feat(tracing): add OpenTelemetry spans to prerenderToStream phases #88914

wyattjoh · 2026-01-22T20:50:55Z

What?

Adds comprehensive OpenTelemetry tracing spans to the App Router render pipeline, covering both build-time prerendering and runtime prefetch scenarios.

Why?

When debugging or optimizing applications, it's helpful to understand where time is being spent during rendering. These spans provide visibility into:

Build-time (Cache Components prerender):

Prospective RSC Payload generation
Prospective Server Prerender (to fill caches)
Cache Ready Wait (waiting for all cache reads)
Prospective Client Prerender (HTML prerender to fill client caches)
Final RSC Payload generation (with warmed caches)
Final Server Prerender (with task-based abort)
Final Client Prerender (with task-based abort)

Runtime (prefetch):

Runtime Prospective RSC Payload
Runtime Prospective Server Prerender
Runtime Cache Ready Wait
Runtime Final RSC Payload
Runtime Final Server Prerender

Dynamic Flight Rendering:

generateDynamicFlightRenderResult span for RSC streaming responses

How?

Added AppRenderSpan.generateDynamicFlightRenderResult for dynamic flight result generation
Added runtime prerender spans to PrerenderSpan enum for prefetch scenarios
Updated span naming to use consistent prospective/final terminology
Wrapped relevant code paths with getTracer().trace() calls

Note: To expose these spans with OpenTelemetry traces, set the NEXT_OTEL_VERBOSE=1 environment variable.

Add tracing spans to visualize the different phases of the Cache Components prerender pipeline in prerenderToStream: - Initial RSC Payload generation - Initial Server Prerender (prospective render to fill caches) - Cache Ready Wait (waiting for all cache reads to complete) - Initial Client Prerender (HTML prerender to fill client caches) - Final RSC Payload generation (with warmed caches) - Final Server Prerender (final RSC render with task-based abort) - Final Client Prerender (final HTML render with task-based abort) These spans help developers understand the performance characteristics of their Cache Components-enabled applications during build time.

nextjs-bot · 2026-01-22T21:19:58Z

Stats from current PR

🔴 1 regression

Metric	Canary	PR	Change	Trend
node_modules Size	460 MB	461 MB	🔴 +120 kB (+0%)	▁▁▁▁▁

📊 All Metrics

📖 Metrics Glossary

Dev Server Metrics:

Listen = TCP port starts accepting connections
First Request = HTTP server returns successful response
Cold = Fresh build (no cache)
Warm = With cached build artifacts

Build Metrics:

Fresh = Clean build (no .next directory)
Cached = With existing .next directory

Change Thresholds:

Time: Changes < 50ms AND < 10%, OR < 2% are insignificant
Size: Changes < 1KB AND < 1% are insignificant
All other changes are flagged to catch regressions

⚡ Dev Server

Metric	Canary	PR	Change	Trend
Cold (Listen)	455ms	455ms	✓	▁▁▁▁▁
Cold (Ready in log)	437ms	437ms	✓	▁▂▁▁▁
Cold (First Request)	1.150s	1.145s	✓	▁▃▂▁▃
Warm (Listen)	455ms	456ms	✓	▁▁▁▂▁
Warm (Ready in log)	443ms	441ms	✓	▁▁▁▁▁
Warm (First Request)	344ms	340ms	✓	▂▁▁▂▂

📦 Dev Server (Webpack) (Legacy)

📦 Dev Server (Webpack)

Metric	Canary	PR	Change	Trend
Cold (Listen)	455ms	455ms	✓	▁▁▁▁▁
Cold (Ready in log)	440ms	439ms	✓	▁▁▁▁▁
Cold (First Request)	1.839s	1.844s	✓	▁▁▁▁▁
Warm (Listen)	455ms	455ms	✓	▁▁▁▁▁
Warm (Ready in log)	440ms	439ms	✓	▁▁▁▁▁
Warm (First Request)	1.836s	1.851s	✓	▁▁▁▁▁

⚡ Production Builds

Metric	Canary	PR	Change	Trend
Fresh Build	4.306s	4.336s	✓	▁▁▁▂▁
Cached Build	4.274s	4.240s	✓	▁▁▁▁▁

📦 Production Builds (Webpack) (Legacy)

📦 Production Builds (Webpack)

Metric	Canary	PR	Change	Trend
Fresh Build	14.094s	14.145s	✓	▄▂▂▂▂
Cached Build	14.212s	14.259s	✓	▄▄▂▂▁
node_modules Size	460 MB	461 MB	🔴 +120 kB (+0%)	▁▁▁▁▁

📦 Bundle Sizes

Bundle Sizes

⚡ Turbopack

Client

Main Bundles: **432 kB** → **432 kB** ✅ -58 B

82 files with content-based hashes (individual files not comparable between builds)

Server

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	761 B	769 B	🔴 +8 B (+1%)
Total	761 B	769 B	⚠️ +8 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	451 B	450 B	✓
Total	451 B	450 B	✅ -1 B

