Prior Arts

AsyncContext-like API exists in languages/runtimes that support await syntax or coroutines.

The following table shows a general landscape of how the API behaves in these languages/runtimes.

Language / API	Continuation feedback	Mutation Scope
dotnet `AsyncLocal`	No implicit feedback	In scope mutation
dotnet `CallContext`	No implicit feedback	In scope mutation
Go `context`	No implicit feedback	In scope mutation
Python `ContextVar`	Both available	In scope mutation
Ruby `Fiber`	No implicit feedback	In scope mutation
Rust `tokio::task_local`	No implicit feedback	New scope mutation
Dart `Zone`	No implicit feedback	New scope mutation
JS `Zone`	No implicit feedback	New scope mutation
Node.js `AsyncLocalStorage`	No implicit feedback	Both available

Explanation:

Continuation feedback
- No implicit feedback: await, or passing context to subtasks, does not feedback mutations to the caller continuation.
- Both available: await may and may not feedback mutations to the caller continuation.
Mutation scope
- In scope mutation: set does not require a new function scope, and can modify in scope.
  - async function-like syntax in these languages usually implies a scope.
- New scope mutation: set requires a new function scope.
- Both available.
  - Node.js has an experimental AsyncLocalStorage.enterWith that mutates in scope. async function in JavaScript does not imply a mutation scope.

AsyncContext in other languages

dotnet

C# on .Net runtime provides syntax support of async/await, with AsyncLocal and CallContext to propagate context variables.

Additional to AsyncLocal's in-process propagation, CallContext also supports propagating context variables via remote procedure calls. So CallContext API requires extra security grants.

Test it yourself: dotnet fiddle.

using System;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;

public class Program
{
  static AsyncLocal<string> _asyncLocal = new AsyncLocal<string>();
  static async Task AsyncMain()
  {
    _asyncLocal.Value = "main";
    var t1 = AsyncTask("task 1", 200);
    Console.WriteLine("Called AsyncTask 1.");
    Console.WriteLine("   AsyncLocal value is '{0}'", _asyncLocal.Value);
    var t2 = AsyncTask("task 2", 100);
    Console.WriteLine("Called AsyncTask 2.");
    Console.WriteLine("   AsyncLocal value is '{0}'", _asyncLocal.Value);

    await Task.WhenAll(new List<Task>{ t1, t2 });
    Console.WriteLine("Awaited tasks.");
    Console.WriteLine("   AsyncLocal value is '{0}'", _asyncLocal.Value);
  }

  static async Task AsyncTask(string expectedValue, Int32 delay)
  {
    _asyncLocal.Value = expectedValue;
    await Task.Delay(delay);
    Console.WriteLine("In AsyncTask, expect '{0}'", expectedValue);
    Console.WriteLine("   AsyncLocal value is '{0}'", _asyncLocal.Value);
  }

  public static void Main()
  {
    AsyncMain().Wait();
  }
}

This prints:

Called AsyncTask 1.
   AsyncLocal value is 'main'
Called AsyncTask 2.
   AsyncLocal value is 'main'
In AsyncTask, expect 'task 2'
   AsyncLocal value is 'task 2'
In AsyncTask, expect 'task 1'
   AsyncLocal value is 'task 1'
Awaited tasks.
   AsyncLocal value is 'main'

From the result, we can tell that:

AsyncLocal can be modified with assignment, without an extra scope.
Modification in a child task does not propagate to its sibling tasks.
Modification to an AsyncLocal does not propagate to the caller continuation, i.e. await in caller.

Go

Go is famous for its deep coroutine integration in the language. As such, is has a conventional context propagation mechanism: by always manual passing the context as the first argument of a function.

Go provides a package context for combining arbitrary values into a single Context opaque bag, so that multiple values can be passed as the first argument of a function.

Test it yourself: Go Playground.

package main

import (
  "context"
  "fmt"
)

func inner_fn(ctx context.Context) context.Context {
  // Context is immutable. Modifying a context creates a new context.
  ctx = context.WithValue(ctx, "FooKey", "inner")
  // Return it explicitly so that modification can be observable from parent scope.
  return ctx
}

func main() {
  ctx := context.WithValue(context.Background(), "FooKey", "main")
  inner := inner_fn(ctx)

  fmt.Println("main:", ctx.Value("FooKey"))
  fmt.Println("inner:", inner.Value("FooKey"))
}

This prints:

main: main
inner: inner

From go's context API, we can tell that:

Context is immutable, and modification creates a new Context.
Modification in a child task does not propagate to its sibling tasks implicitly.
Modification to a Context does not propagate to the caller continuation, i.e. caller's context.

