feat: OpenTelemetry Tracing

package.json

@@ -40,6 +40,7 @@
 			"@parcel/watcher",
 			"esbuild",
 			"netlify-cli",
+			"protobufjs",
 			"rolldown",
 			"sharp",
 			"svelte-preprocess",

packages/kit/package.json

@@ -32,6 +32,7 @@
 		"sirv": "^3.0.0"
 	},
 	"devDependencies": {
+		"@opentelemetry/api": "^1.0.0",
 		"@playwright/test": "catalog:",
 		"@sveltejs/vite-plugin-svelte": "catalog:",
 		"@types/connect": "^3.4.38",
@@ -47,9 +48,15 @@
 	},
 	"peerDependencies": {
 		"@sveltejs/vite-plugin-svelte": "^3.0.0 || ^4.0.0-next.1 || ^5.0.0 || ^6.0.0-next.0",
+		"@opentelemetry/api": "^1.0.0",
 		"svelte": "^4.0.0 || ^5.0.0-next.0",
 		"vite": "^5.0.3 || ^6.0.0 || ^7.0.0-beta.0"
 	},
+	"peerDependenciesMeta": {
+		"@opentelemetry/api": {
+			"optional": true
+		}
+	},
 	"bin": {
 		"svelte-kit": "svelte-kit.js"
 	},

packages/kit/src/core/config/index.spec.js

@@ -76,6 +76,9 @@ const get_defaults = (prefix = '') => ({
 			publicPrefix: 'PUBLIC_',
 			privatePrefix: ''
 		},
+		experimental: {
+			tracing: undefined
+		},
 		files: {
 			assets: join(prefix, 'static'),
 			hooks: {
@@ -404,3 +407,60 @@ test('errors on loading config with incorrect default export', async () => {
 		'The Svelte config file must have a configuration object as its default export. See https://svelte.dev/docs/kit/configuration'
 	);
 });
+
+test('accepts valid tracing values', () => {
+	assert.doesNotThrow(() => {
+		validate_config({
+			kit: {
+				experimental: {
+					tracing: 'server'
+				}
+			}
+		});
+	});
+
+	assert.doesNotThrow(() => {
+		validate_config({
+			kit: {
+				experimental: {
+					tracing: undefined
+				}
+			}
+		});
+	});
+});
+
+test('errors on invalid tracing values', () => {
+	assert.throws(() => {
+		validate_config({
+			kit: {
+				experimental: {
+					// @ts-expect-error - given value expected to throw
+					tracing: true
+				}
+			}
+		});
+	}, /^config\.kit\.experimental\.tracing should be undefined or "server"$/);
+
+	assert.throws(() => {
+		validate_config({
+			kit: {
+				experimental: {
+					// @ts-expect-error - given value expected to throw
+					tracing: false
+				}
+			}
+		});
+	}, /^config\.kit\.experimental\.tracing should be undefined or "server"$/);
+
+	assert.throws(() => {
+		validate_config({
+			kit: {
+				experimental: {
+					// @ts-expect-error - given value expected to throw
+					tracing: 'client'
+				}
+			}
+		});
+	}, /^config\.kit\.experimental\.tracing should be undefined or "server"$/);
+});

packages/kit/src/core/config/options.js

@@ -120,6 +120,15 @@ const options = object(
 				privatePrefix: string('')
 			}),
 
