Laetitia

Why :rpc.call: returns an Inspect.Error?

Hi everybody,

I try to use :rpc.call to create an entity on another node like this:

:rpc.call(:node@me, MyApp, :create_toto, [%{}])

create_toto builds a changeset for Toto Schema and calls Repo.insert.

When the changeset is valid, everything is ok and the result is as expected.

But when the changeset is not valid, i get this error:

{:error,
 %Inspect.Error{
   message: "got UndefinedFunctionError with message \"function Toto.__schema__/1 is undefined (module Toto is not available)\" while inspecting %{__struct__: Ecto.Changeset, action: :insert, changes...... }}

The function works well on its own node.

Can anybody explain this behaviour?

Thanks

11 comments

#rpc

0 1279 11

2021-04-08 09:45:05 UTC

Marked As Solved

voltone

IEx on the local node is calling inspect on the value returned by :rpc.call/4. So by that time the RPC itself was completed, the exception happens locally in the console.

The RPC worked just fine, and if the call didn’t happen in the console but in some local code, you could process the returned changeset normally. It’s just the rendering in the console that fails in this case. You would have similar problems if you were to try and log the changeset, for example.

You could open a ticket on Ecto to see if the team might consider handling this more gracefully. The question then is what fields should be redacted, because that’s what the custom Inspect protocol is trying to do: hide sensitive data. If the struct is not known locally, how can the changeset inspection decide which fields to show? Should it redact all fields, out of an abundance of caution? Maybe…

Post #11

Also Liked

voltone

The calling node is trying to inspect the Ecto.Changeset, which has a custom Inspect protocol implementation that in turn is trying to render the Toto struct that’s embedded inside it. It seems that it does not consider the possibility that the :data field in the changeset is a struct from a module that doesn’t exist locally.

(Edit: it is trying to call __schema__/1 on that module here)

To prevent the error, force the local console to not try and call the Ecto.Changeset inspect implementation, for example by calling IEx.configure(inspect: [structs: false]) before the :rpc call.

Alternatively you could monkey-patch Ecto.Changeset from within iEx:

defimpl Inspect, for: Ecto.Changeset do              
  defdelegate inspect(data, opts), to: Inspect
end

Post #8

Last Post!

Laetitia

Thank you!!!

The RPC worked just fine, and if the call didn’t happen in the console but in some local code, you could process the returned changeset normally. It’s just the rendering in the console that fails in this case.

It really was this and helped me to find out the real problem. The node that called the remote function was trying to cast the changeset (wich was a changeset error) to a string.

Post #12