Error-free Elixir

Error handling can greatly increase the complexity of the system. In Elixir, the default way of reducing the error-handling code is to use the "Let it crash!" approach. But there's another way.

In this talk, you'll learn how to eliminate errors by redefining the operation semantics, all supported by real-life examples. By doing so, you'll be able to not only reduce the need for error handling, but also simplify the entire codebase, make it smaller, more testable and easier to understand.

Transcript

1. mkaszubowski94 Maciej Kaszubowski ERROR-FREE ELIXIR
2. mkaszubowski94 Error handling is crucial
3. mkaszubowski94 Error handling is a big source of complexity
4. mkaszubowski94 def cancel_job(user_id, job_id) do Job  Repo.get(job_id)  Job.cancel_changeset(%{user_id: user_id, canceled_at: now()})  Repo.update() end
5. mkaszubowski94 def cancel_job(user_id, job_id) do with {:ok, job}  fetch_job(job_id), :ok  can_cancel?(job, user_id) do job  Job.cancel_changeset(%{user_id: user_id, canceled_at: now()})  Repo.update() else nil  {:error, :not_found} {:error, :already_canceled}  {:error, :already_canceled} {:error, :already_completed}  {:error, :already_completed} {:error, :no_permissions}  {:error, :no_permissions} {:error, %Changeset{} = changeset}  {:error, changeset} end end
6. mkaszubowski94 When designing, we focus mostly on the happy path.
7. mkaszubowski94 We have to design error-handling too.
8. mkaszubowski94 Not errors are equal
9. mkaszubowski94 Users Sources of errors
10. mkaszubowski94 Users Other services Sources of errors
11. mkaszubowski94 Users Infrastructure Other services Sources of errors
12. mkaszubowski94 </> Developers Users Infrastructure Other services Sources of errors
13. mkaszubowski94 Each requires a different error-handling strategy
14. Strategies
15. mkaszubowski94 Let it crash!
16. mkaszubowski94 Let it crash!
17. mkaszubowski94 Infrastructure Other services Let it crash! • Incorrect behaviour • Bugs hard to reproduce • Bugs hard to recover from What?
18. mkaszubowski94 Infrastructure Other services Let it crash! • Incorrect behaviour • Bugs hard to reproduce • Bugs hard to recover from What? Why? • Protect invariants • Restarting can help
19. mkaszubowski94 :error tuples
20. mkaszubowski94 :error tuples
21. mkaszubowski94 • User/Client errors • Correct behaviour (!) • Predicable errors What? Users Other services :error tuples
22. mkaszubowski94 • User/Client errors • Correct behaviour (!) • Predicable errors What? Why? • Client can interpret the error • Client can recover from errors Users Other services :error tuples
23. mkaszubowski94 </> Developers Users Infrastructure Other services Sources of errors
24. mkaszubowski94 </> Developers Users Infrastructure Other services Sources of errors
25. mkaszubowski94 </> Developers Users Infrastructure Other services Sources of errors
26. mkaszubowski94 </> Developers Users Infrastructure Other services Sources of errors
27. mkaszubowski94 </> Developers Users Infrastructure Other services Sources of errors ?
28. mkaszubowski94 </> Developers Users Infrastructure Other services Sources of errors :(
29. mkaszubowski94 Controller Context Context.Xxx
30. mkaszubowski94 Controller Context Context.Xxx
31. mkaszubowski94 We're too used to the "standard" ways
32. mkaszubowski94 case some_function(args) do {:ok, something}  #  {:error, reason}  {:error, reason} end
33. mkaszubowski94 Things to consider Can the layer above interpret the error? 1 2 How is the error possible? 3 Can the layer above fix the error?
34. mkaszubowski94 Don't deal with errors. Prevent them.
35. Eliminating errors
36. mkaszubowski94 Heuristics Declarative operations 1 2 Reasonable defaults 3 Don't fight the real world
37. mkaszubowski94 Heuristics Declarative operations 1 2 Reasonable defaults 3 Don't fight the real world
38. mkaszubowski94 defmodule Accounts do @spec get_settings(id())  {:ok, t()} | {:error, :not_found} def get_settings(user_id) do case Repo.get_by(Settings, user_id: user_id) do %Settings{} = settings  {:ok, settings} nil  {:error, :not_found} end end end
