Swift Service Lifecycle

Swift Service Lifecycle provides a basic mechanism to cleanly start up and shut down the application, freeing resources in order before exiting. It also provides a Signal-based shutdown hook, to shutdown on signals like TERM or INT.

Swift Service Lifecycle was designed with the idea that every application has some startup and shutdown workflow-like-logic which is often sensitive to failure and hard to get right. The library codes this common need in a safe and reusable way that is non-framework specific, and designed to be integrated with any server framework or directly in an application.

This is the beginning of a community-driven open-source project actively seeking contributions, be it code, documentation, or ideas. What Swift Service Lifecycle provides today is covered in the API docs, but it will continue to evolve with community input.

Getting started

If you have a server-side Swift application or a cross-platform (e.g. Linux, macOS) application, and you would like to manage its startup and shutdown lifecycle, Swift Service Lifecycle is a great idea. Below you will find all you need to know to get started.

Adding the dependency

To add a dependency on the package, declare it in your Package.swift:

.package(url: "https://github.com/swift-server/swift-service-lifecycle.git", from: "1.0.0-alpha"),

and to your application target, add Lifecycle to your dependencies:

.target(name: "MyApplication", dependencies: ["Lifecycle"]),

Defining the lifecycle

// import the package
import Lifecycle

// initialize the lifecycle container
let lifecycle = ServiceLifecycle()

// register a resource that should be shut down when the application exits.
//
// in this case, we are registering a SwiftNIO `EventLoopGroup`
// and passing its `syncShutdownGracefully` function to be called on shutdown
let eventLoopGroup = MultiThreadedEventLoopGroup(numberOfThreads: System.coreCount)
lifecycle.registerShutdown(
    label: "eventLoopGroup",
    .sync(eventLoopGroup.syncShutdownGracefully)
)

// register another resource that should be started when the application starts
// and shut down when the application exits.
//
// in this case, we are registering a contrived `DatabaseMigrator`
// and passing its `migrate` function to be called on startup
// and `shutdown` function to be called on shutdown
let migrator = DatabaseMigrator()
lifecycle.register(
    label: "migrator",
    start: .async(migrator.migrate),
    shutdown: .async(migrator.shutdown)
)

// start the application
//
// start handlers passed using the `register` function
// will be called in the order they were registered in
lifecycle.start { error in
    // start completion handler.
    // if a startup error occurred you can capture it here
    if let error = error {
        logger.error("failed starting \(self) ☠️: \(error)")
    } else {
        logger.info("\(self) started successfully 🚀")
    }
}
// wait for the application to exit
//
// this is a blocking operation that typically waits for a signal
// the signal can be configured at `lifecycle.start`, and defaults to `INT` and `TERM`
// shutdown handlers passed using the `register` or `registerShutdown` functions
// will be called in the reverse order they were registered in
lifecycle.wait()

Detailed design

The main types in the library are ServiceLifecycle and ComponentLifecycle.

ServiceLifecycle is the most commonly used type. It is designed to manage the top level Application (Service) lifecycle, and in addition to managing the startup and shutdown flows it can also set up Signal trap for shutdown and install backtraces.

ComponentLifecycle manages a state machine representing the startup and shutdown logic flow. In larger Applications (Services) ComponentLifecycle can be used to manage the lifecycle of subsystems, such that ServiceLifecycle can start and shutdown ComponentLifecycles.

Registering items

ServiceLifecycle and ComponentLifecycle are containers for LifecycleTasks which need to be registered using a LifecycleHandler - a container for synchronous or asynchronous closures.

Synchronous handlers are defined as () throws -> Void.

Asynchronous handlers defined are as (@escaping (Error?) -> Void) -> Void.

LifecycleHandler comes with static helpers named async and sync designed to help simplify the registration call to:

let foo = ...
lifecycle.register(
    label: "foo",
    start: .sync(foo.syncStart),
    shutdown: .sync(foo.syncShutdown)
)

Or the async version:

let foo = ...
lifecycle.register(
    label: "foo",
    start: .async(foo.asyncStart),
    shutdown: .async(foo.asyncShutdown)
)

or, just shutdown:

let foo = ...
lifecycle.registerShutdown(
    label: "foo",
    .sync(foo.syncShutdown)
)

Or the async version:

let foo = ...
lifecycle.registerShutdown(
    label: "foo",
    .async(foo.asyncShutdown)
)

you can also register a collection of LifecycleTasks (less typical) using:

func register(_ tasks: [LifecycleTask])

func register(_ tasks: LifecycleTask...)

