The behaviour a host application implements to say which paths it owns.

A resolver is asked about a path only when no declared route matched it — which is to say, only about requests that would otherwise be a 404. It answers :pass to let the 404 happen, or {:match, term} to claim the request, and whatever it returns comes back to the controller through DynamicRoutes.resolution/1.

defmodule MyApp.Pages do
  @behaviour DynamicRoutes.Resolver

  @impl true
  def resolve(["blog", slug]) do
    case MyApp.Content.page_by_slug(slug) do
      nil -> :pass
      page -> {:match, page}
    end
  end

  def resolve([slug]) do
    case MyApp.Content.page_by_slug(slug) do
      nil -> :pass
      page -> {:match, page}
    end
  end

  def resolve(_path), do: :pass
end

Returning the record itself, rather than just true, is worth doing: the controller would otherwise look it up a second time on the same request.

Caching

Results are cached, so resolve/1 is not called on every request for the same path — see DynamicRoutes.Cache. Call DynamicRoutes.invalidate/0 when pages change, or DynamicRoutes.invalidate/1 for a single path.

Because the cache holds whatever {:match, term} carries, a resolver that returns a large struct is holding that struct in memory until the entry expires. Return an id, or something small, if the record is big.