39. mkaszubowski94 defmodule Accounts do @spec get_settings(id())  {:ok, t()} | {:error, :not_found} def get_settings(user_id) do case Repo.get_by(Settings, user_id: user_id) do %Settings{} = settings  {:ok, settings} nil  {:error, :not_found} end end end
40. mkaszubowski94 defmodule Accounts do @spec get_settings(id())  {:ok, t()} def get_settings(user_id) do case Repo.get_by(Settings, user_id: user_id) do %Settings{} = settings  {:ok, settings} nil  {:ok, Settings.default()} end end end
41. mkaszubowski94 Heuristics Declarative operations 1 2 Reasonable defaults 3 Don't fight the real world
42. mkaszubowski94 def cancel_job(user_id, job_id) do with {:ok, job}  fetch_job(job_id), :ok  can_cancel?(job, user_id) do job  Job.cancel_changeset(%{user_id: user_id, canceled_at: now()})  Repo.update() else nil  {:error, :not_found} {:error, :already_canceled}  {:error, :already_canceled} {:error, :already_completed}  {:error, :already_completed} {:error, :no_permissions}  {:error, :no_permissions} {:error, %Changeset{} = changeset}  {:error, changeset} end end
43. mkaszubowski94 Describe the end goal, not the steps to reach it
44. mkaszubowski94 def cancel_job(user_id, job_id) do with {:ok, job}  fetch_job(job_id), :ok  can_cancel?(job, user_id) do job  Job.cancel_changeset(%{user_id: user_id, canceled_at: now()})  Repo.update() else nil  {:error, :not_found} {:error, :already_canceled}  {:error, :already_canceled} {:error, :already_completed}  {:error, :already_completed} {:error, :no_permissions}  {:error, :no_permissions} {:error, %Changeset{} = changeset}  {:error, changeset} end end
45. mkaszubowski94 def cancel_job(user_id, job_id) do with {:ok, job}  fetch_job(job_id), :ok  can_cancel?(job, user_id) do job  Job.cancel_changeset(%{user_id: user_id, canceled_at: now()})  Repo.update() else nil  {:error, :not_found} {:error, :already_canceled}  {:error, :already_canceled} {:error, :already_completed}  {:error, :already_completed} {:error, :no_permissions}  {:error, :no_permissions} {:error, %Changeset{} = changeset}  {:error, changeset} end end
46. mkaszubowski94 def cancel_job(user_id, job_id) do with {:ok, job}  fetch_job(job_id), :ok  can_cancel?(job, user_id) do job  Job.cancel_changeset(%{user_id: user_id, canceled_at: now()})  Repo.update() else nil  {:error, :not_found} {:error, :already_canceled}  {:ok, job} {:error, :already_completed}  {:error, :already_completed} {:error, :no_permissions}  {:error, :no_permissions} {:error, %Changeset{} = changeset}  {:error, changeset} end end
47. mkaszubowski94 Heuristics Declarative operations 1 2 Reasonable defaults 3 Don't fight the real world
48. mkaszubowski94 def take_off_shelf(product_id, shelf_id) do if product_on_the_shelf?(product_id, shelf_id) do remove_from_shelf!(product_id, shelf_id) else {:error, :product_missing} end end
49. mkaszubowski94 def take_off_shelf(product_id, shelf_id) do if product_on_the_shelf?(product_id, shelf_id) do remove_from_shelf!(product_id, shelf_id) else {:error, :product_missing} end end
50. mkaszubowski94 def take_off_shelf(product_id, shelf_id) do if product_on_the_shelf?(product_id, shelf_id) do remove_from_shelf!(product_id, shelf_id) else notify_operators(product_id, shelf_id) end end
51. mkaszubowski94 We cannot prevent people from making mistakes
52. mkaszubowski94 Not all errors are bad
53. mkaszubowski94 Error handling has to be designed
54. Why talk about this?
55. mkaszubowski94 Design is not about a single clever idea
56. mkaszubowski94 We cannot rely on intuition
57. mkaszubowski94 You will make wrong decisions Make sure you learn from that.
58. mkaszubowski94 A set of clear and simple heuristics and questions is really helpful
59. mkaszubowski94 Approach design more thoughtfully
60. mkaszubowski94 Use facts instead of opinions
61. mkaszubowski94 Gather as many questions and heuristics as possible.
62. mkaszubowski94 Gather as many questions and heuristics as possible. And share them!
63. mkaszubowski94 Share the process of making decisions.
64. mkaszubowski94 "A Philosophy of Software Design" - John Ousterhout "The art of destroying software" - Greg Young Credits
65. mkaszubowski94 Maciej Kaszubowski THANKS! I hope it was useful