is to create a Domain Service, which has the role of receiving a set of entities and performing some business logic on them. I think it will extremely helpful if you have a GitHub repo demonstrating a real example of using the architectural patterns in this post. │ │ │ └───Domain What I find debatable, is the location of the piece of code that reads events from a queue and delivers them to their listeners: is it a primary or secondary adapter?! It is important to understand this priciple as it was applied to decouple other stuff in our source code. They would make or save money even if they were executed manually. That is the where Dependency Injection is implemented, the concretions are loaded closer to the Main function. My question is, which layer should this component (Input handler) belong to, should I treat as an adapter with a corresponding port in the application layer or something external to my application (just like how you’d treat a framework)? Hi, after reading your article I finally understand the design. On the left we implement the driving actors and on the right we implement the secondary actors. Could you justify why the Event Listeners are inside the Application Core? I may be missing something here though. This layer contains Application Services (and their interfaces) as first class citizens, but it also contains the Ports & Adapters interfaces (ports) which include ORM interfaces, search engines interfaces, messaging interfaces and so on. I preffer to use one language as language of architecture core implementation and don’t like to mix different languages in one scope. Great city, great ppl, great food, so much culture, loved it! Each MS has its own model and its own use cases and defines its own external interfaces/ports (both for supplying data and retrieving data). We also use Solr to improve performance on some of the querying in the application (“LIKE” searches and “JOINS” are not super fast in MongoDB). ├───Core Hi, If i would think of another adventure living abroad, that would be in my thoughts for sure! The Domain Model is also where Domain Events “live”. Any thoughts on this would be much appreciated! In its most simple form you can think of this queue as a database table with the serialized events there, although its best to use something like rabbitqm or kafka. A set of data that it owns and is the only one allowed to change, making it the single source of truth; A set of data that is a copy of other components data, which it can not change on its own, but is needed for the component functionality, and it needs to be updated whenever it changes in the owner component. Hi, awesome article, what I am wondering about for some time is if application layer should be completely free from any external libraries dependencies, such as ready service bus (like MediatR in .NET case), mappers etc. Focus on the business rules, make them clear on your Application and Domain Layers. Not a single line of VS knows about Resharper, but Resharper is developed based on the Visual Studio abstractions. The Primary or Driver Adapters wrap around a Port and use it to tell the Application Core what to do. Using event or interfaces like something Simon did? One more thing: How would the data flow? Does the implementation of the port for driving adapter belong to the application core? If you need to improve the responsiveness of the overall system as well as the resilience, I suggest starting to use a command bus. │ │ └───Domain Though these approximations have their own fancy names and could seem completely different, they share a lot in common. (It is not commercial), Sure, go ahead. Hexagonal architecture was proposed by Alistair Cockburn in 2005. So therefore everything is open to personal adjustments as long as core ideas are kept intact. Just a conceptual doubt regarding Shared Kernel. One is a SaaS e-com platform with thousands of web-shops worldwide, another one is a marketplace, live in 2 countries with a message bus that handles over 20 million messages per month. These policies translate to event listeners: an event is triggered by the UserComponent and listened to by the InvoicingComponent. They are created to fit a very specific entry point to the Application Core, a Port. However, the interface specification they create is actually just mimicking the adapters/tools API. I expanded these ideas a bit more on a followup post: More than concentric layers. On the right side in blue, by adding an IAccountRepository and applying DIP then the AccountSQLRepository has its dependency pointing inwards. Like Simon’s C4 Model and have a unique UI for all of our components. It was coined by Alistar Cockburn more than 13 years ago, and this received improvements with the Onion and Clean Architectures. └───Presentation, I found this sample for that purposehttps://github.com/kgrzybek/modular-monolith-with-ddd/tree/master/src/Modules/Payments.