Python

Python's contextvars.ContextVar provides the ability to propagate context variables.

import asyncio
from contextvars import ContextVar

current_task = ContextVar('current_task')

async def foo():
  print("foo task parent:", current_task.get())
  current_task.set("foo")
  await asyncio.sleep(2)
  print("foo task:", current_task.get())

async def bar():
  print("bar task parent:", current_task.get())
  current_task.set("bar")
  await asyncio.sleep(1)
  print("bar task:", current_task.get())

async def main():
  current_task.set("main")

  await asyncio.gather(
    foo(),
    bar(),
  )
  print("after gather:", current_task.get())

loop = asyncio.get_event_loop()
loop.run_until_complete(main())

This prints:

foo task parent: main
bar task parent: main
bar task: bar
foo task: foo
after gather: main

From the result, we can tell that:

ContextVar can be modified with set method, without an extra scope.
Modification in a child task does not propagate to its sibling tasks.
Modification to an ContextVar does not propagate to the caller continuation, i.e. await in caller.

This is the default asyncio scheduling behavior. Additional to ContextVar, the contextvars package even allow manual context management in Python. This allows userland scheduler to customize the propagation behavior around await with context.copy and context.run. So, if a user run context.run without asyncio on an awaitable object, it can achieve the following behavior:

import asyncio
import contextvars
from contextvars import ContextVar

current_task = ContextVar('current_task')

async def foo():
  print("foo task parent:", current_task.get())
  current_task.set("foo")
  await asyncio.sleep(1)
  print("foo task:", current_task.get())

async def main():
  current_task.set("main")

  ctx = contextvars.copy_context()
  await ctx.run(foo)
  print("after await:", current_task.get())

loop = asyncio.get_event_loop()
loop.run_until_complete(main())

This prints:

foo task parent: main
foo task: foo
after await: foo

This allows userland schedulers to implement different context propagation than the asyncio's default one.

Ruby

Although Ruby's Fiber does not provide a default scheduler, it provides a bracket accessor to get/set context variables, like AsyncContext.Variable does.

Test it yourself: Ruby Playground.

def main
  # Fiber coroutine
  Fiber[:foo] = "main"
  f1 = Fiber.new do
    puts "inner 1 parent: #{Fiber[:foo]}"
    Fiber[:foo] = "1"
    Fiber.current.storage
  end
  f2 = Fiber.new do
    puts "inner 2 parent: #{Fiber[:foo]}"
    Fiber[:foo] = "2"
    Fiber.current.storage
  end
  inner_ctx1 = f1.resume
  inner_ctx2 = f2.resume
  puts "main #{Fiber[:foo]}"
  puts "inner 1 #{inner_ctx1[:foo]}"
  puts "inner 2 #{inner_ctx2[:foo]}"
end

Fiber.new do
  main
end.resume

This prints:

inner 1 parent: main
inner 2 parent: main
main main
inner 1 1
inner 2 2

From the result, we can tell that:

Fiber context variables can be modified with bracket assignment, without an extra scope.
Modification in a child task does not propagates to its sibling tasks.
Modification to a Fiber does not propagate to the caller continuation, i.e. Fiber.resume in caller.

Rust

Rust only provides thread_local in the std crate. tokio.rs is a popular Rust asynchronous applications runtime that provides a task_local, which is similar to AsyncContext.Variable.

Test it yourself: Rust Playground.

use tokio::time::{sleep, Duration};

tokio::task_local! {
  static FOO: &'static str;
}

#[tokio::main]
async fn main() {
  FOO.scope("foo", async move {
    println!("main {}", FOO.get());

    let t1 = FOO.scope("inner1", async move {
      sleep(Duration::from_millis(200)).await;
      println!("inner1: {}", FOO.get());
    });
    let t2 = FOO.scope("inner2", async move {
      sleep(Duration::from_millis(100)).await;
      println!("inner2: {}", FOO.get());
    });
    futures::join!(t1, t2);
    println!("main {}", FOO.get());
  }).await;
}

This prints:

main foo
inner2: inner2
inner1: inner1
main foo

From the tokio API, and the result, we can tell that:

task_local can be only be modified with a sync_scope or a scope.
Modification in a child task does not propagates to its sibling tasks.
Modification to a task_local does not propagate to the caller continuation, i.e. await in caller.

Dart

Dart's Zone provides much more functionality than the AsyncContext.Variable in this proposal. Zone covers the necessary propagation of values that AsyncContext.Variable provides.

Test it yourself: DartPad.

import 'dart:async';

void main() async {
  await runZoned(() async {
    var task1 = runZoned(() async {
      await Future.delayed(Duration(seconds: 2));
      print("Task 1: ${Zone.current[#task]}");
    }, zoneValues: { #task: 'task1' });

    var task2 = runZoned(() async {
      await Future.delayed(Duration(seconds: 1));
      print("Task 2: ${Zone.current[#task]}");
    }, zoneValues: { #task: 'task2' });

    await Future.wait({ task1, task2 });
    print("main : ${Zone.current[#task]}");
  }, zoneValues: { #task: 'main' });
}

This prints:

Task 2: task2
Task 1: task1
main : main

From the Dart Zone API, and the result, we can tell that:

Zone can be only be modified with a new function scope.
Modification in a child task does not propagates to its sibling tasks.
Modification to an Zone does not propagate to the caller continuation, i.e. await in caller.

AsyncContext in real world

OpenTelemetry

Test it yourself: OpenTelemetry Demo. This demo includes more than 10+ services and covers most popular programming languages.

Even though each language or runtime provides different shapes of async context variable API, OpenTelemetry standardized how the tracing context should be like in OpenTelemetry implementations.

