Wow, I didn’t realize that! Thank you.
That ended up leading to this relatively generic solution to allow deferring arbitrary config:
defmodule Foo.Application do
@moduledoc false
use Application
@impl true
def start(_type, _args) do
children = [
FooWeb.Telemetry,
Foo.Repo,
{Ecto.Migrator,
repos: Application.fetch_env!(:fooApp, :ecto_repos),
skip: skip_migrations?()},
#{DNSCluster, query: Application.get_env(:foo, :dns_cluster_query) || :ignore},
{Phoenix.PubSub, name: Foo.PubSub},
{DynamicSupervisor, name: Foo.RoomSupervisor},
FooWeb.Endpoint |> Foo.Util.defer_spec(
secret_key_base: {Foo.Repo, Foo.Repo.Schemas.Secret, &FooWeb.Util.phx_gen_secret/0}
)
]
opts = [strategy: :one_for_one, name: Foo.Supervisor]
Supervisor.start_link(children, opts)
end
…
end
Utility functions / heavy lifting
# lib/foo/util.ex
defmodule Foo.Util do
require Ecto.Query
…
# https://github.com/elixir-ecto/ecto/blob/v3.12.5/lib/ecto/repo/queryable.ex#L153
def one_or_insert_lazy!(repo, query, fun) do
repo.transaction(fn ->
case repo.all(query) do
[value] -> value
[] ->
repo.insert!(fun.())
repo.one!(query)
other -> raise Ecto.MultipleResultsError, queryable: query, count: length(other)
end
end)
end
@spec defer_spec(Supervisor.module_spec(), deferred_opts :: keyword()) :: Supervisor.child_spec()
def defer_spec(module_spec, deferred_opts \\ [])
def defer_spec(module, deferred_opts) when is_atom(module), do: defer_spec({module, []}, deferred_opts)
def defer_spec({module, opts}, deferred_opts)
when is_atom(module) do
%{
id: module,
start: {__MODULE__, :_start_link_helper, [module, opts, deferred_opts]}
}
end
def _start_link_helper(module, opts, deferred_opts) do
access = if is_list(opts), do: Keyword, else: Access
opts = Enum.reduce(deferred_opts, opts, fn
{key, {repo, schema, default_fun}}, acc when is_function(default_fun, 0) ->
{_, acc} = access.get_and_update(acc, key, fn
value when not is_nil(value) ->
{value, value}
_ ->
key = Atom.to_string(key)
value = one_or_insert_lazy!(
repo,
Ecto.Query.from(
s in schema,
where: s.name == ^key,
select: s.value
),
fn -> struct(
schema,
name: key,
value: default_fun.()
) end
)
{nil, value}
end)
acc
{key, fun}, acc when is_function(fun, 0) ->
access.put(acc, key, fun.())
{key, fun}, acc when is_function(fun, 1) ->
{_, acc} = access.get_and_update(acc, key, &{&1, fun.(&1)})
acc
{key, {:get_and_update, fun}}, acc when is_function(fun, 1) ->
{_, acc} = access.get_and_update(acc, key, fun)
acc
end)
module.start_link(opts)
end
end
# lib/foo_web/util.ex
defmodule FooWeb.Util do
def phx_gen_secret(length \\ 64) do
# https://github.com/phoenixframework/phoenix/blob/v1.7.17/lib/mix/tasks/phx.gen.secret.ex#L17
:crypto.strong_rand_bytes(length) |> Base.encode64(padding: false) |> binary_part(0, length)
end
end
