Workers tracing — write custom spans with new startActiveSpan() and span.end() runtime APIs
The Workers runtime now provides built-in tracing.startActiveSpan() and span.end() APIs, allowing you to write custom spans for operations that last beyond a single callback — for example, instrumenting a stream pipeline where the span should stay open until the stream is fully consumed.
This augments the existing API for writing custom spans, tracing.enterSpan(), which automatically ends a span when its callback is returned. With startActiveSpan(), the span remains open after the callback returns, and you call span.end() when the work is complete:
import { tracing } from "cloudflare:workers";
const encoder = new TextEncoder();
export default {
fetch() {
return tracing.startActiveSpan("stream-response", (span) => {
let timer;
const body = new ReadableStream({
start(controller) {
controller.enqueue(encoder.encode("Starting...\n"));
timer = setTimeout(() => {
controller.enqueue(encoder.encode("Complete.\n"));
controller.close();
span.setAttribute("stream.status", "complete");
span.end();
}, 1000);
},
cancel() {
if (timer !== undefined) clearTimeout(timer);
span.setAttribute("stream.status", "cancelled");
span.end();
},
});
return new Response(body, {
headers: { "content-type": "text/plain" },
});
});
},
};import { tracing } from "cloudflare:workers";
const encoder = new TextEncoder();
export default {
fetch(): Response {
return tracing.startActiveSpan("stream-response", (span) => {
let timer: ReturnType<typeof setTimeout> | undefined;
const body = new ReadableStream<Uint8Array>({
start(controller) {
controller.enqueue(encoder.encode("Starting...\n"));
timer = setTimeout(() => {
controller.enqueue(encoder.encode("Complete.\n"));
controller.close();
span.setAttribute("stream.status", "complete");
span.end();
}, 1000);
},
cancel() {
if (timer !== undefined) clearTimeout(timer);
span.setAttribute("stream.status", "cancelled");
span.end();
},
});
return new Response(body, {
headers: { "content-type": "text/plain" },
});
});
},
};For more details, refer to the custom spans documentation.
