When comparing Grails vs Sinatra, the Slant community recommends Sinatra for most people. In the question“What are the best web frameworks to create a web REST API?” Sinatra is ranked 18th while Grails is ranked 28th. The most important reason people chose Sinatra is:
Since it has very few dependencies, the loading time for a Sinatra app is considerably small.
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Pros
Pro Grails is highly scalable
Grails is an abstraction over Spring and Hibernate. This makes Grails applications fast and scalable. This is because both Spring and Hibernate are themselves scalable.
Pro More than 900 plugins
Grails is a full-stack web framework, not just MVC. It contains a lot of stuff out of the box, but it doesn’t enforce it. It contains over 900 plugins which provide a Groovy API for a lot of useful and well-known Java libraries. And what is more important is that they are super easy to install!
Pro Very fast setup and scaffolding
Setting up a new project is quite fast and code generation (scaffolding) saves you a lot of time. It also uses a convention over configuration principle which helps you bypass all the configuration trouble.
Grails also comes with a reloading mechanism out of the box.
Pro Easy to use
Grails is designed to be a rapid development framework with a straight learning curve. It advocates convention over configuration. Extensibility is very simple when using plugins (there is a lot of them). One command in the console – and all the dependencies and configurations are managed for you.
Pro Great UI design alternatives
Do you need something advanced in the UI? You can either create it in GSP with an addon of CSS/JavaScript or you can find a plugin that will do the work for you. Plugins include integration with Bootstrap, jQuery, Yeoman and much more.
Pro Great documentation and community
The Documentation section is actually a wiki, which can be modified by any logged in user. It has an official manual, tutorials, screencasts, a sample app and much more. If that does not do it for you, then there are countless third-party tutorials, more than 12k questions on SO and much much more
Pro Small loading time
Since it has very few dependencies, the loading time for a Sinatra app is considerably small.
Pro Has only the bare minimum needed
Sinatra has taken an approach of having only the most useful components needed to build applications out of the box. It has simple routes along with a Domain Specific Language over a Rack layer.
Cons
Con Grails is a fairly complex framework
Grails is a pretty heavy piece of software. It's functionality is covered by GORM (Grails' Object Relational Mapping) which is a facade for hybernate and by Spring MVC.
Everything is glued by core Spring and furthermore, Grails adds another level of abstraction on top of all this. These things may create some trouble down the road when debugging.
Con Too obtuse and JVM centric
If you're a big fan of the JVM and have mastered all its goofy quirks, Grails might be a good choice, but there are other frameworks out there that are more straightforward and easier to use.
Con Hard to scale well
Because it's rather small and minimalistic, scaling up is not very easy with Sinatra. You need a great deal of knowledge on libraries and modules that may be useful for your particular use-case. As your application grows larger it may be hard to keep things clean and minimalistic, losing a lot of the advantages that Sinatra has.