The Secrets of Hexagonal Architecture

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The secrets of
13
Hexagonal 
Architecture

Separate Domain
& Infrastructure
14
Maintainable Software
=

Twitter  
GitHub  
Medium
16
}@nicoespeon
Nicolas Carlo

18
Seat
Stop
Departure
Roundtrip
Leg
Taxes
Discount code
Fees

We could do our
business without
software!
20

Infrastructure is
how we make it work
22

Infrastructure  
is a detail
23

Put the Domain
at the heart of
the software
24

Problems mixing
Domain & Infra.

Get a poem from a
poetry library, or return
the default one.
26

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class PoetryReader {
giveMeSomePoetry(): string {
}
}

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import * as fs from "fs";
import * as path from "path";
const pathToPoem = path.join(!__dirname, "poem.txt");
let poem = fs.readFileSync(pathToPoem, { encoding: "utf8" });
}
}

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if (!poem) {
poem =
"If you could read a leaf or treernyou’d have no need of books.
rn!-- © Alistair Cockburn (1987)";
}
return poem;
}
}

30
if (!poem) {
poem =
}
return poem;
}
}

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if (!poem) {
poem =
}
return poem;
}
}
Domain

32
if (!poem) {
poem =
}
return poem;
}
}
Infrastructure

33
if (!poem) {
poem =
}
return poem;
}
}

Hard to  
see
the Business
34

Hard to  
test
the Business
35

Hard to  
change
the Business
36

37
import FileReader from "lib/file-reader";
let poem = FileReader.read("poem.txt");
if (!poem) {
poem =
}
return poem;
}
}

import FileReader from "lib/file-reader";
if (!poem) {
poem =
}
return poem;
}
}
Same  
problem
38

The simplest way to
40
Separate Domain  
& Infrastructure

44
Interface
Adapter
use
implement

46
Port
Adapter
Ports & Adapters architecture

49
Left-side Right-side
Adapter
Adapter

50
Left-side Right-side
Adapter
Adapter
Adapter
Adapter

}
Domain
52

poetryLibrary: ObtainPoems;
constructor(poetryLibrary!?: ObtainPoems) {
this.poetryLibrary = poetryLibrary;
}
}
Domain
53

}
}
Domain
54
Dependency
Injection

}
giveMeSomePoetry(): Poem {
if (!this.poetryLibrary) {
return "If you could read a leaf or treernyou’d have no need of books.
}
return this.poetryLibrary.getAPoem();
}
}
Domain
55

}
return "If you could read a leaf or treernyou’d have no need of books.
}
}
}
Domain
56

return "If you could read a leaf or treernyou’d have
no need of books.rn!-- © Alistair Cockburn (1987)";
}
}
57
if (!poem) {
poem =
"If you could read a leaf or treernyou’d have
no need of books.rn!-- © Alistair Cockburn (1987)";
}
return poem;
}
Implementation
Intention

Domain
type Poem = string;
interface ObtainPoems {
getAPoem(): Poem
}
58

import ObtainPoems from "!../domain/obtain-poems";
class ObtainPoemsFromFS implements ObtainPoems {
getAPoem() {
const poem = fs.readFileSync(pathToPoem, { encoding: "utf8" });
return poem;
}
}
Infra.
59

!// 1. Instantiate the right-side adapter(s)
!// 2. Instantiate the hexagon (domain)
!// 3. Instantiate the left-side adapter(s)
60

import ObtainPoemsFromFS from "./infrastructure/obtain-poems-from-fs";
const poetryLibrary = new ObtainPoemsFromFS();
61

import PoetryReader from "./domain/poetry-reader";
const poetryReader = new PoetryReader(poetryLibrary);
62

import ConsoleApi from "./infrastructure/console-api";
const consoleApi = new ConsoleApi(poetryReader);
63

import ConsoleApi from "./infrastructure/console-api";
const consoleApi = new ConsoleApi(poetryReader);
!// App logic is only using left-side adapter(s).
console.log("Here is some poetry:n");
consoleApi.ask();
64

const poetryReader = new PoetryReader();
65

66

This ain't new
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"Program to an Interface" − OOP
"Isolate side effects" − FP
The Onion Architecture

Plug different adapters
to the Domain
69

A good architect
defers decisions
70

Start with
something simple
71

But… it's only
the first step!
72

74
Ubiquitous Language  
Bounded Contexts 
Domain modelling

Separate Domain
& Infrastructure
75
🙏

Twitter  
GitHub  
Medium
}@nicoespeon
Nicolas Carlo

@swcraftmontreal
Software Crafters Montréal
Every 1st Tuesday of the month.

test("give verses when asked for poetry", () !=> {
const poetryReader = new PoetryReader();
const verses = poetryReader.giveMeSomePoetry();
expect(verses).toEqual(
"If you could read a leaf or treernyou’d have no
need of books.rn!-- © Alistair Cockburn (1987)"
);
});
Tests

test("give verses from a PoetryLibrary", () !=> {
const poetryLibrary: ObtainPoems = {
getAPoem() {
return "I want to sleep…rn!-- Masaoka Shiki (1867-1902)";
}
};
const verses = poetryReader.giveMeSomePoetry();
expect(verses).toEqual(
"I want to sleep…rn!-- Masaoka Shiki (1867-1902)"
);
});
Tests

The Secrets of Hexagonal Architecture

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