CNIOAtomics 2.40.0

CNIOAtomics 2.40.0

Maintained by Daniel Alm, Peter Adams, George Barnett, Cory Benfield, Johannes Weiss, Jake Prickett, David Evans.

  • By
  • Apple Inc.



SwiftNIO is a cross-platform asynchronous event-driven network application framework for rapid development of maintainable high performance protocol servers & clients.

It's like Netty, but written for Swift.

Repository organization

The SwiftNIO project is split across multiple repositories:

Repository NIO 2 (Swift 5.2+)
SwiftNIO core
from: "2.0.0"
TLS (SSL) support
from: "2.0.0"
HTTP/2 support
from: "1.0.0"
useful additions around SwiftNIO
from: "1.0.0"
first-class support for macOS, iOS, tvOS, and watchOS
from: "1.0.0"
SSH support
.upToNextMinor(from: "0.2.0")

NIO 2.29.0 and older support Swift 5.0+.

Within this repository we have a number of products that provide different functionality. This package contains the following products:

  • NIO. This is an umbrella module exporting NIOCore, NIOEmbedded and NIOPosix.
  • NIOCore. This provides the core abstractions and types for using SwiftNIO (see "Conceptual Overview" for more details). Most NIO extension projects that provide things like new EventLoops and Channels or new protocol implementations should only need to depend on NIOCore.
  • NIOPosix. This provides the primary [EventLoopGroup], EventLoop, and Channels for use on POSIX-based systems. This is our high performance core I/O layer. In general, this should only be imported by projects that plan to do some actual I/O, such as high-level protocol implementations or applications.
  • NIOEmbedded. This provides EmbeddedChannel and EmbeddedEventLoop, implementations of the NIOCore abstractions that provide fine-grained control over their execution. These are most often used for testing, but can also be used to drive protocol implementations in a way that is decoupled from networking altogether.
  • NIOConcurrencyHelpers. This provides a few low-level concurrency primitives that are used by NIO implementations, such as locks and atomics.
  • NIOFoundationCompat. This extends a number of NIO types for better interoperation with Foundation data types. If you are working with Foundation data types such as Data, you should import this.
  • NIOTLS. This provides a few common abstraction types for working with multiple TLS implementations. Note that this doesn't provide TLS itself: please investigate swift-nio-ssl and swift-nio-transport-services for concrete implementations.
  • NIOHTTP1. This provides a low-level HTTP/1.1 protocol implementation.
  • NIOWebSocket. This provides a low-level WebSocket protocol implementation.
  • NIOTestUtils. This provides a number of helpers for testing projects that use SwiftNIO.

Protocol Implementations

Below you can find a list of a few protocol implementations that are done with SwiftNIO. This is a non-exhaustive list of protocols that are either part of the SwiftNIO project or are accepted into the SSWG's incubation process. All of the libraries listed below do all of their I/O in a non-blocking fashion using SwiftNIO.

Low-level protocol implementations

Low-level protocol implementations are often a collection of ChannelHandlers that implement a protocol but still require the user to have a good understanding of SwiftNIO. Often, low-level protocol implementations will then be wrapped in high-level libraries with a nicer, more user-friendly API.

Protocol Client Server Repository Module Comment
HTTP/1 apple/swift-nio NIOHTTP1 official NIO project
HTTP/2 apple/swift-nio-http2 NIOHTTP2 official NIO project
WebSocket apple/swift-nio NIOWebSocket official NIO project
TLS apple/swift-nio-ssl NIOSSL official NIO project
SSH apple/swift-nio-ssh n/a official NIO project

High-level implementations

High-level implementations are usually libraries that come with an API that doesn't expose SwiftNIO's ChannelPipeline and can therefore be used with very little (or no) SwiftNIO-specific knowledge. The implementations listed below do still do all of their I/O in SwiftNIO and integrate really well with the SwiftNIO ecosystem.

Protocol Client Server Repository Module Comment
HTTP swift-server/async-http-client AsyncHTTPClient SSWG community project
gRPC grpc/grpc-swift GRPC also offers a low-level API; SSWG community project
APNS kylebrowning/APNSwift APNSwift SSWG community project
PostgreSQL vapor/postgres-nio PostgresNIO SSWG community project
Redis mordil/swift-redi-stack RediStack SSWG community project

Supported Versions

SwiftNIO 2

This is the current version of SwiftNIO and will be supported for the foreseeable future.

