Auto-creating Deployments on First Launch

In the standard LTI flow, registrations and deployments are both created out-of-band before the first launch. But many tools don't need per-deployment configuration. They care about the platform (registration) but treat deployments as bookkeeping. For these tools, you can auto-create deployments the first time a new deployment_id appears in a launch.

This works because get_deployment/2 runs after JWT signature verification and nonce validation, so the deployment_id is already trusted by the time your adapter sees it.

Storage adapter

The simplest approach: make get_deployment/2 upsert instead of just querying. The first launch creates the deployment row; subsequent launches find it. No controller changes needed.

@impl true
def get_deployment(%Ltix.Registration{} = reg, deployment_id) do
  registration_id = get_registration_id(reg)

  %PlatformDeployment{registration_id: registration_id, deployment_id: deployment_id}
  |> Repo.insert(on_conflict: :nothing, conflict_target: [:registration_id, :deployment_id])

  case Repo.get_by(PlatformDeployment,
         registration_id: registration_id,
         deployment_id: deployment_id
       ) do
    nil -> {:error, :not_found}
    record -> {:ok, record}
  end
end

The insert-then-select pattern handles the race condition where two concurrent launches for the same new deployment both call get_deployment/2 at the same time. on_conflict: :nothing means the second insert silently succeeds without duplicating the row, and both calls find the same record on the subsequent select.

create unique_index(:platform_deployments, [:registration_id, :deployment_id])

Controller

If deployments carry meaning in your domain (per-deployment settings, onboarding flows, approval steps), handle creation in the controller instead. This lets you show a registration form, collect extra information, or require admin approval before the deployment is active.

def launch(conn, params) do
  state = get_session(conn, :lti_state)

  case Ltix.handle_callback(params, state) do
    {:ok, context} ->
      conn
      |> delete_session(:lti_state)
      |> render(:launch, context: context)

    {:error, %Ltix.Errors.Invalid.DeploymentNotFound{deployment_id: id}} ->
      # Create the deployment, then ask the user to re-launch.
      # We can't retry handle_callback here because the nonce was
      # already consumed (see note below).
      MyApp.Deployments.create!(id)

      conn
      |> delete_session(:lti_state)
      |> render(:deployment_created,
        message: "Setup complete. Return to your LMS and launch again."
      )

    {:error, error} ->
      conn
      |> put_status(400)
      |> text("Launch failed: #{Exception.message(error)}")
  end
end

For an onboarding flow where you need to collect information before creating the deployment:

{:error, %Ltix.Errors.Invalid.DeploymentNotFound{deployment_id: id}} ->
  conn
  |> put_session(:pending_deployment_id, id)
  |> delete_session(:lti_state)
  |> redirect(to: ~p"/lti/onboard")

Which approach to use