Swiftpack.co - Package - slashmo/gsoc-swift-baggage-context

Baggage Context

Swift 5.2 CI

BaggageContext is a minimal (zero-dependency) "context" library meant to "carry" baggage (metadata) for cross-cutting tools such as tracers. It is purposefully not tied to any specific use-case (in the spirit of the Tracing Plane paper's BaggageContext), however it should enable a vast majority of use cases cross-cutting tools need to support. Unlike mentioned in the paper, our BaggageContext does not implement its own serialization scheme (today).

See https://github.com/slashmo/gsoc-swift-tracing for actual instrument types and implementations which can be used to deploy various cross-cutting instruments all reusing the same baggage type.


You can install the BaggageContext library through the Swift Package Manager. The library itself is called Baggage, so that's what you'd import in your Swift files.

dependencies: [
    name: "swift-baggage-context",
    url: "https://github.com/slashmo/gsoc-swift-baggage-context.git",
    from: "0.2.0"

Context-Passing Guidelines

In order for context-passing to feel consistent and Swifty among all server-side (and not only) libraries and frameworks aiming to adopt BaggageContext (or any of its uses, such as Distributed Tracing), we suggest the following set of guidelines:

Argument naming/positioning

In order to propagate baggage through function calls (and asynchronous-boundaries it may often be necessary to pass it explicitly (unless wrapper APIs are provided which handle the propagation automatically).

When passing baggage context explicitly we strongly suggest sticking to the following style guideline:

  • Assuming the general parameter ordering of Swift function is as follows (except DSL exceptions):
    1. Required non-function parameters (e.g. (url: String)),
    2. Defaulted non-function parameters (e.g. (mode: Mode = .default)),
    3. Required function parameters, including required trailing closures (e.g. (onNext elementHandler: (Value) -> ())),
    4. Defaulted function parameters, including optional trailing closures (e.g. (onComplete completionHandler: (Reason) -> ()) = { _ in }).
  • Baggage Context should be passed as: the last parameter in the required non-function parameters group in a function declaration.

This way when reading the call side, users of these APIs can learn to "ignore" or "skim over" the context parameter and the method signature remains human-readable and “Swifty”.


  • func request(_ url: URL, context: BaggageContext ), which may be called as httpClient.request(url, context: context)
  • func handle(_ request: RequestObject, context: FrameworkContext )
    • if a "framework context" exists and carries the baggage context already, it is permitted to pass that context together with the baggage;
    • it is strongly recommended to store the baggage context as baggage property of FrameworkContext in such cases, in order to avoid the confusing spelling of context.context, and favoring the self-explanatory context.baggage spelling when the baggage is contained in a framework context object.
  • func receiveMessage(_ message: Message, context: FrameworkContext)
  • func handle(element: Element, context: FrameworkContext , settings: Settings? = nil)
    • before any defaulted non-function parameters
  • func handle(element: Element, context: FrameworkContext , settings: Settings? = nil, onComplete: () -> ())
    • before defaulted parameters, which themselfes are before required function parameters
  • func handle(element: Element, context: FrameworkContext , onError: (Error) -> (), onComplete: (() -> ())? = nil)

In case there are multiple "framework-ish" parameters, such as passing a NIO EventLoop or similar, we suggest:

  • func perform(_ work: Work, for user: User, frameworkThing: Thing, eventLoop: NIO.EventLoop, context: BaggageContext )
    • pass the baggage as last of such non-domain specific parameters as it will be by far more omnipresent than any specific framework parameter - as it is expected that any framework should be accepting a context if it is able to do so. While not all libraries are necessarily going to be implemented using the same frameworks.

And lastly

We feel it is important to preserve Swift's human-readable nature of function definitions. In other words, we intend to keep the read-out-loud phrasing of methods to remain "request that url (ignore reading out loud the context parameter)" rather than "request (ignore this context parameter when reading) that url".

Existing context argument

When adapting an existing library/framework to support BaggageContext and it already has a "framework context" which is expected to be passed through "everywhere", we suggest to follow these guidelines to adopting BaggageContext:

  1. Add a BaggageContext as a property called baggage to your own context type, so that the call side for your users becomes context.baggage (rather than the confusing context.context)
  2. If you cannot or it would not make sense to carry baggage inside your framework's context object, pass (and accept (!)) the BaggageContext in your framework functions like follows:
  • if they take no framework context, accept a context: BaggageContext which is the same guideline as for all other cases
  • if they already must take a context object and you are out of words (or your API already accepts your framework context as "context"), pass the baggage as last parameter (see above) yet call the parameter baggage to disambiguate your context object from the baggage context object.


  • Lamda.Context may contain baggage and this way offer traceIDs and other values
    • passing context to a Lambda.Context unaware library becomes: http.request(url: "...", context: context.baggage).
    • TODO: We are considering a protocol which would simplify this if it is known that Lambda.Context "carries" baggage...
  • ChannelHandlerContext offers a way to set/get baggage on the underlying channel via context.baggage = ...
    • WorkInProgress, see: https://github.com/apple/swift-nio/pull/1574


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

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

echo './scripts/sanity.sh' > .git/hooks/pre-push


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