The latest released SwiftNIO 2 version supports Swift 5.2+. NIO 2.29.0 and older support Swift 5.0+.

SwiftNIO 1

SwiftNIO 1 is considered end of life - it is strongly recommended that you move to a newer version. The Core NIO team does not actively work on this version. No new features will be added to this version but PRs which fix bugs or security vulnerabilities will be accepted until the end of May 2022.

If you have a SwiftNIO 1 application or library that you would like to migrate to SwiftNIO 2, please check out the migration guide we prepared for you.

The latest released SwiftNIO 1 version supports Swift 4.0, 4.1, 4.2, and 5.0.

Supported Platforms

SwiftNIO aims to support all of the platforms where Swift is supported. Currently, it is developed and tested on macOS and Linux, and is known to support the following operating system versions:


SwiftNIO follows SemVer 2.0.0 with a separate document declaring SwiftNIO's Public API.

What this means for you is that you should depend on SwiftNIO with a version range that covers everything from the minimum SwiftNIO version you require up to the next major version. In SwiftPM that can be easily done specifying for example from: "2.0.0" meaning that you support SwiftNIO in every version starting from 2.0.0 up to (excluding) 3.0.0. SemVer and SwiftNIO's Public API guarantees should result in a working program without having to worry about testing every single version for compatibility.

Conceptual Overview

SwiftNIO is fundamentally a low-level tool for building high-performance networking applications in Swift. It particularly targets those use-cases where using a "thread-per-connection" model of concurrency is inefficient or untenable. This is a common limitation when building servers that use a large number of relatively low-utilization connections, such as HTTP servers.

To achieve its goals SwiftNIO extensively uses "non-blocking I/O": hence the name! Non-blocking I/O differs from the more common blocking I/O model because the application does not wait for data to be sent to or received from the network: instead, SwiftNIO asks for the kernel to notify it when I/O operations can be performed without waiting.

SwiftNIO does not aim to provide high-level solutions like, for example, web frameworks do. Instead, SwiftNIO is focused on providing the low-level building blocks for these higher-level applications. When it comes to building a web application, most users will not want to use SwiftNIO directly: instead, they'll want to use one of the many great web frameworks available in the Swift ecosystem. Those web frameworks, however, may choose to use SwiftNIO under the covers to provide their networking support.

The following sections will describe the low-level tools that SwiftNIO provides, and provide a quick overview of how to work with them. If you feel comfortable with these concepts, then you can skip right ahead to the other sections of this README.

Basic Architecture

The basic building blocks of SwiftNIO are the following 8 types of objects:

All SwiftNIO applications are ultimately constructed of these various components.

EventLoops and EventLoopGroups

The basic I/O primitive of SwiftNIO is the event loop. The event loop is an object that waits for events (usually I/O related events, such as "data received") to happen and then fires some kind of callback when they do. In almost all SwiftNIO applications there will be relatively few event loops: usually only one or two per CPU core the application wants to use. Generally speaking event loops run for the entire lifetime of your application, spinning in an endless loop dispatching events.

Event loops are gathered together into event loop groups. These groups provide a mechanism to distribute work around the event loops. For example, when listening for inbound connections the listening socket will be registered on one event loop. However, we don't want all connections that are accepted on that listening socket to be registered with the same event loop, as that would potentially overload one event loop while leaving the others empty. For that reason, the event loop group provides the ability to spread load across multiple event loops.

In SwiftNIO today there is one EventLoopGroup implementation, and two EventLoop implementations. For production applications there is the MultiThreadedEventLoopGroup, an EventLoopGroup that creates a number of threads (using the POSIX pthreads library) and places one SelectableEventLoop on each one. The SelectableEventLoop is an event loop that uses a selector (either kqueue or epoll depending on the target system) to manage I/O events from file descriptors and to dispatch work. These EventLoops and EventLoopGroups are provided by the NIOPosix module. Additionally, there is the EmbeddedEventLoop, which is a dummy event loop that is used primarily for testing purposes, provided by the NIOEmbedded module.

