Erlang: side effect(s) to calling mnesia:create_schema more than once?

Question

Is there a side effect to calling mnesia:create_schema() on each application start?

From what I keep reading, this function should only be called once per database instance. Is it a big issue to call it more than once on an existing database?

Answer 1

I've done this before in development and it spits out warnings on the tables that already exist. However I wouldn't make it a practice to rerun it in Production since it's possible that it may have some side-effects I'm unaware of and even if it doesn't now there is no guarantee that it won't in future releases.

Why do you want to run it multiple times?

Answer 2

It has no side effect, but later calls will result in {error, {Node,{already_exists,Node}}}. You can use something like

ensure_schema() ->
  Node = node(),
  case mnesia:create_schema([Node]) of
    ok -> ok;
    {error, {Node, {already_exists, Node}}} -> ok;
    Error -> Error
  end.

Answer 3

Well it could throw an exception on the second call. Just catch it.