I recently converted an existing application to an umbrella, and when I compile it - I see the following warning:
warning: failed to load Jason for Phoenix JSON encoding
(module Jason is not available).
Ensure Jason exists in your deps in mix.exs,
and you have configured Phoenix to use it for JSON encoding by
verifying the following exists in your config/config.exs:
config :phoenix, :json_library, Jason
(phoenix) lib/phoenix.ex:40: Phoenix.start/2
(kernel) application_master.erl:277: :application_master.start_it_old/4
In config/config.exs I have config :phoenix, :json_library, Jason
and mix.exs of the web app has {:jason, "~> 1.0"}
Had that problem a while ago, had to move jason to the mix.exs of the Phoenix app. Not in the root, not in any of the other apps in the umbrella. Had to be exactly in the Phoenix app.