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.

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 file 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 ./instrumentation.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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elliott-with-the-longest-name-on-github · 2025-06-17T21:15:44Z

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elliott-with-the-longest-name-on-github · 2025-06-18T23:29:43Z

packages/kit/src/runtime/shared.js

+ * @param {any} data
+ * @param {string} [location_description]
+ */
+export function validate_load_response(data, location_description) {


moved to shared to reduce duplication

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!

Lms24 · 2025-07-09T10:00:23Z

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

+	 * @default undefined
+	 * @since 2.22.0
+	 */
+	tracing?: 'server';


I like it! Is the idea here that something like 'client' or 'serverAndClient' could become additional options when client-side tracing is tackled?

Also, I'm realizing this is far-fetched but do you see a world where we (= tracing SDK like Sentry or OTel) could enable this automatically for users? Just asking because for us, ideally, the fewer files users have to touch when setting up Sentry, the better. Also happy to chat about this as a follow-up item as this is already fine for the beginning.

Yeah, ideally this becomes 'server' | 'client' | 'both' in the future -- hopefully the OTEL folks can stabilize browser tracing enough that we can do it. (AsyncContext pls???)

Let me discuss with the team on the enabling-for-users thing. It's possible we could provide an enableOtel export or something that could be used in instrumentation.ts to turn it on for users. Will think about it!

Lms24 · 2025-07-09T10:27:57Z

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

+			},
+			fn: async () => {
+				return await with_event(event, () =>
+					options.hooks.handle({


idea: If users use the sequence helper for their handle hooks, we could start spans for each handler in the sequence. This is very helpful to group child spans (e.g. db spans) to a specific handler or generally make it more apparent which of their handlers takes up the most time, etc.

Can of course be done in a follow-up :)

Huh, not a bad idea... let me see how weird implementing it would be

Lms24 · 2025-07-09T11:34:34Z

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

+			name: 'sveltekit.handle',
+			tracer,
+			attributes: {
+				'sveltekit.route.id': event.route.id || 'unknown',


This applies to all spans where the sveltekit.route.id attribute is added. There's a standardized semantic attribute called http.route which should be set to the parameterized route. So I'd recommend switching to this:

Suggested change

'sveltekit.route.id': event.route.id || 'unknown',

'http.route': event.route.id || 'unknown',

(You could also keep of course both attributes but we've seen some user confusion in similar cases with "duplicated" attributes in vercel's ai package)

btw, thanks a lot for including the route name -- this is super helpful for us!

Ah see, this is why I "hired" you! 😆 I wasn't sure which attributes might have standardized counterparts I could use. Thanks!

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.

Lms24 · 2025-07-10T09:01:31Z

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

+		name: 'sveltekit.load',
+		tracer,
+		attributes: {
+			'sveltekit.load.node_id': node.server_id || 'unknown',


The node_id already gives us insights into if this load function was in a +layout, +page (or +error) node but it contains the whole path (which is good for this attribute, don't get me wrong 😆). We could go one step further and also add something like a sveltekit.node_type attribute.

So basically sveltekit.node_type: '+page' | '+page.server' | '+layout' | ...

No strong opinion though. The main real benefit here would be that you can query, group and aggregate on spans by a specific type easily.

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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elliott-with-the-longest-name-on-github marked this pull request as ready for review June 17, 2025 21:18

elliott-with-the-longest-name-on-github force-pushed the elliott/trace-fns branch from bf91961 to b504608 Compare June 17, 2025 22:54

elliott-with-the-longest-name-on-github marked this pull request as draft June 18, 2025 19:42

elliott-with-the-longest-name-on-github force-pushed the elliott/init-tracing branch from cc0eef5 to 7f08f16 Compare June 18, 2025 22:46

elliott-with-the-longest-name-on-github force-pushed the elliott/trace-fns branch from b504608 to 74ff497 Compare June 18, 2025 22:46

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

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elliott-with-the-longest-name-on-github force-pushed the elliott/trace-fns branch from 74ff497 to be6f6d2 Compare June 18, 2025 22:53

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elliott-with-the-longest-name-on-github commented Jun 18, 2025

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feat: add universal IDs to client nodes

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

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	'sveltekit.route.id': event.route.id \|\| 'unknown',
	'http.route': event.route.id \|\| 'unknown',

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