yak(boundary-repo-as-sibling-boundary-web-repo-forbidden-tgrc): boundary: Repo as sibling boundary, web->Repo forbidden

app/lib/firehose.ex

@@ -1,4 +1,8 @@
 defmodule Firehose do
+  use Boundary,
+    deps: [Firehose.Repo],
+    exports: :all
+
   @moduledoc """
   Firehose keeps the contexts that define your domain
   and business logic.

app/lib/firehose/application.ex

@@ -3,6 +3,16 @@ defmodule Firehose.Application do
   # for more information on OTP Applications
   @moduledoc false
 
+  # Booting the app is the one job that must reach into the web tier (it starts
+  # FirehoseWeb.Telemetry and FirehoseWeb.Endpoint) while the web tier reaches
+  # back into the contexts. Declaring the Application a top-level boundary of
+  # its own is the layout boundary's README prescribes for `mix phx.new`
+  # projects, and it keeps the graph acyclic:
+  # Firehose.Application -> {Firehose, FirehoseWeb} -> Firehose -> Firehose.Repo.
+  # Putting FirehoseWeb in Firehose's deps instead is a cycle, which boundary
+  # rejects ("dependency cycle found: Firehose -> FirehoseWeb -> Firehose").
+  use Boundary, top_level?: true, deps: [Firehose, FirehoseWeb]
+
   use Application
 
   @impl true

app/lib/firehose/repo.ex

@@ -1,4 +1,6 @@
 defmodule Firehose.Repo do
+  use Boundary, top_level?: true, deps: [], exports: []
+
   use Ecto.Repo,
     otp_app: :firehose,
     adapter: Ecto.Adapters.Postgres

app/lib/firehose_web.ex

@@ -1,4 +1,8 @@
 defmodule FirehoseWeb do
+  use Boundary,
+    deps: [Firehose],
+    exports: [Endpoint, Telemetry]
+
   @moduledoc """
   The entrypoint for defining your web interface, such
   as controllers, components, channels, and so on.