📦 Webpack

Client

Main Bundles

	Canary	PR	Change
2086.HASH.js gzip	169 B	N/A	-
2161-HASH.js gzip	5.41 kB	N/A	-
2747-HASH.js gzip	4.48 kB	N/A	-
4322-HASH.js gzip	52.7 kB	N/A	-
ec793fe8-HASH.js gzip	62.3 kB	N/A	-
framework-HASH.js gzip	59.8 kB	59.8 kB	✓
main-app-HASH.js gzip	251 B	254 B	🔴 +3 B (+1%)
main-HASH.js gzip	38.7 kB	39.1 kB	✓
webpack-HASH.js gzip	1.68 kB	1.68 kB	✓
1596.HASH.js gzip	N/A	169 B	-
2658-HASH.js gzip	N/A	52.4 kB	-
6349-HASH.js gzip	N/A	4.46 kB	-
7019-HASH.js gzip	N/A	5.43 kB	-
b17a3386-HASH.js gzip	N/A	62.3 kB	-
Total	226 kB	226 kB	⚠️ +55 B

Polyfills

	Canary	PR	Change
polyfills-HASH.js gzip	39.4 kB	39.4 kB	✓
Total	39.4 kB	39.4 kB	✓

Pages

	Canary	PR	Change
_app-HASH.js gzip	194 B	193 B	✓
_error-HASH.js gzip	182 B	182 B	✓
css-HASH.js gzip	336 B	335 B	✓
dynamic-HASH.js gzip	1.8 kB	1.8 kB	✓
edge-ssr-HASH.js gzip	256 B	256 B	✓
head-HASH.js gzip	352 B	349 B	✓
hooks-HASH.js gzip	385 B	384 B	✓
image-HASH.js gzip	580 B	580 B	✓
index-HASH.js gzip	259 B	258 B	✓
link-HASH.js gzip	2.5 kB	2.51 kB	✓
routerDirect..HASH.js gzip	319 B	317 B	✓
script-HASH.js gzip	385 B	387 B	✓
withRouter-HASH.js gzip	316 B	315 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Total	7.97 kB	7.96 kB	✅ -8 B

Server

Edge SSR

	Canary	PR	Change
edge-ssr.js gzip	126 kB	126 kB	✓
page.js gzip	244 kB	240 kB	🟢 4.41 kB (-2%)
Total	370 kB	366 kB	✅ -4.2 kB

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	616 B	619 B	✓
middleware-r..fest.js gzip	155 B	156 B	✓
middleware.js gzip	33.2 kB	33.4 kB	✓
edge-runtime..pack.js gzip	842 B	842 B	✓
Total	34.8 kB	35 kB	⚠️ +223 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	738 B	738 B	✓
Total	738 B	738 B	✓

Build Cache

	Canary	PR	Change
0.pack gzip	3.71 MB	3.72 MB	🔴 +10.8 kB (+0%)
index.pack gzip	102 kB	102 kB	✓
index.pack.old gzip	102 kB	101 kB	🟢 1.36 kB (-1%)
Total	3.91 MB	3.92 MB	⚠️ +8.98 kB

🔄 Shared (bundler-independent)

Runtimes

	Canary	PR	Change
app-page-exp...dev.js gzip	306 kB	306 kB	✓
app-page-exp..prod.js gzip	163 kB	163 kB	✓
app-page-tur...dev.js gzip	306 kB	306 kB	✓
app-page-tur..prod.js gzip	163 kB	163 kB	✓
app-page-tur...dev.js gzip	302 kB	303 kB	✓
app-page-tur..prod.js gzip	161 kB	161 kB	✓
app-page.run...dev.js gzip	303 kB	303 kB	✓
app-page.run..prod.js gzip	161 kB	161 kB	✓
app-route-ex...dev.js gzip	69.4 kB	69.5 kB	✓
app-route-ex..prod.js gzip	48.2 kB	48.2 kB	✓
app-route-tu...dev.js gzip	69.4 kB	69.5 kB	✓
app-route-tu..prod.js gzip	48.2 kB	48.2 kB	✓
app-route-tu...dev.js gzip	69 kB	69.1 kB	✓
app-route-tu..prod.js gzip	47.9 kB	47.9 kB	✓
app-route.ru...dev.js gzip	68.9 kB	69.1 kB	✓
app-route.ru..prod.js gzip	47.9 kB	47.9 kB	✓
dist_client_...dev.js gzip	324 B	324 B	✓
dist_client_...dev.js gzip	326 B	326 B	✓
dist_client_...dev.js gzip	318 B	318 B	✓
dist_client_...dev.js gzip	317 B	317 B	✓
pages-api-tu...dev.js gzip	42.4 kB	42.5 kB	✓
pages-api-tu..prod.js gzip	32.2 kB	32.2 kB	✓
pages-api.ru...dev.js gzip	42.4 kB	42.5 kB	✓
pages-api.ru..prod.js gzip	32.2 kB	32.2 kB	✓
pages-turbo....dev.js gzip	51.7 kB	51.8 kB	✓
pages-turbo...prod.js gzip	38.8 kB	38.8 kB	✓
pages.runtim...dev.js gzip	51.7 kB	51.8 kB	✓
pages.runtim..prod.js gzip	38.8 kB	38.8 kB	✓
server.runti..prod.js gzip	62.3 kB	62.3 kB	✓
Total	2.73 MB	2.73 MB	⚠️ +3.02 kB

