Lightweight, HTTP server written in Swift using async/await.

What's New

0.14.0 RangeReplaceableCollection


Build Codecov Platforms Swift 5.10 License Twitter


FlyingFox is a lightweight HTTP server built using Swift Concurrency. The server uses non blocking BSD sockets, handling each connection in a concurrent child Task. When a socket is blocked with no data, tasks are suspended using the shared AsyncSocketPool.


FlyingFox can be installed by using Swift Package Manager.

Note: FlyingFox requires Swift 5.7 on Xcode 14+. It runs on iOS 13+, tvOS 13+, macOS 10.15+ and Linux. Windows 10 support is experimental.

To install using Swift Package Manager, add this to the dependencies: section in your Package.swift file:

.package(url: "", .upToNextMajor(from: "0.14.0"))


Start the server by providing a port number:

import FlyingFox

let server = HTTPServer(port: 80)
try await server.start()

The server runs within the the current task. To stop the server, cancel the task terminating all connections immediatley:

let task = Task { try await server.start() }

Gracefully shutdown the server after all existing requests complete, otherwise forcefully closing after a timeout:

await server.stop(timeout: 3)

Wait until the server is listening and ready for connections:

try await server.waitUntilListening()

Retrieve the current listening address:

await server.listeningAddress

Note: iOS will hangup the listening socket when an app is suspended in the background. Once the app returns to the foreground, HTTPServer.start() detects this, throwing SocketError.disconnected. The server must then be started once more.


Handlers can be added to the server by implementing HTTPHandler:

protocol HTTPHandler {
  func handleRequest(_ request: HTTPRequest) async throws -> HTTPResponse

Routes can be added to the server delegating requests to a handler:

await server.appendRoute("/hello", to: handler)

They can also be added to closures:

await server.appendRoute("/hello") { request in
  try await Task.sleep(nanoseconds: 1_000_000_000)
  return HTTPResponse(statusCode: .ok)

Incoming requests are routed to the handler of the first matching route.

Handlers can throw HTTPUnhandledError if after inspecting the request, they cannot handle it. The next matching route is then used.

Requests that do not match any handled route receive HTTP 404.


Requests can be routed to static files with FileHTTPHandler:

await server.appendRoute("GET /mock", to: .file(named: "mock.json"))

FileHTTPHandler will return HTTP 404 if the file does not exist.


Requests can be routed to static files within a directory with DirectoryHTTPHandler:

await server.appendRoute("GET /mock/*", to: .directory(subPath: "Stubs", serverPath: "mock"))
// GET /mock/fish/index.html  ---->  Stubs/fish/index.html

DirectoryHTTPHandler will return HTTP 404 if a file does not exist.


Requests can be proxied via a base URL:

await server.appendRoute("GET *", to: .proxy(via: ""))
// GET /get?fish=chips  ---->  GET


Requests can be redirected to a URL:

await server.appendRoute("GET /fish/*", to: .redirect(to: ""))
// GET /fish/chips  --->  HTTP 301
//                        Location:


Requests can be routed to a websocket by providing a WSMessageHandler where a pair of AsyncStream<WSMessage> are exchanged:

await server.appendRoute("GET /socket", to: .webSocket(EchoWSMessageHandler()))

protocol WSMessageHandler {
  func makeMessages(for client: AsyncStream<WSMessage>) async throws -> AsyncStream<WSMessage>

enum WSMessage {
  case text(String)
  case data(Data)

Raw WebSocket frames can also be provided.


Multiple handlers can be grouped with requests and matched against HTTPRoute using RoutedHTTPHandler.

var routes = RoutedHTTPHandler()
routes.appendRoute("GET /fish/chips", to: .file(named: "chips.json"))
routes.appendRoute("GET /fish/mushy_peas", to: .file(named: "mushy_peas.json"))
await server.appendRoute(for: "GET /fish/*", to: routes)

HTTPUnhandledError is thrown when it's unable to handle the request with any of its registered handlers.


HTTPRoute is designed to be pattern matched against HTTPRequest, allowing requests to be identified by some or all of its properties.

let route = HTTPRoute("/hello/world")

route ~= HTTPRequest(method: .GET, path: "/hello/world") // true
route ~= HTTPRequest(method: .POST, path: "/hello/world") // true
route ~= HTTPRequest(method: .GET, path: "/hello/") // false

Routes are ExpressibleByStringLiteral allowing literals to be automatically converted to HTTPRoute:

let route: HTTPRoute = "/hello/world"

Routes can include a specific method to match against:

let route = HTTPRoute("GET /hello/world")

route ~= HTTPRequest(method: .GET, path: "/hello/world") // true
route ~= HTTPRequest(method: .POST, path: "/hello/world") // false

They can also use wildcards within the path:

let route = HTTPRoute("GET /hello/*/world")

route ~= HTTPRequest(method: .GET, path: "/hello/fish/world") // true
route ~= HTTPRequest(method: .GET, path: "/hello/dog/world") // true
route ~= HTTPRequest(method: .GET, path: "/hello/fish/sea") // false

Trailing wildcards match all trailing path components:

let route = HTTPRoute("/hello/*")

route ~= HTTPRequest(method: .GET, path: "/hello/fish/world") // true
route ~= HTTPRequest(method: .GET, path: "/hello/dog/world") // true
route ~= HTTPRequest(method: .POST, path: "/hello/fish/deep/blue/sea") // true