EventLoops have a number of important properties. Most vitally, they are the way all work gets done in SwiftNIO applications. In order to ensure thread-safety, any work that wants to be done on almost any of the other objects in SwiftNIO must be dispatched via an EventLoop. EventLoop objects own almost all the other objects in a SwiftNIO application, and understanding their execution model is critical for building high-performance SwiftNIO applications.

Channels, Channel Handlers, Channel Pipelines, and Channel Contexts

While EventLoops are critical to the way SwiftNIO works, most users will not interact with them substantially beyond asking them to create EventLoopPromises and to schedule work. The parts of a SwiftNIO application most users will spend the most time interacting with are Channels and ChannelHandlers.

Almost every file descriptor that a user interacts with in a SwiftNIO program is associated with a single Channel. The Channel owns this file descriptor, and is responsible for managing its lifetime. It is also responsible for processing inbound and outbound events on that file descriptor: whenever the event loop has an event that corresponds to a file descriptor, it will notify the Channel that owns that file descriptor.

Channels by themselves, however, are not useful. After all, it is a rare application that doesn't want to do anything with the data it sends or receives on a socket! So the other important part of the Channel is the ChannelPipeline.

A ChannelPipeline is a sequence of objects, called ChannelHandlers, that process events on a Channel. The ChannelHandlers process these events one after another, in order, mutating and transforming events as they go. This can be thought of as a data processing pipeline; hence the name ChannelPipeline.

All ChannelHandlers are either Inbound or Outbound handlers, or both. Inbound handlers process "inbound" events: events like reading data from a socket, reading socket close, or other kinds of events initiated by remote peers. Outbound handlers process "outbound" events, such as writes, connection attempts, and local socket closes.

Each handler processes the events in order. For example, read events are passed from the front of the pipeline to the back, one handler at a time, while write events are passed from the back of the pipeline to the front. Each handler may, at any time, generate either inbound or outbound events that will be sent to the next handler in whichever direction is appropriate. This allows handlers to split up reads, coalesce writes, delay connection attempts, and generally perform arbitrary transformations of events.

In general, ChannelHandlers are designed to be highly re-usable components. This means they tend to be designed to be as small as possible, performing one specific data transformation. This allows handlers to be composed together in novel and flexible ways, which helps with code reuse and encapsulation.

ChannelHandlers are able to keep track of where they are in a ChannelPipeline by using a ChannelHandlerContext. These objects contain references to the previous and next channel handler in the pipeline, ensuring that it is always possible for a ChannelHandler to emit events while it remains in a pipeline.

SwiftNIO ships with many ChannelHandlers built in that provide useful functionality, such as HTTP parsing. In addition, high-performance applications will want to provide as much of their logic as possible in ChannelHandlers, as it helps avoid problems with context switching.

Additionally, SwiftNIO ships with a few Channel implementations. In particular, it ships with ServerSocketChannel, a Channel for sockets that accept inbound connections; SocketChannel, a Channel for TCP connections; and DatagramChannel, a Channel for UDP sockets. All of these are provided by the NIOPosix module. It also providesEmbeddedChannel, a Channel primarily used for testing, provided by the NIOEmbedded module.

A Note on Blocking

One of the important notes about ChannelPipelines is that they are thread-safe. This is very important for writing SwiftNIO applications, as it allows you to write much simpler ChannelHandlers in the knowledge that they will not require synchronization.

However, this is achieved by dispatching all code on the ChannelPipeline on the same thread as the EventLoop. This means that, as a general rule, ChannelHandlers must not call blocking code without dispatching it to a background thread. If a ChannelHandler blocks for any reason, all Channels attached to the parent EventLoop will be unable to progress until the blocking call completes.

This is a common concern while writing SwiftNIO applications. If it is useful to write code in a blocking style, it is highly recommended that you dispatch work to a different thread when you're done with it in your pipeline.


While it is possible to configure and register Channels with EventLoops directly, it is generally more useful to have a higher-level abstraction to handle this work.

For this reason, SwiftNIO ships a number of Bootstrap objects whose purpose is to streamline the creation of channels. Some Bootstrap objects also provide other functionality, such as support for Happy Eyeballs for making TCP connection attempts.

