yet-another-agent-harness: Skill for Claude Code

Rust Async Patterns

Production patterns for async Rust programming with Tokio runtime, including tasks, channels, streams, and error handling.

When to Use This Skill

• Building async Rust applications
• Implementing concurrent network services
• Using Tokio for async I/O
• Handling async errors properly
• Debugging async code issues
• Optimizing async performance

Core Concepts

1. Async Execution Model

Future (lazy) → poll() → Ready(value) | Pending
                ↑           ↓
              Waker ← Runtime schedules

2. Key Abstractions

Concept	Purpose
Future	Lazy computation that may complete later
async fn	Function returning impl Future
await	Suspend until future completes
Task	Spawned future running concurrently
Runtime	Executor that polls futures

Quick Start

# Cargo.toml
[dependencies]
tokio = { version = "1", features = ["full"] }
futures = "0.3"
async-trait = "0.1"
anyhow = "1.0"
tracing = "0.1"
tracing-subscriber = "0.3"

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

#[tokio::main]
async fn main() -> Result<()> {
    // Initialize tracing
    tracing_subscriber::fmt::init();

    // Async operations
    let result = fetch_data("https://api.example.com").await?;
    println!("Got: {}", result);

    Ok(())
}

async fn fetch_data(url: &str) -> Result<String> {
    // Simulated async operation
    sleep(Duration::from_millis(100)).await;
    Ok(format!("Data from {}", url))
}

Patterns

Pattern 1: Concurrent Task Execution

use tokio::task::JoinSet;
use anyhow::Result;

// Spawn multiple concurrent tasks
async fn fetch_all_concurrent(urls: Vec<String>) -> Result<Vec<String>> {
    let mut set = JoinSet::new();

    for url in urls {
        set.spawn(async move {
            fetch_data(&url).await
        });
    }

    let mut results = Vec::new();
    while let Some(res) = set.join_next().await {
        match res {
            Ok(Ok(data)) => results.push(data),
            Ok(Err(e)) => tracing::error!("Task failed: {}", e),
            Err(e) => tracing::error!("Join error: {}", e),
        }
    }

    Ok(results)
}

// With concurrency limit
use futures::stream::{self, StreamExt};

async fn fetch_with_limit(urls: Vec<String>, limit: usize) -> Vec<Result<String>> {
    stream::iter(urls)
        .map(|url| async move { fetch_data(&url).await })
        .buffer_unordered(limit) // Max concurrent tasks
        .collect()
        .await
}

// Select first to complete
use tokio::select;

async fn race_requests(url1: &str, url2: &str) -> Result<String> {
    select! {
        result = fetch_data(url1) => result,
        result = fetch_data(url2) => result,
    }
}