Dry run mode

[olivier-thatch]

It would be nice if the gem supported a "dry run" mode to test tasks without actually executing side effects.

I used to work at Stripe, which is a large Ruby shop like Shopify but doesn't use Rails, and we had a tool that was very similar to this gem that supported dry run using the following pattern:

def perform(element)
  element.some_field = "foo"
  perform(:save, element)
end

def save_real(element)
  element.save!
end

def save_dry(element)
  logger.info("Would have saved element #{element.id} with the following attributes: #{element.changed_attributes}")
end

Basically, the idea is that any sort of side effects (typically DB writes, but can be anything) are wrapped in do_something_real method, and there is a do_something_dry counterpart method with the same signature but without side effects (typically, outputting a log message with what the real version would have done). Then when running the migration, perform(:method_name, **args) will call the appropriate version depending on whether dry-run mode is enabled or not.

This was very useful to derisk complex or otherwise critical migrations, by allowing you to run the code against production data but without side effects.

Of course this pattern is just one of many ways a dry-run mode could be implemented. I'm opening this discussion mostly to see if there would be an interest in such a feature rather than discuss a particular implementation.