Who can explain exactly the meaning of put_private in Plug.Conn?

Question

  @spec put_private(t, atom, term) :: t
  def put_private(%Conn{private: private} = conn, key, value) when is_atom(key) do
    %{conn | private: Map.put(private, key, value)}
  end

Answer 1

The docs do a pretty good job:

Assigns a new private key and value in the connection.

This storage is meant to be used by libraries and frameworks to avoid writing to the user storage (the :assigns field). It is recommended for libraries/frameworks to prefix the keys with the library name.