polynote / uzhttp   0.2.8

Apache License 2.0 GitHub

Minimal HTTP server for Scala+ZIO

Scala versions: 3.x 2.13 2.12 2.11


Scala CI Maven Central

This (Micro-Z-HTTP, or "uzi-HTTP" if you like) is a minimal HTTP server using ZIO. It has essentially no features. You probably shouldn't use it.


This was made to support the HTTP serving needs of Polynote – which are minimal, because Polynote is a single-page app that only needs to serve some static files and handle websockets. As a result, this is basically all that uzhttp supports.

Should I use it?

Probably not! Here are just a few better options:

  • If you want a full-featured HTTP server which is battle-tested, built on robust technology, supports middleware, and has a purely functional API with a nice DSL, go for http4s – it has pretty good interoperability with ZIO by using zio-interop-cats.
  • If you want the above features but with a native ZIO solution, wait for zio-web.
  • If you want a more minimal solution, that's still got a prinicpled, purely functional API but is production-ready and properly engineered, take a look at finch.

I'm still considering uzhttp. What are its features?

  • Uses 100% non-blocking NIO for its networking (after it's bound, anyway), so it won't gobble up your blocking thread pool.
  • Supports the basic HTTP request types as well as basic websockets.
  • Has no dependencies other than zio and zio-streams.

What important features does it lack?

  • Does not handle fancy new-fangled HTTP 1.1 things like chunked transfer encoding of requests (or responses, unless you build it yourself).
  • Does not support SSL. Nobody really wants to deal with Java's SSL stuff, so the idea is that the app will be behind a reverse proxy that deals with things like SSL termination, SSO, etc.
  • No fancy routing DSL whatsover. It takes a function that gets a request and returns a ZIO[R, HTTPError, Response] and that's basically it.
  • There's nothing else provided for you, either. No authentication stuff built-in (you're using a proxy, remember?). No pluggable middleware or things like that. It won't even parse request URIs into meaningful pieces for you. It's request to response; anything else is yours to deal with.

How do I use it?

To create a server, you use the uzhttp.server.Server.builder constructor. This gives you a builder, which has methods to specify where to listen, how to respond to requests, and how to handle errors. Once you've done that, you call serve which gives you a ZManaged[R with Blocking, Throwable, Server]. You can either useForever this (if you don't need to do anything else with the server), or you can use it as long as you end with awaitShutdown:

serverM.use {
  server => log("It's alive!") *> server.awaitShutdown

The Blocking required for these operations is used for:

  • Binding to the given port
  • Selecting from NIO
  • Some file operations (for generating responses with the Response API) which are more efficient when using blocking (You can avoid these if you wish, and generate your own responses).

Here's an example:

import java.net.InetSocketAddress
import uzhttp.server.Server
import uzhttp.{Request, Response, RefineOps}
import uzhttp.websocket.Frame
import zio.{App, ZIO, Task}

object ExampleServer extends App {
  override def run(args:  List[String]): ZIO[zio.ZEnv, Nothing, Int] =
    Server.builder(new InetSocketAddress("", 8080))
      .handleSome {
        case req if req.uri.getPath startsWith "/static" =>
          // deliver a static file from an application resource
          Response.fromResource(s"staticFiles${req.uri}", req).refineHTTP(req)
        case req if req.uri.getPath == "/" =>
          // deliver a constant HTML response
          ZIO.succeed(Response.html("<html><body><h1>Hello world!</h1></body></html>"))
        case req@Request.WebsocketRequest(_, uri, _, _, inputFrames)
          if uri.getPath startsWith "/ws" =>
          // inputFrames are the incoming frames; construct a websocket session
          // by giving a stream of output frames
          Response.websocket(req, inputFrames.mapM(respondToWebsocketFrame))

  def respondToWebsocketFrame(frame: Frame): Task[Frame] = ???

Can I make a pull request?

Absolutely! Please follow the Polynote contributing guide. We'll gladly accept bugfixes, performance improvements, and tests; and we'd love to see features like:

  • Better support of HTTP features (e.g. Transfer-Encoding).
  • Better support of websockets (e.g. permessage-deflate).

uzhttp would like to stay reasonably minimal. At the time of initial commit, uzhttp was under 1k lines of code! If you've got a great routing DSL or other conveniences, we'll gladly take a look if it's pretty small. But we might suggest that it live as a separate library.


This project is licensed under the Apache 2 license:

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