I have a grails project that is at version 2.3.4 and I need to upgrade to the latest version possible. Looking at the documentation I realized that from 2.x to 3.x there are huge changes.
The question is: is it ideal for me to do this gradual upgrade from 2 to 3 from 3 to 4 and from 4 to 5 or is it better/possible to do this from 2 to 5 at once?
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Jeff is wrong in saying its easier to go from 2 to 5. There are swathes of code and classes removed and changed. 2 to 3 is a far easier transition. PLUS, 3 is based on Springboot so if you have any problems, you can easily transition into the most widely used Java Framework and just use Groovy with it. :)
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If your end-goal is to have a modern Grails application, I would go from 2 to 5 directly. There have been numerous upgrades from 2 to 4 recently, and probably from 2 to 5 onwards - so there is a decent amount of information about it to be found.
Doing it stepwise means you have to first find info for 2 to 3, then 3 to 4 and finally 4 to 5. I think that will cause you more trouble than it is worth.
Generally speaking it will be better to go from 2 to 5. The effort to go from 2 to 3 is about the same as going from 2 to 5. Introducing the interim upgrades will be more work to end up at the same place.