Configuration

ServiceLifecycle initializer takes optional ServiceLifecycle.Configuration to further refine the ServiceLifecycle behavior:

• callbackQueue: Defines the DispatchQueue on which startup and shutdown handlers are executed. By default, DispatchQueue.global is used.

• shutdownSignal: Defines what, if any, signals to trap for invoking shutdown. By default, INT and TERM are trapped.

• installBacktrace: Defines if to install a crash signal trap that prints backtraces. This is especially useful for application running on Linux since Swift does not provide backtraces on Linux out of the box. This functionality is provided via the Swift Backtrace library.

Starting the lifecycle

Use start function to start the application. Start handlers passed using the register function will be called in the order the items were registered in.

start is an asynchronous operation. If a startup error occurred, it will be logged and the startup sequence will halt on the first error, and bubble it up to the provided completion handler.

lifecycle.start { error in
    if let error = error {
        logger.error("failed starting \(self) ☠️: \(error)")
    } else {
        logger.info("\(self) started successfully 🚀")
    }
}

Shutdown

Typical use of the library is to call on wait after calling start.

lifecycle.start { error in
  ...   
}
lifecycle.wait() // <-- blocks the thread

If you are not interested in handling start completion, there is also a convenience method:

lifecycle.startAndWait() // <-- blocks the thread

Both wait and startAndWait are blocking operations that wait for the lifecycle library to finish the shutdown sequence. The shutdown sequence is typically triggered by the shutdownSignal defined in the configuration. By default, INT and TERM are trapped.

During shutdown, the shutdown handlers passed using the register or registerShutdown functions are called in the reverse order of the registration. E.g.

lifecycle.register("1", ...)
lifecycle.register("2", ...)
lifecycle.register("3", ...)

startup order will be 1, 2, 3 and shutdown order will be 3, 2, 1.

If a shutdown error occurred, it will be logged and the shutdown sequence will continue to the next item, and attempt to shut it down until all registered items that have been started are shut down.

In more complex cases, when Signal-trapping-based shutdown is not appropriate, you may pass nil as the shutdownSignal configuration, and call shutdown manually when appropriate. This is designed to be a rarely used pressure valve.

shutdown is an asynchronous operation. Errors will be logged and bubbled up to the provided completion handler.

Complex Systems and Nesting of Subsystems

In larger Applications (Services) ComponentLifecycle can be used to manage the lifecycle of subsystems, such that ServiceLifecycle can start and shutdown ComponentLifecycles.

In fact, since ComponentLifecycle conforms to LifecycleTask, it can start and stop other ComponentLifecycles, forming a tree. E.g.:

struct SubSystem {
    let lifecycle = ComponentLifecycle(label: "SubSystem")
    let subsystem: SubSubSystem

    init() {
        self.subsystem = SubSubSystem()
        self.lifecycle.register(self.subsystem.lifecycle)
    }

    struct SubSubSystem {
        let lifecycle = ComponentLifecycle(label: "SubSubSystem")

        init() {
            self.lifecycle.register(...)
        }
    }
}

let lifecycle = ServiceLifecycle()
let subsystem = SubSystem()
lifecycle.register(subsystem.lifecycle)

lifecycle.start { error in
    ...
}
lifecycle.wait()

Compatibility with SwiftNIO Futures

SwiftNIO is a popular networking library that among other things provides Future abstraction named EventLoopFuture.

Swift Service Lifecycle comes with a compatibility module designed to make managing SwiftNIO based resources easy.

Once you import LifecycleNIOCompat module, LifecycleHandler gains a static helper named eventLoopFuture designed to help simplify the registration call to:

let foo = ...
lifecycle.register(
    label: "foo",
    start: .eventLoopFuture(foo.start),
    shutdown: .eventLoopFuture(foo.shutdown)
)

or, just shutdown:

let foo = ...
lifecycle.registerShutdown(
    label: "foo",
    .eventLoopFuture(foo.shutdown)
)

Do not hesitate to get in touch as well, over on https://forums.swift.org/c/server.