NIOAsyncRuntime

NIOAsyncRuntime provides a lightweight implementation of MultiThreadedEventLoopGroup and NIOThreadPool that can be used as a drop-in replacement for the original implementations in NIOPosix.

NIOAsyncRuntime is powered by Swift Concurrency and avoids low-level operating system C API calls. This enables compiling to WebAssembly using the Swift SDK for WebAssembly

Highlights

• Drop-in MultiThreadedEventLoopGroup and NIOThreadPool implementations that enable avoiding NIOPosix dependencies.
• Uses Swift Concurrency tasks under the hood.
• Matches the existing NIOPosix APIs, making adoption straightforward.

Known Limitations

• NIOPosix currently provides significantly faster performance in benchmarks for heavy-load event enqueuing. See the benchmarks below for details.

Getting Started

Requirements

• Swift 6.0 or later toolchain
• Any platform supporting Swift Concurrency
• Minimum supported platforms: macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, WASI 0.1

Swift Package Manager

NIOAsyncRuntime + NIOPosix side-by-side

This is the preferred way to use NIOAsyncRuntime. Use NIOAsyncRuntime only for targets where NIOPosix is unsupported.

Add the package to your Package.swift:

dependencies: [
    .package(
        url: "https://github.com/apple/swift-nio.git",
        from: "2.89.0"
    ),
    .package(
        url: "https://github.com/passivelogic/nio-async-runtime.git",
        from: "0.0.1"
    ),
],
targets: [
    .target(
        name: "YourTarget",
        dependencies: [
            // WASI targets use NIOAsyncRuntime
            .product(
                name: "NIOAsyncRuntime",
                package: "nio-async-runtime",
                condition: .when(platforms: [.wasi])
            ),

            // All other targets use NIOPosix
            .product(
                name: "NIOPosix",
                package: "swift-nio",
                condition: .when(platforms: [.macOS, .linux, .iOS])
            ),
        ]
    ),
]

NIOAsyncRuntime standalone, no NIOPosix

Add the package to your Package.swift:

dependencies: [
    .package(
        url: "https://github.com/passivelogic/nio-async-runtime.git",
        from: "0.0.1"
    ),
],
targets: [
    .target(
        name: "YourTarget",
        dependencies: [
            .product(name: "NIOAsyncRuntime", package: "nio-async-runtime")
        ]
    ),
]

Importing

You can opt in to the async runtime or fall back to NIOPosix with a simple conditional import.

#if canImport(NIOAsyncRuntime)
import class NIOAsyncRuntime.MultiThreadedEventLoopGroup
#elseif canImport(NIOPosix)
import class NIOPosix.MultiThreadedEventLoopGroup
#endif

let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)

Usage Examples

Event loops with MultiThreadedEventLoopGroup

See ExampleMTELG for full example.

import protocol NIOCore.EventLoopGroup
import class NIOAsyncRuntime.MultiThreadedEventLoopGroup

let group = MultiThreadedEventLoopGroup()

let loop = group.next()
let future = loop.submit {
    "Hello World!"
}

future.whenSuccess { value in
    print(value)
}

// Shutdown when done
do {
    try await group.shutdownGracefully()
    print("Shutdown status: OK")
} catch {
    print("Shutdown status:", error)
}

Thread pool work with NIOThreadPool

See ExampleNIOThreadPool for full example.

import protocol NIOCore.EventLoopGroup
import class NIOAsyncRuntime.AsyncEventLoop
import class NIOAsyncRuntime.NIOThreadPool

let pool = NIOThreadPool()
pool.start()

let loop = AsyncEventLoop()
let future = pool.runIfActive(eventLoop: loop) {
    return "Welcome to the Future!"
}

let result = try await future.get()
print("Result:", result)

// Clean up
do {
    try await loop.shutdownGracefully()
    try await pool.shutdownGracefully()
    print("Shutdown status: OK")
} catch {
    print("Shutdown status:", error)
}

Benchmarks

NIOAsyncRuntime is currently significantly less performant than NIOPosix. Below are benchmark results run against both frameworks.

These benchmarks were last run and updated on December 4, 2025.

Benchmark	NIOPosix	NIOAsyncRuntime
Jump to EL and back using actor with EL executor	1.44x faster	1.00x
Jump to EL and back using execute and unsafecontinuation	1.31x faster	1.00x
MTELG.scheduleCallback(in:)	11.71x faster	1.00x
MTELG.scheduleTask(in:)	4.06x faster	1.00x
MTELG.immediateTasksThroughput	4.92x faster	1.00x