When comparing Restify vs Koa, the Slant community recommends Koa for most people. In the question“What are the best node.js web frameworks?” Koa is ranked 5th while Restify is ranked 9th. The most important reason people chose Koa is:
Using generators (a bleeding edge feature, even for Node.js) would clean up your code from the mess caused by all those callbacks; making your code more manageable.
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Pros
Pro Automatic DTrace support
DTrace helps you with troubleshooting and building robust RESTful applications.
Pro Specialized for Restful APIs
As the name suggests, Restify is devoted solely to creating an elegant REST API. The API is the core of your site or service, so it makes sense to use a framework that excels at that.
Pro Generator support from ground up
Using generators (a bleeding edge feature, even for Node.js) would clean up your code from the mess caused by all those callbacks; making your code more manageable.
Pro Extremely lightweight
Koa is very lightweight with just 550 lines of code.
Pro async/await keywords are supported and has transcended beyond generator functions
Generators functions are of course a huge plus, but at the time Koa team has transcended generations functions and shifted towards async/await style programming. It has made the Koa best framework available in the market.
Pro Development team has a proven track record
Koa is developed by the team behind a widely used node.js framework (express.js).
Pro Built for ES6
Cons
Con Performance is not very optimum
As most of the benchmarks have already revealed, for large inflows of traffic, apps powered by Restify perform below par and are easily beaten by Express.js in terms of load time (in heavy traffic).
Con Community is relatively small
Con Not compatible with express style middleware
Koa uses generators which are not compatible with any other type of Node.js framework middleware.