feat: Add tracing to `load`, server actions, and `handle`/`resolve` #13900

elliott-with-the-longest-name-on-github · 2025-06-17T21:15:27Z

Adds spans to load (on the server), server actions, and handle/resolve. It also adds a tracing object with rootSpan and currentSpan to the RequestEvent, ServerLoadEvent, and LoadEvent types. This means you can get access to a span to annotate it pretty much anywhere -- including using getRequestEvent (this could be quite nice for library integrations!).

We're punting on clientside tracing for now, as we do not feel the o11y community has sufficiently converged on approaches (see https://github.com/open-telemetry/community/blob/main/projects/browser-phase-1.md). When OTEL provides a stable and truly browser-native tracing platform, we'll be all over it.

Want to play around with it?

Clone the repo: https://github.com/elliott-with-the-longest-name-on-github/test-tracing

Set up something like Jaeger (you can copy and run the Docker command at the top of this page to get it up and running with UI and ingestion ports): https://www.jaegertracing.io/docs/2.7/getting-started/

Then build and run your app:

pnpm build && node --import ./instrument.server.mjs build/index.js

Visit localhost:3000/one/two/three/four, click some buttons to see different scenarios. This will generate traces you can view in Jaeger.

TODO:

Configuration to enable globally enabling or disabling telemetry
Tests
Ask for comments from community -- esp. around what data we're capturing in our attributes and how it's structured, whether we need any additional API surface for annotating spans
Test on serverless platforms

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packages/kit/src/runtime/shared.js

elliott-with-the-longest-name-on-github · 2025-06-18T23:29:58Z

packages/kit/types/index.d.ts

+		 * - `'server'` - Enable tracing only on the server side
+		 * - `'client'` - Enable tracing only on the client side
+		 * @default false
+		 * @since 2.22.0


Lms24

This looks really great, thanks for adding the spans. In fact, you're including more information in the attributes than what I currently add in our build-time load function instrumentation in the Sentry SDK 😅 Really looking forward to removing this in favour of just picking up the new spans!

I Had some suggestions for attribute names and additional spans for sequential handle hooks.

While probably out of scope for this PR: Given the future of load functions, we should probably think about how we can collect spans for async svelte components and remote functions. Though of course this is something for later. Happy to help out!

packages/kit/src/exports/public.d.ts

packages/kit/src/runtime/server/respond.js

Lms24

Another round:

I think for additional spans, it would be great to also capture spans for +server routes (i.e. the GET/POST etc functions). Technically, the handle spans will already tell us by the route id that a +server route was accessed but I'd see a span for the specfic GET/POST/etc handler as quite valuable, similarly to how OTel express instrumentation would record spans for individual request handlers.

Also left a comment about a node_type attribute.

packages/kit/src/runtime/server/page/load_data.js

benmccann

Could we put this behind an experimental flag like we've been doing with the other larger features recently? It seems like it'd be a nice way to gather feedback about the spans that we're providing

It'd also probably be helpful to add a docs page for this

packages/kit/src/runtime/server/respond.js

elliott-with-the-longest-name-on-github · 2025-07-17T03:24:23Z

packages/kit/src/exports/public.d.ts

+		/**
+		 * Whether to enable serverside OpenTelemetry tracing for SvelteKit operations including handle hooks, load functions, and form actions.
+		 * @default undefined
+		 * @since 2.22.0 // TODO: update this before publishing


TODO, need to update this directly prior to merging

packages/kit/src/runtime/server/respond.js

elliott-with-the-longest-name-on-github · 2025-07-17T03:31:45Z

packages/kit/test/apps/basics/src/hooks.server.js

@@ -49,6 +50,13 @@ export const handleError = ({ event, error: e, status, message }) => {
 };

 export const handle = sequence(
+	({ event, resolve }) => {


Some things we want to test, like navigation, actually involve two requests, so the traces will have two separate trace IDs. This allows us to set a stable ID across all of the traces for a given test so that we can pull them out of the file we write to the disk. We put them on the root span (rather than the span associated with this specific handle function) so that we can easily find the root span, then rebuild the span tree from there.

elliott-with-the-longest-name-on-github · 2025-07-17T03:32:54Z

packages/kit/test/apps/basics/src/lib/tracing-config.js

+import fs from 'node:fs';
+
+/** @implements {SpanExporter} */
+class FilesystemSpanExporter {


I am reasonably sure this can replace our error serialization stuff, because the OTEL integration captures error spans. Will do it in a followup PR sometime in the future, or never, depending on how motivated I am and how much more-important work there is to do 😆

elliott-with-the-longest-name-on-github · 2025-07-17T03:33:43Z

packages/kit/test/apps/basics/src/lib/tracing-config.js

+			return JSON.stringify(span_data);
+		});
+
+		fs.appendFileSync(this.#path, serialized_spans.join('\n') + '\n');


using jsonl conventions so that we can just spam the lines into the file without having to parse/reserialize it all the time

elliott-with-the-longest-name-on-github · 2025-07-17T03:35:08Z

packages/kit/test/utils.js

+	read_traces: async ({}, use) => {
+		/** @param {string} test_id */
+		function read_traces(test_id) {
+			const raw = fs.readFileSync('test/spans.jsonl', 'utf8').split('\n').filter(Boolean);


I could see this becoming inefficient / slowing down tests if we start using it more widely -- it may need optimization in the future

elliott-with-the-longest-name-on-github · 2025-07-17T03:35:57Z

packages/kit/test/utils.js

+			if (root_traces.length === 0) {
+				return [];
+			}
+			return root_traces.map((root_trace) => {


this should significantly increase the efficiency of building the tree as the number of spans associated with a given trace is quite small compared to the total number of spans

elliott-with-the-longest-name-on-github mentioned this pull request Jun 17, 2025

feat: add universal IDs to client nodes #13898

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elliott-with-the-longest-name-on-github mentioned this pull request Jun 17, 2025

feat: Initial tracing setup (peer deps + utils) #13899

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feat: add universal IDs to client nodes

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Merge branch 'main' into elliott/trace-fns

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feat: Add tracing to load, server actions, and handle/resolve #13900

feat: Add tracing to load, server actions, and handle/resolve #13900

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feat: Add tracing to `load`, server actions, and `handle`/`resolve` #13900

feat: Add tracing to `load`, server actions, and `handle`/`resolve` #13900

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