Turbopack: Implement server hmr in nodejs dev runtime by wbinnssmith · Pull Request #89130 · vercel/next.js

wbinnssmith · 2026-01-27T22:22:50Z

This implements the remainder of functionality needed for Server HMR (Fast Refresh).

How It Works

Turbopack detects file changes in RSC modules
Rust sends HMR update payloads to the Node.js runtime via the existing chunk update system
A Node-specific HMR client evaluates changed modules using shared HMR runtime logic (extracted earlier in this stack)
Only modified modules are re-evaluated. Unmodified modules retain their state
Next.js hot-reloader forwards updates to RSC workers, triggering automatic re-renders in clients

Test Plan: Added an e2e test confirming that unaffected modules don't get re-evaluated when others change.

wbinnssmith · 2026-01-27T22:23:28Z

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nextjs-bot · 2026-01-27T22:39:57Z

Tests Passed

nextjs-bot · 2026-01-27T22:45:21Z

Stats from current PR

✅ No significant changes detected

📊 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)	438ms	439ms	✓	▁▆▅▂▆
Cold (First Request)	1.185s	1.165s	✓	▁██▁█
Warm (Listen)	456ms	456ms	✓	▁▁▁▁▁
Warm (Ready in log)	443ms	442ms	✓	▁▁▁▂▁
Warm (First Request)	338ms	339ms	✓	▁▁▆▅▅

📦 Dev Server (Webpack) (Legacy)

📦 Dev Server (Webpack)

Metric	Canary	PR	Change	Trend
Cold (Listen)	456ms	456ms	✓	▁▁▁▅▁
Cold (Ready in log)	440ms	440ms	✓	▄▃▄▅▃
Cold (First Request)	1.849s	1.841s	✓	▂▂▃▆▁
Warm (Listen)	456ms	456ms	✓	▁▁▁▅▁
Warm (Ready in log)	440ms	438ms	✓	▃▃▃▅▃
Warm (First Request)	1.849s	1.875s	✓	▂▂▃▅▂

⚡ Production Builds

Metric	Canary	PR	Change	Trend
Fresh Build	3.829s	3.799s	✓	▁▁▁▃▁
Cached Build	3.810s	3.773s	✓	▁▁▁▃▁

📦 Production Builds (Webpack) (Legacy)

📦 Production Builds (Webpack)

Metric	Canary	PR	Change	Trend
Fresh Build	13.909s	13.873s	✓	▁▂▃▅▁
Cached Build	13.925s	13.931s	✓	▁▂▃▅▁
node_modules Size	467 MB	467 MB	✓	▁▁▁▁▁

📦 Bundle Sizes

Bundle Sizes

⚡ Turbopack

Client

Main Bundles: **437 kB** → **437 kB** ⚠️ +8 B

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

Server

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	758 B	757 B	✓
Total	758 B	757 B	✅ -1 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	451 B	449 B	✓
Total	451 B	449 B	✅ -2 B

📦 Webpack

Client

Main Bundles

	Canary	PR	Change
5528-HASH.js gzip	5.47 kB	N/A	-
6280-HASH.js gzip	57 kB	N/A	-
6335.HASH.js gzip	169 B	N/A	-
912-HASH.js gzip	4.53 kB	N/A	-
e8aec2e4-HASH.js gzip	62.5 kB	N/A	-
framework-HASH.js gzip	59.7 kB	59.7 kB	✓
main-app-HASH.js gzip	255 B	254 B	✓
main-HASH.js gzip	39.1 kB	39.1 kB	✓
webpack-HASH.js gzip	1.68 kB	1.68 kB	✓
262-HASH.js gzip	N/A	4.53 kB	-
2889.HASH.js gzip	N/A	169 B	-
5602-HASH.js gzip	N/A	5.49 kB	-
6948ada0-HASH.js gzip	N/A	62.5 kB	-
9544-HASH.js gzip	N/A	57.7 kB	-
Total	230 kB	231 kB	⚠️ +631 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	194 B	✓
_error-HASH.js gzip	183 B	180 B	🟢 3 B (-2%)
css-HASH.js gzip	331 B	330 B	✓
dynamic-HASH.js gzip	1.81 kB	1.81 kB	✓
edge-ssr-HASH.js gzip	256 B	256 B	✓
head-HASH.js gzip	351 B	352 B	✓
hooks-HASH.js gzip	384 B	383 B	✓
image-HASH.js gzip	580 B	581 B	✓
index-HASH.js gzip	260 B	260 B	✓
link-HASH.js gzip	2.49 kB	2.49 kB	✓
routerDirect..HASH.js gzip	320 B	319 B	✓
script-HASH.js gzip	386 B	386 B	✓
withRouter-HASH.js gzip	315 B	315 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Total	7.97 kB	7.97 kB	✅ -1 B

