BaggageContext library, extracted from slashmo/gsoc-swift-tracing

What's New


  • Split Baggage & BaggageContext into separate libraries (#34)

Baggage Context

Swift 5.2 Swift 5.1 Swift 5.0 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. More information can be found in the SSWG meeting notes.


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-context",
    url: "https://github.com/slashmo/gsoc-swift-baggage-context.git",
    from: "0.5.0"


BaggageContext is intended to be used in conjunction with the instrumentation of distributed systems. To make this instrumentation work, all parties involved operate on the same BaggageContext type. These are the three common parties, in no specific order, and guidance on how to use BaggageContext:

End Users - explicit context passing

You'll likely interact with some API that takes a context. In most cases you already have a context at hand so you should pass that along. If you're certain you don't have a context at hand, pass along an empty one after thinking about why that's the case.

TODO: Document the reasoning behind .background & .TODO once merged (#26)

While this might seem like a burden to take on, this will allow you to immediately add instrumentation (e.g. tracing) once your application grows. Let's say your profiling some troublesome performance regressions. You won't have the time to go through the entire system to start passing contexts around.

TL;DR: You should always pass around BaggageContext, so that you're ready for when you need it.

Once you are ready to instrument your application, you already have everything in place to get going. Instead of each instrument operating on its own context type they'll be using the same BaggageContext that you're already passing around to the various instrumentable libraries & frameworks you make use of, so you're free to mix & match any compatible instrument(s) 🙌 Check out the swift-tracing repository for instructions on how to get up & running.

Library & Framework Authors - passing context and instrumenting libraries

Developers creating frameworks/libraries (e.g. NIO, gRPC, AsyncHTTPClient, ...) which benefit from being instrumented should adopt BaggageContext as part of their public API. AsyncHTTPClient for example might accept a context like this:

let context = BaggageContext()
client.get(url: "https://swift.org", context: context)

For more information on where to place this argument and how to name it, take a look at the Context-Passing Guidelines.

Generally speaking, frameworks and libraries should treat baggage as an opaque container and simply thread it along all asynchronous boundaries a call may have to go through. Libraries and frameworks should not attempt to reuse context as a means of passing values that they need for "normal" operation.

At cross-cutting boundaries, e.g. right before sending an HTTP request, they inject the BaggageContext into the HTTP headers, allowing context propagation. On the receiving side, an HTTP server should extract the request headers into a BaggageContext. Injecting/extracting is part of the swift-tracing libraries and documented in its own repository.

Instrumentation Authors - defining, injecting and extracting baggage

When implementing instrumentation for cross-cutting tools, BaggageContext becomes the way you propagate metadata such as trace ids. Because each instrument knows what values might be added to the BaggageContext they are the ones creating BaggageContextKey types dictating the type of value associated with each key added to the context. To make accessing values a bit more convenient, we encourage you to add computed properties to BaggageContextProtocol:

private enum TraceIDKey: BaggageContextKey {
  typealias Value = String

extension BaggageContextProtocol {
  var traceID: String? {
    get {
      return self[TraceIDKey.self]
    set {
      self[TraceIDKey.self] = newValue

var context = BaggageContext()
context.traceID = "4bf92f3577b34da6a3ce929d0e0e4736"
print(context.traceID ?? "new trace id")

Context-Passing Guidelines

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

Propagating baggage context through your system is to be done explicitly, meaning as a parameter in function calls, following the "flow" of execution.

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: BaggageContextCarrier )
    • 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: BaggageContextCarrier , settings: Settings? = nil)
    • before any defaulted non-function parameters
  • func handle(element: Element, context: BaggageContextCarrier , settings: Settings? = nil, onComplete: () -> ())
    • before defaulted parameters, which themselfes are before required function parameters
  • func handle(element: Element, context: BaggageContextCarrier , 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 can do so. While not all libraries are necessarily going to be implemented using the same frameworks.

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".

When to use what context type?

This library defines the following context (carrier) types:

  • struct BaggageContext - which is the actual context object,
  • protocol BaggageContextCarrier - which should be used whenever a library implements an API and does not necessarily care where it gets a context value from
    • this pattern enables other frameworks to pass their FrameworkContext, like so: get(context: MyFrameworkContext()) if they already have such context in scope (e.g. Vapor's Request object is a good example, or Lambda Runtime's Lambda.Context
  • protocol LoggingBaggageContextCarrier - which in addition exposes a logger bound to the passed context

Finally, some frameworks will have APIs which accept the specific MyFrameworkContext, withing frameworks specifically a lot more frequently than libraries one would hope. It is important when designing APIs to keep in mind -- can this API work with any context, or is it always going to require my framework context, and erring on accepting the most general type possible.

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 for 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 = ...


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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Last updated: Fri May 26 2023 04:17:59 GMT-0500 (GMT-05:00)