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
   

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2. mkaszubowski94
   Error handling is crucial
   

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3. mkaszubowski94
   Error handling is a big source of
   complexity
   

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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
   

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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
   

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6. mkaszubowski94
   When designing, we focus mostly
   on the happy path.
   

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7. mkaszubowski94
   We have to design error-handling
   too.
   

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8. mkaszubowski94
   Not errors are equal
   

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9. mkaszubowski94
   Users
   Sources of errors
   

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10. mkaszubowski94
    Users
    Other services
    Sources of errors
    

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11. mkaszubowski94
    Users Infrastructure
    Other services
    Sources of errors
    

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12. mkaszubowski94
    > Developers
    Users Infrastructure
    Other services
    Sources of errors
    

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13. mkaszubowski94
    Each requires a different
    error-handling strategy
    

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14. Strategies
    

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15. mkaszubowski94
    Let it crash!
    

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16. mkaszubowski94
    Let it crash!
    

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17. mkaszubowski94
    Infrastructure
    Other services
    Let it crash!
    • Incorrect behaviour
    • Bugs hard to reproduce
    • Bugs hard to recover from
    What?
    

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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
    

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19. mkaszubowski94
    :error tuples
    

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20. mkaszubowski94
    :error tuples
    

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21. mkaszubowski94
    • User/Client errors
    • Correct behaviour (!)
    • Predicable errors
    What?
    Users
    Other services
    :error tuples
    

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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
    

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23. mkaszubowski94
    > Developers
    Users Infrastructure
    Other services
    Sources of errors
    

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24. mkaszubowski94
    > Developers
    Users Infrastructure
    Other services
    Sources of errors
    

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25. mkaszubowski94
    > Developers
    Users Infrastructure
    Other services
    Sources of errors
    

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26. mkaszubowski94
    > Developers
    Users Infrastructure
    Other services
    Sources of errors
    

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27. mkaszubowski94
    > Developers
    Users Infrastructure
    Other services
    Sources of errors
    ?
    

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28. mkaszubowski94
    > Developers
    Users Infrastructure
    Other services
    Sources of errors
    :(
    

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29. mkaszubowski94
    Controller
    Context
    Context.Xxx
    

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30. mkaszubowski94
    Controller
    Context
    Context.Xxx
    

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31. mkaszubowski94
    We're too used to the "standard" ways
    

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32. mkaszubowski94
    case some_function(args) do
    {:ok, something} 
    # 
    {:error, reason} 
    {:error, reason}
    end
    

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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?
    

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34. mkaszubowski94
    Don't deal with errors.
    Prevent them.
    

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35. Eliminating errors
    

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36. mkaszubowski94
    Heuristics
    Declarative operations
    1
    2
    Reasonable defaults
    3 Don't fight the real world
    

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37. mkaszubowski94
    Heuristics
    Declarative operations
    1
    2
    Reasonable defaults
    3 Don't fight the real world
    

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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
    

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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
    

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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
    

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41. mkaszubowski94
    Heuristics
    Declarative operations
    1
    2
    Reasonable defaults
    3 Don't fight the real world
    

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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
    

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43. mkaszubowski94
    Describe the end goal,
    not the steps to reach it
    

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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
    

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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
    

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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
    

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47. mkaszubowski94
    Heuristics
    Declarative operations
    1
    2
    Reasonable defaults
    3 Don't fight the real world
    

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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
    

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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
    

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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
    

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51. mkaszubowski94
    We cannot prevent people
    from making mistakes
    

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52. mkaszubowski94
    Not all errors are bad
    

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53. mkaszubowski94
    Error handling has to be
    designed
    

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54. Why talk about this?
    

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55. mkaszubowski94
    Design is not about a single
    clever idea
    

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56. mkaszubowski94
    We cannot rely on intuition
    

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57. mkaszubowski94
    You will make wrong decisions
    Make sure you learn from that.
    

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58. mkaszubowski94
    A set of clear and simple
    heuristics and questions is really
    helpful
    

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59. mkaszubowski94
    Approach design more
    thoughtfully
    

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60. mkaszubowski94
    Use facts instead of opinions
    

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61. mkaszubowski94
    Gather as many questions and
    heuristics as possible.
    

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62. mkaszubowski94
    Gather as many questions and
    heuristics as possible.
    And share them!
    

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63. mkaszubowski94
    Share the process of making
    decisions.
    

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64. mkaszubowski94
    "A Philosophy of Software Design" - John Ousterhout
    "The art of destroying software" - Greg Young
    Credits
    

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65. mkaszubowski94
    Maciej Kaszubowski
    THANKS!
    I hope it was useful
    

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