Use behaviours to declare interfaces and decouple implementations from business logic

active_users.ex

defmodule Project.ActiveUsers do
  @behaviour Project.UseCase

  def run(params, deps) do
    deps[:repo].fetch_users(params)
  end
end

controller.ex

defmodule ProjectWeb.Controller do
  def index(conn, params) do
    {:ok, users} = Project.ActiveUsers.run(params, repo: Project.StaticUserRepository)
    conn.assing(:users, users)
  end
end

static_user_repository.ex

defmodule Project.StaticUserRepository do
  @behaviour Project.UserRepository

  def fetch_users(_params) do
    users = ["Joao", "Pedro"]
    {:ok, users}
  end
end

use_case.ex

defmodule Project.UseCase do
  @callback run(params :: map(), dependencies :: Keyword.t()) :: {:ok, any()} | {:error, any()}
end

user_repository.ex

defmodule Project.UserRepository do
  @callback fetch_users(params :: map()) :: {:ok, list()} | {:error, any()}
end