The document discusses hexagonal architecture and domain-driven design in Elixir, emphasizing the separation of business logic (domain) from technical details (infrastructure) to create maintainable software. It highlights the importance of using interfaces and behaviors, illustrating implementation with Elixir code snippets to demonstrate how to structure applications effectively. Additionally, it raises questions about the readability and testability of business logic while advocating for flexibility in infrastructure integration.

1. Hexagonal
Architecture
in Elixir

2. Buzzwords
🐝

3. Hexagonal
Architecture
Domain-Driven Design
Clean Architecture
Ports & Adapters

4. Lots of
Buzzwords
This is not about them…

5. Separate Domain
& Infrastructure
= maintainable software

6. @nicoespeon

7. Domain
vs
Infrastructure

8. Domain
Business Logic
Infra.
Technical Details

9. Software is
a mean
to a
Business end

10. Infrastructure is
a detail

11. Business
doesn't care!
Infrastructure can change
shouldn't

12. Can your PM
🌟 understand 🌟
the source code?

13. Problems mixing
Domain & Infra.

14. def prs_open_time_until_today() do
today = Date.utc_today() !|> Date.to_iso8601()
{200, prs, _} =
Tentacat.Client.new(%{access_token: @access_token})
!|> Tentacat.Pulls.list("nicoespeon", "my-repo")
prs
!|> Enum.map(fn pr !-> pr["created_at"] end)
!|> Enum.flat_map(fn created_at !->
case DaysDiff.between(today, created_at) do
{:ok, open_time} !-> [open_time]
{:error, _} !-> []
end
end)
end

15. def prs_open_time_until_today() do
today = Date.utc_today() !|> Date.to_iso8601()
{200, prs, _} =
Tentacat.Client.new(%{access_token: @access_token})
!|> Tentacat.Pulls.list("nicoespeon", "my-repo")
prs
!|> Enum.map(fn pr !-> pr["created_at"] end)
!|> Enum.flat_map(fn created_at !->
case DaysDiff.between(today, created_at) do
{:ok, open_time} !-> [open_time]
{:error, _} !-> []
end
end)
end

16. Hard to
see
the Business

17. Hard to
test
the Business

18. Hard to
change
the Business

19. Hard to change the Infra.
without breaking the Business

20. Is your Business
easy to read?
📖

21. Is your Business
easy to test?
🚦

22. Hexagonal
Architecture

23. The simplest way to
Separate Domain
& Infrastructure

24. Infra.
Domain

25. The outside depends
on the inside.
Never the opposite.
Rule

26. Infra.
Domain
Dependency

27. Infra.
Domain
Dependency

28. Interface
Adapter
use
implement

29. Port
Adapter
use
implement
Ports & Adapters Architecture

30. Inside the Hexagon
Business language
only

31. %
Left-side Right-side
⚙
W
eb
UI
REST
API
Database
Send
event

32. '(Benefits

33. Flexibility
REST API CLI CRON
FTP File System HTTP
Console Sentry Postgres
…

34. You can plug another
Infra. to the Domain
🔌

35. A good software
architect defers
technical decisions

36. You can start with
something simple✨

37. In Elixir

38. Interfaces in Elixir
But we have
Behaviours

39. # i_read_prs.ex (Behavior)
defmodule IReadPrs do
@callback opened_prs() !:: [OpenTime.pr()]
end
# i_give_dates.ex (Behavior)
defmodule IGiveDates do
@callback today() !:: String.t()
end

40. defmodule OpenTime do
@type pr !:: %{created_at: String.t()}
def prs_open_time_until_today() do
end
end

41. defmodule OpenTime do
@type pr !:: %{created_at: String.t()}
@dates_giver Application.get_env(:pr_metrics, :dates_giver)
def prs_open_time_until_today() do
today = @dates_giver.today()
end
end

42. defmodule OpenTime do
@type pr !:: %{created_at: String.t()}
@dates_giver Application.get_env(:pr_metrics, :dates_giver)
@prs_reader Application.get_env(:pr_metrics, :prs_reader)
def prs_open_time_until_today() do
today = @dates_giver.today()
prs = @prs_reader.opened_prs()
end
end

43. defmodule OpenTime do
@type pr !:: %{created_at: String.t()}
@dates_giver Application.get_env(:pr_metrics, :dates_giver)
@prs_reader Application.get_env(:pr_metrics, :prs_reader)
def prs_open_time_until_today() do
today = @dates_giver.today()
prs = @prs_reader.opened_prs()
prs
!|> Enum.map(fn pr !-> pr.created_at end)
!|> Enum.flat_map(fn created_at !->
case DaysDiff.between(today, created_at) do
{:ok, open_time} !-> [open_time]
{:error, _} !-> []
end
end)
end
end

44. # dates_giver.system.ex (Adapter)
defmodule DatesGiver.System do
@behaviour IGiveDates
end
# prs_reader.github.ex (Adapter)
defmodule PrsReader.GitHub do
@behaviour IReadPrs
end

45. # dates_giver.system.ex (Adapter)
defmodule DatesGiver.System do
@behaviour IGiveDates
def today, do: Date.utc_today() !|> Date.to_iso8601()
end
# prs_reader.github.ex (Adapter)
defmodule PrsReader.GitHub do
@behaviour IReadPrs
end

46. # dates_giver.system.ex (Adapter)
defmodule DatesGiver.System do
@behaviour IGiveDates
def today, do: Date.utc_today() !|> Date.to_iso8601()
end
# prs_reader.github.ex (Adapter)
defmodule PrsReader.GitHub do
@behaviour IReadPrs
@access_token Application.get_env(:pr_metrics, :github_access_token)
def opened_prs do
{200, prs, _} =
Tentacat.Client.new(%{access_token: @access_token})
!|> Tentacat.Pulls.list("nicoespeon", "my-repo")
prs
!|> Enum.map(fn pr !-> %{created_at: pr["created_at"]} end)
end
end

47. # config/config.exs
use Mix.Config
config :pr_metrics,
dates_giver: DatesGiver.System,
prs_reader: PrsReader.GitHub

48. # config/config.test.exs
use Mix.Config
config :pr_metrics,
dates_giver: Mock.DatesGiver,
prs_reader: Mock.PrsReader

49. defmodule OpenTimeTest do
use ExUnit.Case
import Mox
end

50. defmodule OpenTimeTest do
use ExUnit.Case
import Mox
test "returns an empty array if there is no opened PR" do
open_times = OpenTime.prs_open_time_until_today()
assert open_times !== []
end
end

51. defmodule OpenTimeTest do
use ExUnit.Case
import Mox
@today "2011-01-10"
test "returns an empty array if there is no opened PR" do
Mock.DatesGiver
!|> stub(:today, fn !-> @today end)
open_times = OpenTime.prs_open_time_until_today()
assert open_times !== []
end
end

52. defmodule OpenTimeTest do
use ExUnit.Case
import Mox
@today "2011-01-10"
test "returns an empty array if there is no opened PR" do
Mock.DatesGiver
!|> stub(:today, fn !-> @today end)
Mock.PrsReader
!|> stub(:opened_prs, fn !-> [] end)
open_times = OpenTime.prs_open_time_until_today()
assert open_times !== []
end
end

53. "Program to a
behaviour"
Elixir

54. Use
Domain language
in Behaviours

55. +
Limits,

56. It doesn't guide you
to scale 🚀

57. Compatible
Has more layers

58. Compatible
Ubiquitous Language
Bounded Contexts
Domain modelling

59. Separate Domain
& Infrastructure
🙏

60. Twitter
GitHub
Medium
}@nicoespeon
Nicolas Carlo