The OpenTelemetry Context Specification requires that each write operation to a Context must result in the creation of a new Context. This eliminates the confusion could be caused by language context APIs that if a mutation happens after an async operation, if the mutation can be observed by prior async operations.

This requirement asserts that mutation in a child scope can not be propagated to its immutable caller continuation as well.

The following list shows the underlying language constructs of each OpenTelemetry language SDK:

JavaScript: OpenTelemetry JS provides both web (zone.js based) and Node.js context implementations:
- AsyncLocalStorageContextManager.
- ZoneContextManager.
dotnet: OpenTelemetry dotnet provides both AsyncLocal based and CallContext based context implementations.
- OpenTelemetry.Context.AsyncLocalRuntimeContextSlot.
- OpenTelemetry.Context.RemotingRuntimeContextSlot.
Go: uses go.context directly.
Python ContextVarsRuntimeContext.
Ruby Context, based on Ruby's Fiber.
Rust Context, does not support tokio yet.
Swift:
- ActivityContextManager.
- TaskLocalContextManager.

JavaScript prior arts

Node.js AsyncLocalStorage

Node.js provides a stable API AsyncLocalStorage that supports implicit context propagation across await and runtime APIs.

class AsyncLocalStorage<ValueType> {
  static bind<T extends Function>(fn: T): T;
  static snapshot(): () => void;

  constructor();

  getStore(): ValueType;

  run<T extends Function, ReturnType = GetReturnType<T>>(store: ValueType, callback: T, ...args: never[]): ReturnType;

  /** @experimental */
  enterWith(store: ValueType);
}

The AsyncContext.Variable is significantly inspired by AsyncLocalStorage. However, AsyncContext.Variable only provides an essential subset of AsyncLocalStorage, with a follow-up extension for set semantic with scope enforcement like using _ = asyncVar.withValue(val), as described in mutation-scope.md.

Additionally, as AsyncContext.Variable is built in the language, it also support language constructs like (async) generators.

zones.js

zone.js provides a Zone object, which has the following API:

class Zone {
  constructor({ name, parent });

  name;
  get parent();

  fork({ name });
  run(callback);
  wrap(callback);

  static get current();
}

The concept of the current zone, reified as Zone.current, is crucial. Both run and wrap are designed to manage running the current zone:

z.run(callback) will set the current zone to z for the duration of callback, resetting it to its previous value afterward. This is how you "enter" a zone.
z.wrap(callback) produces a new function that essentially performs z.run(callback) (passing along arguments and this, of course).

The current zone is the async context that propagates with all our operations. In our above example, sites (1) through (6) would all have the same value of Zone.current. If a developer had done something like:

const loadZone = Zone.current.fork({ name: "loading zone" });
window.onload = loadZone.wrap(e => { ... });

then at all those sites, Zone.current would be equal to loadZone.

Notably, zone.js features like monitoring or intercepting async tasks scheduled in a zone are not in the scope of this proposal.

Other JavaScript APIs on async tasks

Node.js `domain` module

Domain's global central active domain can be consumed by multiple endpoints and be exchanged in any time with synchronous operation (domain.enter()). Since it is possible that some third party module changed active domain on the fly and application owner may unaware of such change, this can introduce unexpected implicit behavior and made domain diagnosis hard.

Check out Domain Module Postmortem for more details.

Node.js `async_hooks`

This is what the proposal evolved from. async_hooks in Node.js enabled async resources tracking for APM vendors. On which Node.js also implemented AsyncLocalStorage.

Chrome Async Stack Tagging API

Frameworks can schedule tasks with their own userland queues. In such case, the stack trace originated from the framework scheduling logic tells only part of the story.

Error: Call stack
  at someTask (example.js)
  at loop (framework.js)

The Chrome Async Stack Tagging API introduces a new console method named console.createTask(). The API signature is as follows:

interface Console {
  createTask(name: string): Task;
}

interface Task {
  run<T>(f: () => T): T;
}

console.createTask() snapshots the call stack into a Task record. And each Task.run() restores the saved call stack and append it to newly generated call stacks.

Error: Call stack
  at someTask (example.js)
  at loop (framework.js)          // <- Task.run
  at async someTask               // <- Async stack appended
  at schedule (framework.js)      // <- console.createTask
  at businessLogic (example.js)

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Prior Arts

AsyncContext in other languages

dotnet

Go

Python

Ruby

Rust

Dart

AsyncContext in real world

OpenTelemetry

JavaScript prior arts

Node.js AsyncLocalStorage

zones.js

Other JavaScript APIs on async tasks

Node.js `domain` module

Node.js `async_hooks`

Chrome Async Stack Tagging API

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Prior Arts

AsyncContext in other languages

dotnet

Go

Python

Ruby

Rust

Dart

AsyncContext in real world

OpenTelemetry

JavaScript prior arts

Node.js AsyncLocalStorage

zones.js

Other JavaScript APIs on async tasks

Node.js domain module

Node.js async_hooks

Chrome Async Stack Tagging API

Node.js `domain` module

Node.js `async_hooks`