Currently SwiftNIO ships with three Bootstrap objects in the NIOPosix module: ServerBootstrap, for bootstrapping listening channels; ClientBootstrap, for bootstrapping client TCP channels; and DatagramBootstrap for bootstrapping UDP channels.


The majority of the work in a SwiftNIO application involves shuffling buffers of bytes around. At the very least, data is sent and received to and from the network in the form of buffers of bytes. For this reason it's very important to have a high-performance data structure that is optimized for the kind of work SwiftNIO applications perform.

For this reason, SwiftNIO provides ByteBuffer, a fast copy-on-write byte buffer that forms a key building block of most SwiftNIO applications. This type is provided by the NIOCore module.

ByteBuffer provides a number of useful features, and in addition provides a number of hooks to use it in an "unsafe" mode. This turns off bounds checking for improved performance, at the cost of potentially opening your application up to memory correctness problems.

In general, it is highly recommended that you use the ByteBuffer in its safe mode at all times.

For more details on the API of ByteBuffer, please see our API documentation, linked below.

Promises and Futures

One major difference between writing concurrent code and writing synchronous code is that not all actions will complete immediately. For example, when you write data on a channel, it is possible that the event loop will not be able to immediately flush that write out to the network. For this reason, SwiftNIO provides EventLoopPromise<T> and EventLoopFuture<T> to manage operations that complete asynchronously. These types are provided by the NIOCore module.

An EventLoopFuture<T> is essentially a container for the return value of a function that will be populated at some time in the future. Each EventLoopFuture<T> has a corresponding EventLoopPromise<T>, which is the object that the result will be put into. When the promise is succeeded, the future will be fulfilled.

If you had to poll the future to detect when it completed that would be quite inefficient, so EventLoopFuture<T> is designed to have managed callbacks. Essentially, you can hang callbacks off the future that will be executed when a result is available. The EventLoopFuture<T> will even carefully arrange the scheduling to ensure that these callbacks always execute on the event loop that initially created the promise, which helps ensure that you don't need too much synchronization around EventLoopFuture<T> callbacks.

Another important topic for consideration is the difference between how the promise passed to close works as opposed to closeFuture on a Channel. For example, the promise passed into close will succeed after the Channel is closed down but before the ChannelPipeline is completely cleared out. This will allow you to take action on the ChannelPipeline before it is completely cleared out, if needed. If it is desired to wait for the Channel to close down and the ChannelPipeline to be cleared out without any further action, then the better option would be to wait for the closeFuture to succeed.

There are several functions for applying callbacks to EventLoopFuture<T>, depending on how and when you want them to execute. Details of these functions is left to the API documentation.

Design Philosophy

SwiftNIO is designed to be a powerful tool for building networked applications and frameworks, but it is not intended to be the perfect solution for all levels of abstraction. SwiftNIO is tightly focused on providing the basic I/O primitives and protocol implementations at low levels of abstraction, leaving more expressive but slower abstractions to the wider community to build. The intention is that SwiftNIO will be a building block for server-side applications, not necessarily the framework those applications will use directly.

Applications that need extremely high performance from their networking stack may choose to use SwiftNIO directly in order to reduce the overhead of their abstractions. These applications should be able to maintain extremely high performance with relatively little maintenance cost. SwiftNIO also focuses on providing useful abstractions for this use-case, such that extremely high performance network servers can be built directly.

The core SwiftNIO repository will contain a few extremely important protocol implementations, such as HTTP, directly in tree. However, we believe that most protocol implementations should be decoupled from the release cycle of the underlying networking stack, as the release cadence is likely to be very different (either much faster or much slower). For this reason, we actively encourage the community to develop and maintain their protocol implementations out-of-tree. Indeed, some first-party SwiftNIO protocol implementations, including our TLS and HTTP/2 bindings, are developed out-of-tree!


Example Usage

There are currently several example projects that demonstrate how to use SwiftNIO.

