dagx/docs/TRACING.md

Tracing Support

dagx provides optional observability through the tracing crate with zero runtime overhead when disabled. The tracing instrumentation is conditionally compiled using feature flags, meaning when the feature is off, the logging code doesn't exist in the compiled binary at all.

Enabling Tracing

[dependencies]
dagx = { version = "0.3", features = ["tracing"] }
tracing-subscriber = "0.3"

Usage Example

use tracing_subscriber::{fmt, EnvFilter};

// Initialize tracing subscriber
fmt()
    .with_env_filter(
        EnvFilter::try_from_default_env()
            .unwrap_or_else(|_| EnvFilter::new("dagx=info"))
    )
    .init();

// Your DAG code here - tracing will log execution details

Log Levels

• INFO: DAG execution start/completion
• DEBUG: Task additions, dependency wiring, layer computation
• TRACE: Individual task execution (inline vs spawned), detailed execution flow
• ERROR: Task panics, concurrent execution attempts

Run with different levels:

RUST_LOG=dagx=info  cargo run    # High-level execution info
RUST_LOG=dagx=debug cargo run    # Task and layer details
RUST_LOG=dagx=trace cargo run    # All execution details

See examples/tracing_example.rs for a complete working example.

Zero-Cost Guarantee

When the tracing feature is disabled (the default), there is literally 0ns overhead:

• Logging code is removed at compile time via #[cfg(feature = "tracing")]
• No branches, no function calls, nothing
• The tracing crate isn't even linked
• Benchmarks verify identical performance with/without the feature

This follows the same pattern used by Tokio, Hyper, and other performance-critical Rust async libraries.