Stateful operation is probably not safe, and should be marked as such

[johnynek]

http://docs.cascading.org/cascading/1.2/javadoc/cascading/operation/BaseOperation.html#isSafe()

By default, cascading (and hence scalding) assumes you can rerun the same map operation over and over (that is, idempotency is assumed). For a stateful operation, you might be talking to a server and doing some non-idempotent operation.

We should either make it clearer in the docs that idempotency is assumed and/or allow the programmer to signal to cascading that it is not (and we set isSafe == false).

I prefer to tell everyone that scalding addresses the idempotent case, and if you want to go beyond, write your own operation and use .each (and you take responsibility). I don't like encouraging non-idempotent operations.