📝 Changed Files (16 files)

Files with changes:

app-page-exp..ntime.dev.js
app-page-exp..time.prod.js
app-page-tur..ntime.dev.js
app-page-tur..time.prod.js
app-page-tur..ntime.dev.js
app-page-tur..time.prod.js
app-page.runtime.dev.js
app-page.runtime.prod.js
app-route-ex..ntime.dev.js
app-route-tu..ntime.dev.js
app-route-tu..ntime.dev.js
app-route.runtime.dev.js
pages-api-tu..ntime.dev.js
pages-api.runtime.dev.js
pages-turbo...ntime.dev.js
pages.runtime.dev.js

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- Add span for generateDynamicFlightRenderResult - Add runtime prerender spans for prefetch scenarios - Improve span naming consistency (prospective/final pattern)

nextjs-bot · 2026-01-23T06:17:01Z

Failing test suites

Commit: edd96f3 | About building and testing Next.js

pnpm test-start test/e2e/app-dir/segment-cache/prefetch-runtime/prefetch-runtime.test.ts (job)

runtime prefetching > in a private cache > can completely prefetch a page that uses cookies and no uncached IO (DD)

Expand output

● runtime prefetching › in a private cache › can completely prefetch a page that uses cookies and no uncached IO

apiRequestContext.fetch: socket hang up
Call log:
  - → GET http://localhost:37901/in-private-cache/cookies-only?_rsc=180y0
  -   user-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) HeadlessChrome/130.0.6723.31 Safari/537.36
  -   accept: */*
  -   accept-encoding: gzip,deflate,br
  -   cookie: testCookie=initialValue
  -   next-test-fetch-priority: low
  -   referer: http://localhost:37901/
  -   next-router-prefetch: 1
  -   next-router-segment-prefetch: /!KGRlZmF1bHQp/in-private-cache
  -   next-url: /
  -   rsc: 1
  -   sec-ch-ua: "Chromium";v="130", "HeadlessChrome";v="130", "Not?A_Brand";v="99"
  -   sec-ch-ua-mobile: ?0
  -   sec-ch-ua-platform: "Linux"

  225 |             // server; we pass the request to the server the immediately.
  226 |             result: (async () => {
> 227 |               const originalResponse = await page.request.fetch(request, {
      |                                                           ^
  228 |                 maxRedirects: 0,
  229 |               })
  230 |

  at fetch (lib/router-act.ts:227:59)
  at lib/router-act.ts:245:13
  at routeHandler (lib/router-act.ts:257:7)

● runtime prefetching › in a private cache › can completely prefetch a page that uses cookies and no uncached IO

apiRequestContext.fetch: socket hang up
Call log:
  - → GET http://localhost:37901/in-private-cache/cookies-only?_rsc=180y0
  -   user-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) HeadlessChrome/130.0.6723.31 Safari/537.36
  -   accept: */*
  -   accept-encoding: gzip,deflate,br
  -   cookie: testCookie=initialValue
  -   next-test-fetch-priority: low
  -   referer: http://localhost:37901/
  -   next-router-prefetch: 1
  -   next-router-segment-prefetch: /!KGRlZmF1bHQp/in-private-cache
  -   next-url: /
  -   rsc: 1
  -   sec-ch-ua: "Chromium";v="130", "HeadlessChrome";v="130", "Not?A_Brand";v="99"
  -   sec-ch-ua-mobile: ?0
  -   sec-ch-ua-platform: "Linux"

  225 |             // server; we pass the request to the server the immediately.
  226 |             result: (async () => {
> 227 |               const originalResponse = await page.request.fetch(request, {
      |                                                           ^
  228 |                 maxRedirects: 0,
  229 |               })
  230 |

  at fetch (lib/router-act.ts:227:59)
  at lib/router-act.ts:245:13
  at routeHandler (lib/router-act.ts:257:7)

nextjs-bot added created-by: Next.js team PRs by the Next.js team. type: next labels Jan 22, 2026

wyattjoh force-pushed the wyattjoh/app-render-tracing branch from e1b0e42 to 53e7e26 Compare January 22, 2026 20:58

feat(tracing): expand OpenTelemetry coverage for app render

edd96f3

- Add span for generateDynamicFlightRenderResult - Add runtime prerender spans for prefetch scenarios - Improve span naming consistency (prospective/final pattern)

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feat(tracing): add OpenTelemetry spans to prerenderToStream phases #88914

feat(tracing): add OpenTelemetry spans to prerenderToStream phases #88914

wyattjoh commented Jan 22, 2026 •

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nextjs-bot commented Jan 22, 2026

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nextjs-bot commented Jan 22, 2026

Stats from current PR

🔴 1 regression

⚡ Dev Server

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⚡ Production Builds

📦 Production Builds (Webpack)

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wyattjoh commented Jan 22, 2026 •

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