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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.
Pro Small loading time
Since it has very few dependencies, the loading time for a Sinatra app is considerably small.
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.