+			experimental: object({
+				tracing: validate(undefined, (input, keypath) => {
+					if (input !== 'server') {
+						throw new Error(`${keypath} should be undefined or "server"`);
+					}
+					return input;
+				})
+			}),
+
 			files: object({
 				assets: string('static'),
 				hooks: object({

packages/kit/src/core/sync/write_server.js

@@ -33,6 +33,11 @@ import { set_building, set_prerendering } from '__sveltekit/environment';
 import { set_assets } from '__sveltekit/paths';
 import { set_manifest, set_read_implementation } from '__sveltekit/server';
 import { set_private_env, set_public_env, set_safe_public_env } from '${runtime_directory}/shared-server.js';
+import { get_tracer, enable_tracing } from '${runtime_directory}/telemetry/get_tracer.js';
+
+if (${s(config.kit.experimental.tracing === 'server')}) {
+	enable_tracing();
+}
 
 export const options = {
 	app_template_contains_nonce: ${template.includes('%sveltekit.nonce%')},
@@ -60,6 +65,7 @@ export const options = {
 			.replace(/%sveltekit\.status%/g, '" + status + "')
 			.replace(/%sveltekit\.error\.message%/g, '" + message + "')}
 	},
+	tracer: get_tracer(),
 	version_hash: ${s(hash(config.kit.version.name))}
 };
 

