Your app's answer to "may this actor do this here?", and to "what is this document?". Issho has no roles and no user model: it asks, and your app answers with whatever it already uses.

defmodule MyApp.Collaboration do
  @behaviour Issho.Policy

  @impl true
  def authorize(%Issho.Identity{} = who, "board:" <> id, action) do
    if MyApp.Boards.allowed?(who.claims["user_id"], id, action),
      do: :ok,
      else: {:error, :forbidden}
  end

  @impl true
  def document(_who, "board:" <> id) do
    board = MyApp.Boards.get!(id)
    {:ok, %{engine: Issho.Engine.Map, initial: board.content}}
  end
end

Actions

authorize/3 gets the room (the topic, such as "board:42") and one of:

• :join: to enter the room, see its people and their places;
• :read: to receive its documents and their changes, asked again for the changes others make, so that a person whose access you take away stops receiving them;
• :write: to change its documents;
• :lock: to hold an advisory lock on a place.

Issho asks on join and on every message in both directions, so keep it fast: put what it needs in the identity's claims when you sign the token (Issho.Token.sign/4), or cache.

Documents

document/2 says what a document is when it opens for the first time on a node: its engine (Issho.Engine), its initial content from your server (a browser never seeds a document), and the engine's options. A room's documents are the room itself ("board:42") and paths under it ("board:42/n7/body"), so the nested documents of a canvas share the room's people and authorization.