Phoenix JSON API

Canonical FP bar: [docs/fcis-engineering-rules.md](../../docs/fcis-engineering-rules.md) — Functional Core, Imperative Shell: pure domain modules; side effects at edges. Keep LiveView/controller callbacks thin; delegate business rules to contexts/pure modules.

RULES — Follow these with no exceptions

1. Use the :api pipeline — don't mix HTML and JSON pipelines; API routes skip CSRF and sessions 2. Render errors as structured JSON — {:error, changeset} must become {"errors": {...}} 3. Version APIs via URL prefix (/api/v1/) — not headers; URL versioning is visible and cacheable 4. Use FallbackController for consistent error handling — every action returns {:ok, result} or {:error, reason}

Build Workflow

Follow these steps in order when constructing a new API endpoint:

1. Define the route in the :api (or :api_auth) pipeline scope with a versioned URL prefix
2. Create the controller with actionfallback MyAppWeb.FallbackController and return {:ok, } / {:error, _} from every action
3. Verify error responses — confirm that invalid input and missing resources return structured JSON (e.g., {"errors": {...}}) before proceeding
4. Add the auth plug (ApiAuth) to protected scopes; confirm that missing/invalid tokens yield 401 with a JSON body
5. Paginate list endpoints — ensure index accepts page/per_page params and never returns an unbounded collection

API Pipeline Setup

❌ Bad — one mixed pipeline (drags CSRF/session into JSON) and header-based versioning:

scope "/api", MyAppWeb do
  pipe_through :browser  # CSRF + session cookies on a JSON API

  # version selected from a header — invisible and hard to cache
  resources "/posts", PostController
end

✅ Good — a dedicated :api pipeline with a versioned URL prefix:

defmodule MyAppWeb.Router do
  use MyAppWeb, :router

  pipeline :api do
    plug :accepts, ["json"]
  end

  pipeline :api_auth do
    plug MyAppWeb.Plugs.ApiAuth
  end

  # Public endpoints
  scope "/api/v1", MyAppWeb.API.V1, as: :api_v1 do
    pipe_through :api

    post "/auth/login", AuthController, :login
    post "/auth/register", AuthController, :register
  end

  # Protected endpoints
  scope "/api/v1", MyAppWeb.API.V1, as: :api_v1 do
    pipe_through [:api, :api_auth]

    resources "/posts", PostController, except: [:new, :edit]
  end
end

Controller Pattern

❌ Bad — no action_fallback, error handling duplicated inline, unstructured error body:

def create(conn, %{"post" => post_params}) do
  case Blog.create_post(post_params) do
    {:ok, post} ->
      json(conn, %{data: post})

    {:error, _changeset} ->
      # plain string, no status control, inconsistent shape
      json(conn, %{error: "could not create"})
  end
end

✅ Good — actionfallback handles every {:error, }; actions stay focused on the happy path:

defmodule MyAppWeb.API.V1.PostController do
  use MyAppWeb, :controller

  alias MyApp.Blog
  alias MyApp.Blog.Post

  action_fallback MyAppWeb.FallbackController

  def index(conn, params) do
    page = parse_int(params["page"], 1)
    per_page = parse_int(params["per_page"], 20) |> min(100)

    {posts, total} = Blog.list_posts(page: page, per_page: per_page)

    conn
    |> put_resp_header("x-total-count", to_string(total))
    |> json(%{
      data: Enum.map(posts, &post_json/1),
      meta: %{page: page, per_page: per_page, total: total}
    })
  end

  def create(conn, %{"post" => post_params}) do
    with {:ok, %Post{} = post} <- Blog.create_post(post_params) do
      conn
      |> put_status(:created)
      |> put_resp_header("location", ~p"/api/v1/posts/#{post}")
      |> json(%{data: post_json(post)})
    end
  end

  defp post_json(post) do
    %{
      id: post.id,
      title: post.title,
      body: post.body,
      inserted_at: post.inserted_at
    }
  end

  defp parse_int(nil, default), do: default
  defp parse_int(value, default) when is_binary(value) do
    case value |> String.trim() |> Integer.parse() do
      {n, ""} -> max(n, 1)
      _ -> default
    end
  end
  defp parse_int(value, default) when is_integer(value), do: max(value, 1)
  defp parse_int(_, default), do: default
end

FallbackController

defmodule MyAppWeb.FallbackController do
  use MyAppWeb, :controller

  def call(conn, {:error, :not_found}) do
    conn
    |> put_status(:not_found)
    |> json(%{errors: %{detail: "Resource not found"}})
  end

  def call(conn, {:error, :unauthorized}) do
    conn
    |> put_status(:unauthorized)
    |> json(%{errors: %{detail: "Not authorized"}})
  end

  def call(conn, {:error, %Ecto.Changeset{} = changeset}) do
    conn
    |> put_status(:unprocessable_entity)
    |> json(%{errors: format_changeset_errors(changeset)})
  end

  defp format_changeset_errors(changeset) do
    Ecto.Changeset.traverse_errors(changeset, fn {msg, opts} ->
      Enum.reduce(opts, msg, fn {key, value}, acc ->
        String.replace(acc, "%{#{key}}", to_string(value))
      end)
    end)
  end
end

Bearer Token Authentication

defmodule MyAppWeb.Plugs.ApiAuth do
  import Plug.Conn
  import Phoenix.Controller

  def init(opts), do: opts

  def call(conn, _opts) do
    with ["Bearer " <> token] <- get_req_header(conn, "authorization"),
         {:ok, user} <- MyApp.Accounts.get_user_by_api_token(token) do
      assign(conn, :current_user, user)
    else
      _ ->
        conn
        |> put_status(:unauthorized)
        |> json(%{errors: %{detail: "Invalid or missing token"}})
        |> halt()
    end
  end
end

Common Pitfalls

Integration