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Swift Service Context

ServiceContext is a minimal (zero-dependency) context propagation container, intended to "carry" items for purposes of cross-cutting tools to be built on top of it.

It is modeled after the concepts explained in W3C Baggage and the in the spirit of Tracing Plane 's "Baggage Context" type, although by itself it does not define a specific serialization format.

See https://github.com/apple/swift-distributed-tracing for actual instrument types and implementations which can be used to deploy various cross-cutting instruments all reusing the same baggage type. More information can be found in the SSWG meeting notes.

Overview

ServiceContext serves as currency type for carrying around additional contextual information between Swift tasks and functions.

One generally starts from a "top level" (empty) or the "current" (ServiceContext.current) context and then adds values to it.

The context is a value type and is propagated using task-local values so it can be safely used from concurrent contexts like this:

var context = ServiceContext.topLevel
context[FirstTestKey.self] = 42

func exampleFunction() async -> Int {
    guard let context = ServiceContext.current {
        return 0
    }
    guard let value = context[FirstTestKey.self] {
        return 0
    }
    print("test = \(value)") // test = 42
    return value
}

let c = ServiceContext.withValue(context) {
    await exampleFunction()
}
assert(c == 42)

ServiceContext is a fundamental building block for how distributed tracing propagates trace identifiers.

Dependency

In order to depend on this library you can use the Swift Package Manager, and add the following dependency to your Package.swift:

dependencies: [
  .package(
    url: "https://github.com/apple/swift-service-context.git",
    from: "1.0.0"
  )
]

and depend on the module in your target:

targets: [
    .target(
        name: "MyAwesomeApp",
        dependencies: [
            .product(
              name: "ServiceContextModule",
              package: "swift-service-context"
            ),
        ]
    ),
    // ...
]

Contributing

Please make sure to run the ./scripts/soundness.sh script when contributing, it checks formatting and similar things.

You can ensure it always runs and passes before you push by installing a pre-push hook with git:

echo './scripts/soundness.sh' > .git/hooks/pre-push
chmod +x .git/hooks/pre-push