fastrace-tower

fastrace-tower is a middleware library that connects fastrace, a distributed tracing library, with tower, modular and reusable components for building robust networking clients and servers. This integration enables seamless trace context propagation across microservice boundaries in applications based on tower.

What is Context Propagation?

Context propagation is a fundamental concept in distributed tracing that enables the correlation of operations spanning multiple services. When a request moves from one service to another, trace context information needs to be passed along, ensuring that all operations are recorded as part of the same trace.

fastrace-tower implements the W3C Trace Context standard for propagating trace information between services. This ensures compatibility with other tracing systems that follow the same standard.

Features

• Automatic Context Propagation: Automatically inject trace context into outgoing HTTP requests.
• Seamless Integration: Works seamlessly with the fastrace library for complete distributed tracing.
• Full Compatibility: Works with fastrace's collection and reporting capabilities.

Custom span context extractor

By default, the server layer reads the traceparent header and starts a new trace when it is missing or invalid. To customize extraction (for example, keep noop when it is missing), configure an extractor. Return None to keep noop:

use fastrace_tower::TRACEPARENT_HEADER;

let layer = fastrace_tower::FastraceServerLayer::default()
    .with_span_context_extractor(|headers| {
        headers
            .get(TRACEPARENT_HEADER)
            .and_then(|traceparent| {
                fastrace::prelude::SpanContext::decode_w3c_traceparent(
                    traceparent.to_str().ok()?,
                )
            })
    });

How It Works

1. When a client makes a request, FastraceClientLayer detects if there's an active trace and adds a traceparent HTTP header with the trace context.
2. When a server receives the request, FastraceServerLayer runs the span context extractor. By default, it decodes the traceparent header, otherwise starts a new trace.
3. If the extractor returns None, a noop span is used.
4. When a context is available, the server creates a new root span for the request using the method as the name.

This process ensures that all operations across services are properly connected in the resulting trace, providing visibility into the entire request lifecycle.

License

This project is licensed under the Apache-2.0 license.