Deploying Swift on Kubernetes: A Basic Guide for Developers

Introduction: Why Swift and Kubernetes?

Swift is not just a language for iOS and macOS. Thanks to the Swift on Server project and frameworks like Vapor and SwiftNIO, Swift developers can build high-performance server-side applications. However, for such an application to run stably and scale, a modern infrastructure is required.

Kubernetes (K8s) is the de facto standard for container orchestration. In this guide, we will show how to package a Swift application into a Docker image, write Kubernetes manifests, and deploy it in a cluster. You will learn how to use SwiftNIO to create a lightweight HTTP server and configure it for cloud deployment.

1. Preparing the Swift Application for Containerization

1.1. Creating a SwiftNIO Project

For demonstration, we will write a minimal HTTP server using SwiftNIO. This avoids unnecessary dependencies and shows how low-level networking works. Open a terminal and create a new project:

mkdir SwiftK8sDemocd SwiftK8sDemoswift package init --type executable

Add the SwiftNIO dependency to Package.swift:

// swift-tools-version:5.7import PackageDescription

let package = Package(    name: "SwiftK8sDemo",    dependencies: [        .package(url: "https://github.com/apple/swift-nio.git", from: "2.0.0")    ],    targets: [        .executableTarget(            name: "SwiftK8sDemo",            dependencies: [                .product(name: "NIO", package: "swift-nio"),                .product(name: "NIOHTTP1", package: "swift-nio")            ]        )    ])

1.2. Server Code with Health Check

Let's create a simple server that responds to GET requests and has a health check endpoint (/health). This is necessary for Kubernetes liveness probes. Replace the contents of Sources/SwiftK8sDemo/main.swift:

import NIOimport NIOHTTP1

final class HTTPHandler: ChannelInboundHandler {    typealias InboundIn = HTTPServerRequestPart    typealias OutboundOut = HTTPServerResponsePart

    func channelRead(context: ChannelHandlerContext, data: NIOAny) {        let req = unwrapInboundIn(data)        switch req {        case .head(let header):            let response: String            let status: HTTPResponseStatus            if header.uri == "/health" {                response = "{\\"status\\":\\"ok\\"}"                status = .ok            } else {                response = "Hello from Swift on K8s!"                status = .ok            }            var buffer = context.channel.allocator.buffer(capacity: response.utf8.count)            buffer.writeString(response)            let head = HTTPResponseHead(version: .init(major: 1, minor: 1), status: status)            context.write(wrapOutboundOut(.head(head)), promise: nil)            context.write(wrapOutboundOut(.body(.byteBuffer(buffer))), promise: nil)            context.writeAndFlush(wrapOutboundOut(.end(nil)), promise: nil)        default:            break        }    }}

let group = MultiThreadedEventLoopGroup(numberOfThreads: System.coreCount)defer { try! group.syncShutdownGracefully() }

let bootstrap = ServerBootstrap(group: group)    .serverChannelOption(ChannelOptions.backlog, value: 256)    .serverChannelOption(ChannelOptions.socketOption(.so_reuseaddr), value: 1)    .childChannelInitializer { channel in        channel.pipeline.configureHTTPServerPipeline().flatMap {            channel.pipeline.addHandler(HTTPHandler())        }    }

let host = ProcessInfo.processInfo.environment["HOST"] ?? "0.0.0.0"let port = Int(ProcessInfo.processInfo.environment["PORT"] ?? "8080") ?? 8080

print("Server starting on \\(host):\\(port)")try bootstrap.bind(host: host, port: port).wai