Super lightweight async HTTP server library in pure Swift runs in iOS / MacOS / Linux

What's New



What's Changed

New Contributors

Full Changelog: v4.1.5...v4.1.6


Build Status Carthage compatible SwiftPM compatible CocoaPods Swift Version Plaform GitHub license

Super lightweight async HTTP server in pure Swift.

Please read: Embedded web server for iOS UI testing.

See also: Our lightweight web framework Ambassador based on Embassy


  • Swift 4 & 5
  • iOS / tvOS / MacOS / Linux
  • Super lightweight, only 1.5 K of lines
  • Zero third-party dependency
  • Async event loop based HTTP server, makes long-polling, delay and bandwidth throttling all possible
  • HTTP Application based on SWSGI, super flexible
  • IPV6 ready, also supports IPV4 (dual stack)
  • Automatic testing covered


Here's a simple example shows how Embassy works.

let loop = try! SelectorEventLoop(selector: try! KqueueSelector())
let server = DefaultHTTPServer(eventLoop: loop, port: 8080) {
        environ: [String: Any],
        startResponse: ((String, [(String, String)]) -> Void),
        sendBody: ((Data) -> Void)
    ) in
    // Start HTTP response
    startResponse("200 OK", [])
    let pathInfo = environ["PATH_INFO"]! as! String
    sendBody(Data("the path you're visiting is \(pathInfo.debugDescription)".utf8))
    // send EOF

// Start HTTP server to listen on the port
try! server.start()

// Run event loop

Then you can visit http://[::1]:8080/foo-bar in the browser and see

the path you're visiting is "/foo-bar"

By default, the server will be bound only to the localhost interface (::1) for security reasons. If you want to access your server over an external network, you'll need to change the bind interface to all addresses:

let server = DefaultHTTPServer(eventLoop: loop, interface: "::", port: 8080)

Async Event Loop

To use the async event loop, you can get it via key embassy.event_loop in environ dictionary and cast it to EventLoop. For example, you can create an SWSGI app which delays sendBody call like this

let app = { (
    environ: [String: Any],
    startResponse: ((String, [(String, String)]) -> Void),
    sendBody: @escaping ((Data) -> Void)
) in
    startResponse("200 OK", [])

    let loop = environ["embassy.event_loop"] as! EventLoop

    loop.call(withDelay: 1) {
        sendBody(Data("hello ".utf8))
    loop.call(withDelay: 2) {
        sendBody(Data("baby ".utf8))
    loop.call(withDelay: 3) {

Please notice that functions passed into SWSGI should be only called within the same thread for running the EventLoop, they are all not threadsafe, therefore, you should not use GCD for delaying any call. Instead, there are some methods from EventLoop you can use, and they are all threadsafe

call(callback: (Void) -> Void)

Call given callback as soon as possible in the event loop

call(withDelay: TimeInterval, callback: (Void) -> Void)

Schedule given callback to withDelay seconds then call it in the event loop.

call(atTime: Date, callback: (Void) -> Void)

Schedule given callback to be called at atTime in the event loop. If the given time is in the past or zero, this methods works exactly like call with only callback parameter.

What's SWSGI (Swift Web Server Gateway Interface)?

SWSGI is a hat tip to Python's WSGI (Web Server Gateway Interface). It's a gateway interface enables web applications to talk to HTTP clients without knowing HTTP server implementation details.

It's defined as

public typealias SWSGI = (
    [String: Any],
    @escaping ((String, [(String, String)]) -> Void),
    @escaping ((Data) -> Void)
) -> Void


It's a dictionary contains all necessary information about the request. It basically follows WSGI standard, except wsgi.* keys will be swsgi.* instead. For example:

  "SERVER_NAME": "[::1]",
  "SERVER_PORT" : "53479",
  "SCRIPT_NAME" : "",
  "PATH_INFO" : "/",
  "HTTP_HOST": "[::1]:8889",
  "HTTP_USER_AGENT" : "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/50.0.2661.102 Safari/537.36",
  "HTTP_ACCEPT_LANGUAGE" : "en-US,en;q=0.8,zh-TW;q=0.6,zh;q=0.4,zh-CN;q=0.2",
  "HTTP_CONNECTION" : "keep-alive",
  "HTTP_ACCEPT" : "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8",
  "HTTP_ACCEPT_ENCODING" : "gzip, deflate, sdch",
  "swsgi.version" : "0.1",
  "swsgi.input" : (Function),
  "swsgi.error" : "",
  "swsgi.multiprocess" : false,
  "swsgi.multithread" : false,
  "swsgi.url_scheme" : "http",
  "swsgi.run_once" : false

To read request from body, you can use swsgi.input, for example

let input = environ["swsgi.input"] as! SWSGIInput
input { data in
    // handle the body data here

An empty Data will be passed into the input data handler when EOF reached. Also please notice that, request body won't be read if swsgi.input is not set or set to nil. You can use swsgi.input as bandwidth control, set it to nil when you don't want to receive any data from client.

Some extra Embassy server specific keys are

  • embassy.connection - HTTPConnection object for the request
  • embassy.event_loop - EventLoop object
  • embassy.version - Version of embassy as a String, e.g. 3.0.0


Function for starting to send HTTP response header to client, the first argument is status code with message, e.g. "200 OK". The second argument is headers, as a list of key value tuple.

To response HTTP header, you can do

startResponse("200 OK", [("Set-Cookie", "foo=bar")])

startResponse can only be called once per request, extra call will be simply ignored.


Function for sending body data to client. You need to call startResponse first in order to call sendBody. If you don't call startResponse first, all calls to sendBody will be ignored. To send data, here you can do


To end the response data stream simply call sendBody with an empty Data.




To install with CocoaPod, add Embassy to your Podfile:

pod 'Embassy', '~> 4.1'


To install with Carthage, add Embassy to your Cartfile:

github "envoy/Embassy" ~> 4.1

Package Manager

Add it this Embassy repo in Package.swift, like this

import PackageDescription

let package = Package(
    name: "EmbassyExample",
    dependencies: [
        .package(url: "https://github.com/envoy/Embassy.git",
                 from: "4.1.4"),

You can read this example project here.


  • Swift Tools 4.2.0
View More Packages from this Author


  • None
Last updated: Wed Jun 05 2024 05:33:06 GMT-0900 (Hawaii-Aleutian Daylight Time)