Collaborative Code Editor Design

Setup

Mix.install([
  {:phoenix_live_view, "~> 0.20.17"},
  {:jason, "~> 1.4"},
  {:kino, "~> 0.11.0"},
  {:burrito, "~> 1.0"},  # For deployments
  {:rewire, "~> 0.9"},   # For testing/mocking
  {:diff_match_patch, "~> 0.2"} # For text diffing
])

defmodule CollabEditor.Design do
  @moduledoc """
  Design patterns for collaborative editing features
  """
  
  def architecture do
    """
    # Collaborative Editor Architecture
    
    ## Core Components:
    
    1. EditorPresence - Tracks active users and cursors
    2. DocumentState - Manages document state and operations
    3. OperationalTransform - Handles concurrent edits
    4. SyncManager - Manages state synchronization
    
    ## Data Flow:
                    ┌─────────────────┐
                    │   LiveView UI   │
                    └────────┬────────┘
                             │
                    ┌────────┴────────┐
                    │  Phoenix PubSub │
                    └────────┬────────┘
                             │
              ┌──────────────┴──────────────┐
              │                             │
    ┌─────────┴───────────┐    ┌──────────┴──────────┐
    │    DocumentState    │    │   EditorPresence    │
    │     (GenServer)    │    │     (Presence)      │
    └─────────┬──────────┘    └──────────┬──────────┘
              │                           │
              └──────────────┬────────────┘
                            │
                   ┌────────┴────────┐
                   │   SyncManager   │
                   └────────┬────────┘
                            │
                   ┌────────┴────────┐
                   │    Database     │
                   └─────────────────┘
    """
  end
end

defmodule CollabEditor.DocumentState do
  @moduledoc """
  Manages document state and operations
  """
  use GenServer
  
  defstruct [:id, :content, :version, operations: [], users: %{}]
  
  def start_link(opts) do
    GenServer.start_link(__MODULE__, opts, name: via_tuple(opts[:id]))
  end
  
  def init(opts) do
    {:ok, %__MODULE__{
      id: opts[:id],
      content: opts[:initial_content] || "",
      version: 0
    }}
  end
  
  def handle_cast({:apply_operation, operation, user_id}, state) do
    # Apply operational transform
    {transformed_op, new_content} = 
      OperationalTransform.apply(operation, state.content, state.operations)
    
    new_state = %{state |
      content: new_content,
      version: state.version + 1,
      operations: [transformed_op | state.operations]
    }
    
    broadcast_update(new_state)
    {:noreply, new_state}
  end
  
  defp via_tuple(id), do: {:via, Registry, {CollabEditor.Registry, id}}
  
  defp broadcast_update(state) do
    Phoenix.PubSub.broadcast(
      CollabEditor.PubSub,
      "document:#{state.id}",
      {:doc_updated, state}
    )
  end
end

defmodule CollabEditor.OperationalTransform do
  @moduledoc """
  Handles concurrent edit operations
  """
  
  def apply(operation, content, previous_ops) do
    # Transform operation against previous operations
    transformed = transform_operation(operation, previous_ops)
    # Apply transformed operation to content
    new_content = apply_operation(transformed, content)
    {transformed, new_content}
  end
  
  defp transform_operation(op, []), do: op
  defp transform_operation(op, [prev | rest]) do
    transformed = transform_pair(op, prev)
    transform_operation(transformed, rest)
  end
  
  defp transform_pair(op1, op2) do
    # Implement operational transform algorithm
    # This is a simplified version
    case {op1, op2} do
      {{:insert, pos1, text1}, {:insert, pos2, _}} when pos1 > pos2 ->
        {:insert, pos1 + String.length(text1), text1}
      {{:delete, pos1, len1}, {:insert, pos2, text2}} when pos1 > pos2 ->
        {:delete, pos1 + String.length(text2), len1}
      # Add more transformation rules
      _ -> op1
    end
  end
end

LiveView Implementation

defmodule CollabEditorWeb.EditorLive do
  use Phoenix.LiveView
  
  def mount(%{"id" => id}, _session, socket) do
    if connected?(socket) do
      Phoenix.PubSub.subscribe(CollabEditor.PubSub, "document:#{id}")
      CollabEditor.EditorPresence.track_user(self(), id, socket.assigns.current_user)
    end
    
    {:ok, assign(socket,
      document_id: id,
      content: CollabEditor.DocumentState.get_content(id),
      users: CollabEditor.EditorPresence.list_users(id),
      cursor_positions: %{}
    )}
  end
  
  def handle_event("edit", %{"operation" => operation}, socket) do
    CollabEditor.DocumentState.apply_operation(
      socket.assigns.document_id,
      operation,
      socket.assigns.current_user.id
    )
    {:noreply, socket}
  end
  
  def handle_event("cursor_move", %{"position" => position}, socket) do
    CollabEditor.EditorPresence.update_cursor(
      socket.assigns.document_id,
      socket.assigns.current_user.id,
      position
    )
    {:noreply, socket}
  end
  
  def handle_info({:doc_updated, new_state}, socket) do
    {:noreply, assign(socket, content: new_state.content)}
  end
  
  def handle_info({:presence_diff, diff}, socket) do
    {:noreply, update_presence(socket, diff)}
  end
  
  def render(assigns) do
    ~H"""
    
      
        <%= for {user_id, user} <- @users do %>
          
            <%= user.name %>
          
        <% end %>
      
      
      
        <%= @content %>
      
      
      <%= for {user_id, pos} <- @cursor_positions do %>
        
      <% end %>
    
    """
  end
end

Burrito Deployment

# config/burrito.exs
config :burrito,
  app: :collab_editor,
  output_dir: "rel/burrito",
  steps: [
    :assemble,
    :compress
  ],
  targets: [
    macos: [os: :darwin, cpu: :x86_64],
    linux: [os: :linux, cpu: :x86_64],
    windows: [os: :windows, cpu: :x86_64]
  ]

# mix.exs
def project do
  [
    app: :collab_editor,
    deps: [
      {:burrito, "~> 1.0", runtime: false}
    ],
    releases: [
      collab_editor: [
        steps: [:assemble, :tar],
        include_executables_for: [:unix, :windows]
      ]
    ]
  ]
end

Testing with Rewire

defmodule CollabEditor.DocumentStateTest do
  use ExUnit.Case
  use Rewire
  
  # Rewire allows us to mock dependencies
  rewire CollabEditor.DocumentState, OperationalTransform: MockOperationalTransform
  
  test "applies operations with transformation" do
    operation = {:insert, 0, "Hello"}
    
    # Mock the transformation
    MockOperationalTransform
    |> expect(:apply, fn ^operation, "", [] ->
      {operation, "Hello"}
    end)
    
    {:ok, pid} = DocumentState.start_link(id: "test-doc")
    :ok = DocumentState.apply_operation("test-doc", operation, "user-1")
    
    assert DocumentState.get_content("test-doc") == "Hello"
  end
end

Key features:

1. Real-time Collaboration:

   • Operational Transform for concurrent edits
   • Presence tracking for active users
   • Cursor position synchronization

2. State Management:

   • GenServer-based document state
   • PubSub for real-time updates
   • Registry for document process lookup

3. Deployment:

   • Burrito for creating standalone executables
   • Cross-platform support
   • Easy distribution

4. Testing:

   • Rewire for dependency mocking
   • Comprehensive test coverage
   • Isolated component testing

To use:

1. Start the server:

   mix phx.server

2. Create standalone executable:

   mix burrito.build

3. Run tests:

   mix test

Would you like me to expand on any particular aspect?