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apple/swift-nio

Cross-platform, non-blocking event-driven networking framework in Swift for building high-performance protocol servers and clients, similar to Netty.

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What is it?

SwiftNIO is a low-level networking framework for Swift that uses non-blocking I/O and event loops to build protocol servers and clients. Instead of dedicating a thread to each connection, it multiplexes many connections across a small number of event loops, which suits servers that hold large numbers of relatively idle connections. It is described as a Swift counterpart to Netty.

The framework provides the core building blocks: EventLoopGroup and EventLoop for I/O dispatch, Channel and ChannelPipeline with ChannelHandlers for processing data, ByteBuffer for byte handling, and futures and promises for asynchronous results. Modules cover POSIX I/O (NIOPosix), testing helpers (NIOEmbedded, NIOTestUtils), and low-level HTTP/1.1 and WebSocket protocol implementations. TLS, HTTP/2, SSH and other protocols live in companion repositories.

SwiftNIO is aimed at framework authors and teams that need fine control over networking performance, rather than end-user web development. Higher-level libraries such as AsyncHTTPClient, gRPC Swift and PostgresNIO are built on top of it.

Key features

  • 01Non-blocking I/O built on event loops, using kqueue or epoll depending on the platform.
  • 02Channel and ChannelPipeline model for composing reusable protocol handlers.
  • 03Low-level HTTP/1.1 and WebSocket protocol implementations.
  • 04ByteBuffer for efficient copy-on-write byte handling.
  • 05EmbeddedChannel and EmbeddedEventLoop for testing protocol code without real networking.
  • 06Supports macOS and Linux, with experimental OpenBSD support.

Use cases

  • Building a custom protocol server or client in Swift that must handle many concurrent connections.
  • Developing a higher-level Swift networking library or web framework on top of a non-blocking core.
  • Writing performance-sensitive services where thread-per-connection models are too costly.
  • Unit testing protocol handlers in isolation using the embedded channel.

Quick start

  1. 1

    Add SwiftNIO to your Package.swift dependencies using a version range such as from: "2.0.0"

  2. 2

    Add the NIO product to your target and import NIO in your Swift code

Things to know

  • It is a low-level networking toolkit, not a high-level web framework, so it requires good understanding of its event-loop and channel model.
  • Recent releases require a newer Swift toolchain, for example Swift 6.1 for version 2.98.0 and later.
  • SwiftNIO 1 is end of life and receives no new features.

Topics

#swift#networking#non-blocking-io#event-loop#async-io#protocol-server#http#websocket#asynchronous-io#event-driven

FAQ

What is swift-nio?
Cross-platform, non-blocking event-driven networking framework in Swift for building high-performance protocol servers and clients, similar to Netty. SwiftNIO is a low-level networking framework for Swift that uses non-blocking I/O and event loops to build protocol servers and clients. Instead of dedicating a thread to each connection, it multiplexes many connections across a small number of event loops, which suits servers that hold large numbers of relatively idle connections. It is described as a Swift counterpart to Netty.
Is swift-nio free to use? What license does it have?
swift-nio is open source under the Apache-2.0 license. Check the license terms before using it in commercial products.
Is swift-nio still maintained?
Actively maintained. The last commit to swift-nio was on Oct 19, 2026.
What can I use swift-nio for?
Building a custom protocol server or client in Swift that must handle many concurrent connections. Developing a higher-level Swift networking library or web framework on top of a non-blocking core. Writing performance-sensitive services where thread-per-connection models are too costly. Unit testing protocol handlers in isolation using the embedded channel.
How do I install swift-nio?
Add SwiftNIO to your Package.swift dependencies using a version range such as from: "2.0.0" Add the NIO product to your target and import NIO in your Swift code
Where can I get swift-nio?
The source code and installation instructions are on GitHub: https://github.com/apple/swift-nio

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