Clone or download



Swift License Slack Travis Codebeat

Why Zewo?SupportCommunityContributing


Zewo is a lightweight library for web applications in Swift.

What sets Zewo apart?

Zewo is not a web framework. Zewo is a lightweight library for web applications in Swift. Most server-side Swift projects use Grand Central Dispatch (GCD) as the concurrency library of choice. The drawback of using GCD is that its APIs are asynchronous. With async code comes callback hell and we all know it, it's no fun.

Node.js is the best example of how callbacks can be frustrating. Express.js creator TJ Holowaychuk wrote a blog post about Callback vs Coroutines in 2013 and one year later left the Node.js community in favor of Go. There were many reasons for that but one of the main reasons was the concurrency model. Sure we have futures and promises and functional reactive programming. They all mitigate the problem, but the async nature of the code will always be there.

At Zewo we use coroutines. Coroutines allow concurrency while maintaining synchronous APIs. We all learn how to program with synchronous code. We're used to reason about our code synchronously. Being able to use synchronous APIs makes the code much more readable and understandable. Coroutines are also faster than threads, because they live in user-space, unlike threads which are managed by the kernel.

Our implementation of coroutines (which is based on libdill) is single-threaded. This means that you don't have to worry about locks or race conditions. So your code is safer by default. To use all the CPU power available all you have to do is to replicate the work according to the number of logical CPUs available. As an example, this could mean running as many processes of your server as cores in your machine. Rob Pike, one of the creators of Go had a talk called Concurrency is not Parallelism that explains this concept very well. Go also has the philosophy:

Don't communicate by sharing memory. Share memory by communicating.

Like Go, instead of sharing memory and handling state we promote the use of CSP-style concurrency using channels. This pattern brings the abstractions used on the development of distributed systems closer to the way we're used to think about communication. It also aligns well with Swift's mindset of immutability and value types. All of these things contributes to a distinct experince on the server-side Swift.

With Zewo you get:

  • Go-style concurrency
  • Synchronous APIs
  • Incredible performance
  • Safer applications
  • Scalable systems
  • Cleaner code
  • Proper error handling
  • No callback hell
  • No race conditions


If you have any trouble create a Github issue and we'll do everything we can to help you. When stating your issue be sure to add enough details and reproduction steps so we can help you faster. If you prefer you can join our Slack and go to the #help channel too.



We have an amazing community of open and welcoming developers. Join us on Slack to get to know us!


All Zewo modules are released under the MIT license. See LICENSE for details.