packages/kit/src/exports/hooks/sequence.js

@@ -1,3 +1,9 @@
+/** @import { Handle, RequestEvent, ResolveOptions } from '@sveltejs/kit' */
+/** @import { MaybePromise } from 'types' */
+import { with_event } from '../../runtime/app/server/event.js';
+import { get_tracer } from '../../runtime/telemetry/get_tracer.js';
+import { record_span } from '../../runtime/telemetry/record_span.js';
+
 /**
  * A helper function for sequencing multiple `handle` calls in a middleware-like manner.
  * The behavior for the `handle` options is as follows:
@@ -66,56 +72,77 @@
  * first post-processing
  * ```
  *
- * @param {...import('@sveltejs/kit').Handle} handlers The chain of `handle` functions
- * @returns {import('@sveltejs/kit').Handle}
+ * @param {...Handle} handlers The chain of `handle` functions
+ * @returns {Handle}
  */
 export function sequence(...handlers) {
 	const length = handlers.length;
 	if (!length) return ({ event, resolve }) => resolve(event);
 
-	return ({ event, resolve }) => {
+	return async ({ event, resolve }) => {
+		// there's an assumption here that people aren't doing something insane like sequence(() => {}, sequence(() => {}))
+		// worst case there is that future spans get a lower-down span as their root span -- the tracing would still work,
+		// it'd just look a little weird
+		const { rootSpan } = event.tracing;
+		const tracer = await get_tracer();
 		return apply_handle(0, event, {});
 
 		/**
 		 * @param {number} i
-		 * @param {import('@sveltejs/kit').RequestEvent} event
-		 * @param {import('@sveltejs/kit').ResolveOptions | undefined} parent_options
-		 * @returns {import('types').MaybePromise<Response>}
+		 * @param {RequestEvent} event
+		 * @param {ResolveOptions | undefined} parent_options
+		 * @returns {MaybePromise<Response>}
 		 */
 		function apply_handle(i, event, parent_options) {
 			const handle = handlers[i];
 
-			return handle({
-				event,
-				resolve: (event, options) => {
-					/** @type {import('@sveltejs/kit').ResolveOptions['transformPageChunk']} */
-					const transformPageChunk = async ({ html, done }) => {
-						if (options?.transformPageChunk) {
-							html = (await options.transformPageChunk({ html, done })) ?? '';
-						}
+			return record_span({
+				tracer,
+				name: 'sveltekit.handle.child',
+				attributes: {
+					'sveltekit.handle.child.index': i
+				},
+				fn: async (span) => {
+					const traced_event = { ...event, tracing: { rootSpan, currentSpan: span } };
+					return await with_event(traced_event, () =>
+						handle({
+							event: traced_event,
+							resolve: (event, options) => {
+								/** @type {ResolveOptions['transformPageChunk']} */
+								const transformPageChunk = async ({ html, done }) => {
+									if (options?.transformPageChunk) {
+										html = (await options.transformPageChunk({ html, done })) ?? '';
+									}
 
-						if (parent_options?.transformPageChunk) {
-							html = (await parent_options.transformPageChunk({ html, done })) ?? '';
-						}
+									if (parent_options?.transformPageChunk) {
+										html = (await parent_options.transformPageChunk({ html, done })) ?? '';
+									}
 
-						return html;
-					};
+									return html;
+								};
 
-					/** @type {import('@sveltejs/kit').ResolveOptions['filterSerializedResponseHeaders']} */
-					const filterSerializedResponseHeaders =
-						parent_options?.filterSerializedResponseHeaders ??
-						options?.filterSerializedResponseHeaders;
+								/** @type {ResolveOptions['filterSerializedResponseHeaders']} */
+								const filterSerializedResponseHeaders =
+									parent_options?.filterSerializedResponseHeaders ??
+									options?.filterSerializedResponseHeaders;
 
-					/** @type {import('@sveltejs/kit').ResolveOptions['preload']} */
-					const preload = parent_options?.preload ?? options?.preload;
+								/** @type {ResolveOptions['preload']} */
+								const preload = parent_options?.preload ?? options?.preload;
 
-					return i < length - 1
-						? apply_handle(i + 1, event, {
-								transformPageChunk,
-								filterSerializedResponseHeaders,
-								preload
-							})
-						: resolve(event, { transformPageChunk, filterSerializedResponseHeaders, preload });
+								return i < length - 1
+									? apply_handle(i + 1, event, {
+											transformPageChunk,
+											filterSerializedResponseHeaders,
+											preload
+										})
+									: resolve(event, {
+											transformPageChunk,
+											filterSerializedResponseHeaders,
+											preload
+										});
+							}
+						})
+					);
 				}
 			});
 		}

packages/kit/src/exports/hooks/sequence.spec.js

@@ -4,6 +4,10 @@ import { installPolyfills } from '../node/polyfills.js';
 
 installPolyfills();
 
+const dummy_event = /** @type {import('@sveltejs/kit').RequestEvent} */ ({
+	tracing: { rootSpan: {} }
+});
+
 test('applies handlers in sequence', async () => {
 	/** @type {string[]} */
 	const order = [];
@@ -29,10 +33,9 @@ test('applies handlers in sequence', async () => {
 		}
 	);
 
-	const event = /** @type {import('@sveltejs/kit').RequestEvent} */ ({});
 	const response = new Response();
 
-	assert.equal(await handler({ event, resolve: () => response }), response);
+	assert.equal(await handler({ event: dummy_event, resolve: () => response }), response);
 	expect(order).toEqual(['1a', '2a', '3a', '3b', '2b', '1b']);
 });
 
@@ -47,9 +50,8 @@ test('uses transformPageChunk option passed to non-terminal handle function', as
 		async ({ event, resolve }) => resolve(event)
 	);
 
-	const event = /** @type {import('@sveltejs/kit').RequestEvent} */ ({});
 	const response = await handler({
-		event,
+		event: dummy_event,
 		resolve: async (_event, opts = {}) => {
 			let html = '';
 
@@ -84,9 +86,8 @@ test('merges transformPageChunk option', async () => {
 		}
 	);
 
-	const event = /** @type {import('@sveltejs/kit').RequestEvent} */ ({});
 	const response = await handler({
-		event,
+		event: dummy_event,
 		resolve: async (_event, opts = {}) => {
 			let html = '';
 
@@ -117,9 +118,8 @@ test('uses first defined preload option', async () => {
 		}
 	);
 
-	const event = /** @type {import('@sveltejs/kit').RequestEvent} */ ({});
 	const response = await handler({
-		event,
+		event: dummy_event,
 		resolve: (_event, opts = {}) => {
 			let html = '';
 
@@ -150,9 +150,8 @@ test('uses first defined filterSerializedResponseHeaders option', async () => {
 		}
 	);
 
-	const event = /** @type {import('@sveltejs/kit').RequestEvent} */ ({});
 	const response = await handler({
-		event,
+		event: dummy_event,
 		resolve: (_event, opts = {}) => {
 			let html = '';