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0.1.1 Mar 1, 2023
0.1.0 Feb 28, 2023

#241 in Debugging

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Crate Docs

The Futures in Async Rust can be arbitrarily composited or nested to achieve a variety of control flows. Assuming that the execution of each Future is represented as a node, then the asynchronous execution of an async task can be organized into a logical tree, which is constantly transformed over the polling, completion, and cancellation of Futures.

await-tree allows developers to dump this execution tree at runtime, with the span of each Future annotated by instrument_await. A basic example is shown below, and more examples of complex control flows can be found in the examples directory.

async fn bar(i: i32) {
    // `&'static str` span
    baz(i).instrument_await("baz in bar").await

async fn baz(i: i32) {
    // runtime `String` span is also supported
        .instrument_await(format!("pending in baz {i}"))

async fn foo() {
    // spans of joined futures will be siblings in the tree

let root = register("foo");

let tree = get_tree("foo");

// foo [1.006s]
//   bar [1.006s]
//     baz in bar [1.006s]
//       pending in baz 3 [1.006s]
//   baz [1.006s]
//     pending in baz 2 [1.006s]

Compared to async-backtrace

tokio-rs/async-backtrace is a similar crate that also provides the ability to dump the execution tree of async tasks. Here are some differences between await-tree and async-backtrace:

Pros of await-tree:

  • await-tree support customizing the span with runtime String, while async-backtrace only supports function name and line number.

    This is useful when we want to annotate the span with some dynamic information, such as the identifier of a shared resource (e.g., a lock), to see how the contention happens among different tasks.

  • await-tree support almost all kinds of async control flows with arbitrary Future topology, while async-backtrace fails to handle some of them.

    For example, it's common to use &mut impl Future as an arm of select to avoid problems led by cancellation unsafety. To further resolve this Future after the select completes, we may move it to another place and await it there. async-backtrace fails to track this Future again due to the change of its parent. See examples/detach.rs for more details.

  • await-tree maintains the tree structure with an arena-based data structure, with zero extra unsafe code. For comparison, async-backtrace crafts it by hand and there's potential memory unsafety for unhandled topologies mentioned above.

    It's worth pointing out that await-tree has been applied in the production deployment of RisingWave, a distributed streaming database, for a long time.

  • await-tree maintains the tree structure separately from the Future itself, which enables developers to dump the tree at any time with nearly no contention, no matter the Future is under active polling or has been pending. For comparison, async-backtrace has to wait for the polling to complete before dumping the tree, which may cause a long delay.

Pros of async-backtrace:

  • async-backtrace is under the Tokio organization.


await-tree is distributed under the Apache License (Version 2.0). Please refer to LICENSE for more information.


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