This topic is taken from Solid Book - The Software Architecture & Design Handbook w/ TypeScript + Node.js. Check it out if you like this post.

How many times have you been working on an application, and in the middle of writing some logic, asked yourself:

"Is this the right place to put this logic?"

There's a lot that goes into figuring out where something belongs.

First, you have to understand the domain that you're working in. That's how you figure out the subdomains in your app.

Then, for each subdomain, you have to identify all the use cases and which actors are allowed to execute them.

This is Conway's Law in action.

What I've found to be the most challenging part , especially for developers new to enterprise application development that have moved well-past simple CRUD MVC apps, is implementing a layered architecture to separate the concerns of app development.

The challenge seems to be understanding which layer exactly to place logic.

In this article, we'll learn about the following:

Clean Architecture, and why we should separate the concerns of large applications into layers.

The 6 most common types of application logic and which layer they belong to.

The Clean Architecture / Layered Architecture

Robert Martin wrote about Clean Architecture in his book of the same name. Although a bit of a challenging read (took me 2 really good reads), it's amazing. It teaches you about how to organize and group code into components, and then how to organize an application that connects those components to things like databases, APIs, web servers and other external things that we need to power applications.

A really simplified version of what the clean architecture might look like:

Domain Layer

In one layer (domain) we have all of the important stuff: the entities, business logic, rules and events. This is the irreplaceable stuff in our software that we can't just swap out for another library or framework. It's also the stuff that's much less likely to need to change because it represents what our business does.

For example, if we're an app that sells books, we sell books. If we're an app that finds parking spots, the domain layer contains the core logic to find parking spots.

Because we're unlikely to change the core essense of what our business does, the domain layer is the most stable layer.

The domain layer assumes a kind of higher-level policy that everything else relies on.

Infra (everything-else) layer

The other layer (infra) contains everything that actually spins up the code in the domain layer to execute.

Clean architecture expanded

We can generally express the clean architecture as domain and infrastructure.

Stuff that's specific to our business is domain, and the adapter-stuff that just hooks into the technologies that enable us to run a web app (database, web servers, controllers, caches, etc) is the infrastructure.

But the devil is in the details.

If we wipe our glasses off, a more detailed view of the clean architecture would look like this.

For trivially small applications, this might seem like overkill. But for applications that are expected to live a long life, be maintained by larger teams, and make / save a company money, it's vital to figure out how to separate the concerns and where to put logic.

Failure to address that complexity and solve it with a better architecture can turn a project legacy-mode, real quick.

Without further ado, here are the 6 most common types of application logic that find their way into large applications.

1. Presentation Logic

Logic that's concerned with how we present something to the user.

Most apps used by consumers have front-ends. This type of logic is entirely concerned with how we present something to the user.

It's the HTML, CSS and JavaScript that we write that turns a blank page into a well-designed, living, breathing front-end application.

.container background : white; color : blue;

Dumb UIs should contain no business rules

There's a principle called Dumb UI.

The idea is to keep the UI logic divorced from any domain-layer logic, because the domain-layer logic acts as dependency to everything else in the architecture.

The front-end is volatile. That means it's constantly requiring changes. Due to that, it wouldn't be a good idea to locate logic (domain logic, specifically) that was important to other components in the architecture, because that would have the potential to consistently break the app.

Stable Dependency Principle: If you're interested in this phenomenon of what makes a dependency stable and what makes it unstable, check this out.

Smart and dumb components

Modern front-end JavaScript frameworks like React and Angular have popularized smart (container) components and dumb (functional) components.

There may be a good separation of concerns between those two, where smart components hold the state and methods that manipulate the state, leaving dumb components to simply project the view, but front-end is still primarily presentation logic and should contain little to no other types of logic (besides some validation logic).

To the backend, front-end is an infrastructural concern that we provide data access / adapters to through RESTful APIs and the like.

2. Data Access / Adapter Logic

Logic concerned with how we enable access to infrastructure layer concern like caches, databases, front-ends, etc.

We can't get very far with just a plain ol' JavaScript/TypeScript objects that represent our domain layer.

We need to hook this thing up to the internet and enable a front-end to connect to it! Which web server do we want to use? Express.js? Hapi?

We also need to figure out how we'll persist our domain objects. Want to use a SQL database? NoSQL?

What about caching?

Oh, and how do make use of cool external services like Stripe for billing or Pusher for real-time chat? We need to write adapters for those so that our inner layers can use them.

That's right. This layer is all about defining the adapters to the outside world. Simplify usage for the inner layers by encapsulating the complexity of persisting an aggregate to a database by creating a Repository class to do that.

Here are several common things to do at this layer:

RESTful APIs : Define a RESTful API with Express.js and create controllers to accept requests.

: Define a RESTful API with Express.js and create controllers to accept requests. Production middleware :Write Express.js controller middleware to protect your API from things like DDos and brute force login attempts.

:Write Express.js controller middleware to protect your API from things like DDos and brute force login attempts. Database Access : Create repositories that contain methods that perform CRUD on a database. Use either an ORM like Sequelize and TypeORM or raw queries to do this.

: Create repositories that contain methods that perform CRUD on a database. Use either an ORM like Sequelize and TypeORM or raw queries to do this. Billing Integrations: Create an adapter to a payment processor like Stripe or Paypal so that it can be used by inner layers.