To build & run them, run following command, replace TARGET_NAME with the folder name under ./Sources

swift run TARGET_NAME

For example, to run NIOHTTP1Server, run following command:

swift run NIOHTTP1Server

Getting Started

SwiftNIO primarily uses SwiftPM as its build tool, so we recommend using that as well. If you want to depend on SwiftNIO in your own project, it's as simple as adding a dependencies clause to your Package.swift:

dependencies: [
    .package(url: "", from: "2.0.0")

and then adding the appropriate SwiftNIO module(s) to your target dependencies. The syntax for adding target dependencies differs slightly between Swift versions. For example, if you want to depend on the NIOCore, NIOPosix and NIOHTTP1 modules, specify the following dependencies:

Swift 5.2 and newer (swift-tools-version:5.2)

dependencies: [.product(name: "NIOCore", package: "swift-nio"),
               .product(name: "NIOPosix", package: "swift-nio"),
               .product(name: "NIOHTTP1", package: "swift-nio")]

Using Xcode Package support

If your project is set up as an Xcode project and you're using Xcode 11+, you can add SwiftNIO as a dependency to your Xcode project by clicking File -> Swift Packages -> Add Package Dependency. In the upcoming dialog, please enter and click Next twice. Finally, select the targets you are planning to use (for example NIOCore, NIOHTTP1, and NIOFoundationCompat) and click finish. Now will be able to import NIOCore (as well as all the other targets you have selected) in your project.

To work on SwiftNIO itself, or to investigate some of the demonstration applications, you can clone the repository directly and use SwiftPM to help build it. For example, you can run the following commands to compile and run the example echo server:

swift build
swift test
swift run NIOEchoServer

To verify that it is working, you can use another shell to attempt to connect to it:

echo "Hello SwiftNIO" | nc localhost 9999

If all goes well, you'll see the message echoed back to you.

To work on SwiftNIO in Xcode 11+, you can just open the Package.swift file in Xcode and use Xcode's support for SwiftPM Packages.

If you want to develop SwiftNIO with Xcode 10, you have to generate an Xcode project:

swift package generate-xcodeproj

An alternative: using docker-compose

Alternatively, you may want to develop or test with docker-compose.

First make sure you have Docker installed, next run the following commands:

  • docker-compose -f docker/docker-compose.yaml run test

    Will create a base image with Swift runtime and other build and test dependencies, compile SwiftNIO and run the unit and integration tests

  • docker-compose -f docker/docker-compose.yaml up echo

    Will create a base image, compile SwiftNIO, and run a sample NIOEchoServer on localhost:9999. Test it by echo Hello SwiftNIO | nc localhost 9999.

  • docker-compose -f docker/docker-compose.yaml up http

    Will create a base image, compile SwiftNIO, and run a sample NIOHTTP1Server on localhost:8888. Test it by curl http://localhost:8888

  • docker-compose -f docker/docker-compose.yaml -f docker/docker-compose.2004.54.yaml run test

    Will create a base image using Ubuntu 20.04 and swift 5.4, compile SwiftNIO and run the unit and integration tests. Files exist for other ubuntu and swift versions in the docker directory.

Developing SwiftNIO

Note: This section is only relevant if you would like to develop SwiftNIO yourself. You can ignore the information here if you just want to use SwiftNIO as a SwiftPM package.

For the most part, SwiftNIO development is as straightforward as any other SwiftPM project. With that said, we do have a few processes that are worth understanding before you contribute. For details, please see in this repository.


SwiftNIO's main branch is the development branch for the next releases of SwiftNIO 2, it's Swift 5-only.

To be able to compile and run SwiftNIO and the integration tests, you need to have a few prerequisites installed on your system.


  • Xcode 11.4 or newer, Xcode 12 recommended.


  • Swift 5.2, 5.3, or 5.4 from We always recommend to use the latest released version.
  • netcat (for integration tests only)
  • lsof (for integration tests only)
  • shasum (for integration tests only)

Ubuntu 18.04

# install swift tarball from
apt-get install -y git curl libatomic1 libxml2 netcat-openbsd lsof perl

Fedora 28+

dnf install swift-lang /usr/bin/nc /usr/bin/lsof /usr/bin/shasum

Speeding up testing

It's possible to run the test suite in parallel, it can save significant time if you have a larger multi-core machine, just add --parallel when running the tests. This can speed up the run time of the test suite by 30x or more.

swift test --parallel