When to use Modules / Concerns?

Whenever your model gets too heavy?

The easiest way to clean up your classes might be to create smaller, more concise methods. The next easiest way of tiding up your models is moving stuff to modules (whether they are 'Concerns' or not). Modules can then be included in the final classes. It will lead to a crowded list of methods exposed on these classes, for which alternative solutions exist (Presenters, Decorators), but if you shield off private methods nicely and have a consistent way of naming things, I wouldn't be too concerned about that. Note that having many modules used in only a single class might be a code smell: perhaps you're trying to do too much with that single class.

Concerns or Modules?

When you're using Rails, you can make use of Concerns. They offer a few advantages over traditional modules, so use it whenever you're bothering recreating the same behaviour using plain old ruby Modules. I prefer consistency, so if you've adopted Concerns, use con…

When to use Form-objects?

When necessary.

It depends. By default I would advise against them; not creating Form objects to receive and validate data that could be validated by the Model directly. Even when you have a few nested attributes that belong to the main model modified, I would advise against Form objects. Keep It Simple.

But… sometimes you have more complex forms that don't fit the database-mirroring model as nicely.

When to use Job (or Worker) objects?

Always.

When you are able to do stuff async (not blocking the web-request), make it async. It will also reduce the need for a category of Service-objects. Worker or Job objects can often be called inline if desired.

Sidenote: I personally prefer the "Job" object name, a Job that needs to be performed. Worker is a name that was popularised by Sidekiq, but Sidekiq moved to Jobs as well.

When to use Service-objects?

Never.

There is of course never an absolute answer to stuff but if you are running it in a background job anyway have you considered directly writing it in a Worker or Job-object? Note that you can always run jobs async when needed.

My main objection against service objects is that all too often they are ill defined as a category. So while having fat controllers or fat models may be a bad thing, just creating a bunch of somewhat arbitrary 'Services' is not making the code more manageable.

When considering adding a 'services' directory to your app, try to think of what class of problems you want to tackle. And when in doubt, just keep messing around with the somewhat fatter models & controllers.