3. Application Logic / Use Cases

Logic that defines the actual features of our app

Use cases are the features of our app.

Once all the use-cases of our application have been identified and then developed, we are objectively done.

Done: "Done" is a word that I don't throw around often. It has different meanings to everyone. However, we can get closer to a shared understanding of done if we: figure out the use cases, and figure out who or what should be able to execute those use cases (actors or agents- because robots/servers are actors too). Once that happens, I'm more confident about saying we're done.

CQS / CQRS

Use Cases are either COMMANDS or QUERIES if we're following the Command-Query Segregation principle.

Use Cases are application specific

Your company might have several applications within it.

Take Google for example. Google has Google Drive, Google Docs, Google Maps, etc.

Each of these applications has their own set of use cases, like this:

Google Drive

shareFolder(folderId: FolderId, users: UserCollection) : Shares a folder with other users in the Google enterprise.

: Shares a folder with other users in the Google enterprise. createFolder(parentFolderId: FolderId, name: string) : Creates a folder.

: Creates a folder. createDocument(parentFolderId: FolderId) : Creates a Google Docs document.

Google Docs

shareDocument(users: UserCollection, visibility: VisibilityType) : Shares a document with several users.

: Shares a document with several users. deleteDocument(documentId: DocumentId) : Deletes a document.

Google Maps

getTripRoutes (start: Location, end: Location, time?: Date) : Gets all routes for a trip.

: Gets all routes for a trip. startTrip (start: Location, end: Location) : Starts a trip now.

A more common and simple example is deploying an Admin panel. You might need a separate dashboard from your main app in order to do a couple of admin-y things.

I usually plan and build software around use cases because it makes the code a lot easier to reason about, and improves how rapidly you can complete projects.



I have a free ebook called "Name, Construct & Structure" where you can learn more about this approach to designing readable codebases.



If you're more into the nitty-gritty of how to actually learn to code using Use-Case Driven Development, first read this article on Use Cases (it's my favourite article).



If you want more, check out my book on software design and architecture where I walk you through the process of Use-Case Driven Development.

4. Domain Service logic

Core business logic that doesn't quite fit within the confines of a single entity.

Now we're in the domain layer. And Domain-Driven Design is the best way to approach creating a rich domain model.

In DDD, we always try to locate domain logic closest to the entity it pertains to.

There comes situations where that logic spills into perhaps two or more entities, and it doesn't quite seem to make sense to locate that logic in one or the other.

We use domain services to ensure that we don't lose that business rule within one specific application's use case, but instead keep it within the domain layer so that it can be used by every application that relies on it.

5. Validation logic

Logic that dictates what it means for a domain object to be valid.

Validation logic is another domain layer concern, not an infrastructure one.

Let's say that we wanted to create a User entity. And User contained a property called email:string .

interface UserProps { userEmail : string ; } class User extends Entity < UserProps > { private constructor ( props : UserProps , id ? : UniqueEntityId ) { super ( props , id ) } public static create ( props : UserProps , id ? : UniqueEntityId ) : Result < User > { const propsResult = Guard . againstNullOrUndefined ( props . userEmail ) ; if ( ! propsResult . succeeded ) { return Result . fail < User > ( propsResult . error ) ; } return Result . ok < User > ( new User ( props , id ) ) } }

What's stopping someone from creating a User with an invalid userEmail:string ?

const userOrError : Result < User > = User . create ( { userEmail : 'diddle' } ) ; userOrError . isSuccess

That's what we use Value Objects for. We can encapsulate the validation rule with a Value Object for userEmail .

If we change the type of userEmail to be strictly-typed instead of string-ly typed like this:

interface UserProps { userEmail : UserEmail ; }

And then create a UserEmail value object...

import { TextUtil } from '../utils' import { Result , Guard } from '../../core' interface UserEmailProps { email : string ; } export class UserEmail extends ValueObject < UserEmailProps > { private constructor ( props : UserEmailProps ) { super ( props ) ; } public static create ( props : UserEmailProps ) : Result < UserEmail > { if ( Guard . againstNullOrUndefined ( props . email ) || ! TextUtil . isValidEmail ( props . email ) ) { return Result . fail < UserEmail > ( "Email not provided or not valid." ) ; } else { return Result . ok < UserEmail > ( new UserEmail ( props ) ) ; } } }

Now, there's no way to create an invalid User .

6. Core business logic / entity logic

Logic that belongs to a single entity.

The most important is where the family jewels of the application are: the entity.

And if the entity has a reference to other related entities, the aggregate root.

Core business logic that lives here are:

Initial / default values

Protecting class invariants (what changes are allowed, and when)

Creating Domain Events for changes, creations, deletions, and anything else relevant to the business. It's through domain events that complex business logic can be chained.

Some principles to follow

Never over-engineer

You should know when you need to use a layered architecture. Typically, it makes most sense when your application has a lot of business rules. In that case, it's a good idea to implement a layered architecture in order to keep the concerns of persistence logic (for example), separate from validation logic and the core business rules of the Domain Layer.

Conclusion

In this article, we covered the 6 main types of logic in large application development.

Here they are in summary: