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.
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.
Dit artikel van murblog van Maarten Brouwers (murb) is in licentie gegeven volgens een Creative Commons Naamsvermelding 3.0 Nederland licentie .