Specific query items can be matched:

let route = HTTPRoute("/hello?time=morning")

route ~= HTTPRequest(method: .GET, path: "/hello?time=morning") // true
route ~= HTTPRequest(method: .GET, path: "/hello?count=one&time=morning") // true
route ~= HTTPRequest(method: .GET, path: "/hello") // false
route ~= HTTPRequest(method: .GET, path: "/hello?time=afternoon") // false

Query item values can include wildcards:

let route = HTTPRoute("/hello?time=*")

route ~= HTTPRequest(method: .GET, path: "/hello?time=morning") // true
route ~= HTTPRequest(method: .GET, path: "/hello?time=afternoon") // true
route ~= HTTPRequest(method: .GET, path: "/hello") // false

HTTP headers can be matched:

let route = HTTPRoute("*", headers: [.contentType: "application/json"])

route ~= HTTPRequest(headers: [.contentType: "application/json"]) // true
route ~= HTTPRequest(headers: [.contentType: "application/xml"]) // false

Header values can be wildcards:

let route = HTTPRoute("*", headers: [.authorization: "*"])

route ~= HTTPRequest(headers: [.authorization: "abc"]) // true
route ~= HTTPRequest(headers: [.authorization: "xyz"]) // true
route ~= HTTPRequest(headers: [:]) // false

Body patterns can be created to match the request body data:

public protocol HTTPBodyPattern: Sendable {
  func evaluate(_ body: Data) -> Bool

Darwin platforms can pattern match a JSON body with an NSPredicate:

let route = HTTPRoute("POST *", body: .json(where: "food == 'fish'"))
{"side": "chips", "food": "fish"}


HTTPResponse can switch the connection to the WebSocket protocol by provding a WSHandler within the response payload.

protocol WSHandler {
  func makeFrames(for client: AsyncThrowingStream<WSFrame, Error>) async throws -> AsyncStream<WSFrame>

WSHandler facilitates the exchange of a pair AsyncStream<WSFrame> containing the raw websocket frames sent over the connection. While powerful, it is more convenient to exchange streams of messages via WebSocketHTTPHandler.

Preview Macro Handler

The branch preview/macro contains an experimental preview implementation where handlers can annotate functions with routes:

struct MyHandler {

  func ping() { }

  func getPong(_ request: HTTPRequest) -> HTTPResponse {
    HTTPResponse(statusCode: .accepted)

  @JSONRoute("POST /account")
  func createAccount(body: AccountRequest) -> AccountResponse {
    AccountResponse(id: UUID(), balance: body.balance)

let server = HTTPServer(port: 80, handler: MyHandler())
try await server.start()

The annotations are implemented via SE-0389 Attached Macros available in Swift 5.9 and later.

Read more here.


Internally, FlyingFox uses a thin wrapper around standard BSD sockets. The FlyingSocks module provides a cross platform async interface to these sockets;

import FlyingSocks

let socket = try await AsyncSocket.connected(to: .inet(ip4: "", port: 80))
try await socket.write(Data([0x01, 0x02, 0x03]))
try socket.close()


Socket wraps a file descriptor and provides a Swift interface to common operations, throwing SocketError instead of returning error codes.

public enum SocketError: LocalizedError {
  case blocked
  case disconnected
  case unsupportedAddress
  case failed(type: String, errno: Int32, message: String)

When data is unavailable for a socket and the EWOULDBLOCK errno is returned, then SocketError.blocked is thrown.


AsyncSocket simply wraps a Socket and provides an async interface. All async sockets are configured with the flag O_NONBLOCK, catching SocketError.blocked and then suspending the current task using an AsyncSocketPool. When data becomes available the task is resumed and AsyncSocket will retry the operation.


protocol AsyncSocketPool {
  func prepare() async throws
  func run() async throws

  // Suspends current task until a socket is ready to read and/or write
  func suspendSocket(_ socket: Socket, untilReadyFor events: Socket.Events) async throws


SocketPool<Queue> is the default pool used within HTTPServer. It suspends and resume sockets using its generic EventQueue depending on the platform. Abstracting kqueue(2) on Darwin platforms and epoll(7) on Linux, the pool uses kernel events without the need to continuosly poll the waiting file descriptors.

Windows uses a queue backed by a continuous loop of poll(2) / Task.yield() to check all sockets awaiting data at a supplied interval.


The sockaddr cluster of structures are grouped via conformance to SocketAddress

  • sockaddr_in
  • sockaddr_in6
  • sockaddr_un

This allows HTTPServer to be started with any of these configured addresses:

// only listens on localhost 8080
let server = HTTPServer(address: .loopback(port: 8080))

It can also be used with UNIX-domain addresses, allowing private IPC over a socket:

// only listens on Unix socket "Ants"
let server = HTTPServer(address: .unix(path: "Ants"))

You can then netcat to the socket:

% nc -U Ants

Command line app

An example command line app FlyingFoxCLI is available here.


FlyingFox is primarily the work of Simon Whitty.

(Full list of contributors)


  • Swift Tools 5.8.0
View More Packages from this Author


  • None
Last updated: Mon Apr 15 2024 16:04:22 GMT-0900 (Hawaii-Aleutian Daylight Time)