---
title: "Custom Instrumentation"
url: https://docs.sentry.io/platforms/rust/guides/actix-web/tracing/trace-propagation/custom-instrumentation/
---

# Custom Instrumentation | Sentry for Actix Web

On this page you will learn how to manually propagate trace information into and out of your Rust application.

To obtain trace headers from a span, use the `iter_headers()` method. Then pass these to the downstream service. If the communication happens over HTTP, it is recommended to attach all headers to the outgoing HTTP request.

For example when using the `reqwest` crate:

```rust
let client = reqwest::Client::new();

let mut request = client.get("downstream-url");
if let Some(span) = sentry::configure_scope(|scope| scope.get_span()) {
    for (k, v) in span.iter_headers() {
        request = request.header(k, v);
    }
}

let response = request.send().await?;
```

To create a span as a continuation of the trace retrieved from the upstream service, pass an iterator of the incoming headers to the transaction context:

```rust
// This needs to be a `impl Iterator<Item = (&str, &str)>` of header key/value pairs.
let headers = request.headers();
let tx_ctx = sentry::TransactionContext::continue_from_headers(
    "transaction name",
    "http.server",
    headers,
);
let transaction = sentry::start_transaction(tx_ctx);
```

For usage with the `tower` crate, the `SentryHttpLayer` handles trace continuations automatically when constructed via `SentryHttpLayer::with_transaction()`.