Server

Edge SSR

	Canary	PR	Change
edge-ssr.js gzip	126 kB	126 kB	✓
page.js gzip	249 kB	250 kB	✓
Total	375 kB	376 kB	⚠️ +394 B

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	613 B	615 B	✓
middleware-r..fest.js gzip	156 B	155 B	✓
middleware.js gzip	33.2 kB	33.2 kB	✓
edge-runtime..pack.js gzip	842 B	842 B	✓
Total	34.8 kB	34.8 kB	✅ -79 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	733 B	735 B	✓
Total	733 B	735 B	⚠️ +2 B

Build Cache

	Canary	PR	Change
0.pack gzip	3.84 MB	3.85 MB	🔴 +5.54 kB (+0%)
index.pack gzip	103 kB	102 kB	🟢 1.09 kB (-1%)
index.pack.old gzip	103 kB	103 kB	✓
Total	4.05 MB	4.06 MB	⚠️ +4.63 kB

🔄 Shared (bundler-independent)

Runtimes

	Canary	PR	Change
app-page-exp...dev.js gzip	316 kB	316 kB	✓
app-page-exp..prod.js gzip	168 kB	168 kB	✓
app-page-tur...dev.js gzip	315 kB	315 kB	✓
app-page-tur..prod.js gzip	167 kB	167 kB	✓
app-page-tur...dev.js gzip	312 kB	312 kB	✓
app-page-tur..prod.js gzip	166 kB	166 kB	✓
app-page.run...dev.js gzip	312 kB	312 kB	✓
app-page.run..prod.js gzip	166 kB	166 kB	✓
app-route-ex...dev.js gzip	70.5 kB	70.5 kB	✓
app-route-ex..prod.js gzip	49 kB	49 kB	✓
app-route-tu...dev.js gzip	70.5 kB	70.5 kB	✓
app-route-tu..prod.js gzip	49 kB	49 kB	✓
app-route-tu...dev.js gzip	70.1 kB	70.1 kB	✓
app-route-tu..prod.js gzip	48.8 kB	48.8 kB	✓
app-route.ru...dev.js gzip	70.1 kB	70.1 kB	✓
app-route.ru..prod.js gzip	48.8 kB	48.8 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	43.2 kB	43.2 kB	✓
pages-api-tu..prod.js gzip	32.9 kB	32.9 kB	✓
pages-api.ru...dev.js gzip	43.2 kB	43.2 kB	✓
pages-api.ru..prod.js gzip	32.8 kB	32.8 kB	✓
pages-turbo....dev.js gzip	52.5 kB	52.5 kB	✓
pages-turbo...prod.js gzip	39.4 kB	39.4 kB	✓
pages.runtim...dev.js gzip	52.5 kB	52.5 kB	✓
pages.runtim..prod.js gzip	39.4 kB	39.4 kB	✓
server.runti..prod.js gzip	62.7 kB	62.7 kB	✓
Total	2.8 MB	2.8 MB	✅ -1 B

codspeed-hq · 2026-01-28T00:55:55Z

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 3 skipped benchmarks¹

_{Comparing wbinnssmith/server-hmr-runtime-nodejs (6ca6196) with canary (486f674)}

3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

sokra · 2026-02-10T20:03:32Z

+ * Contains chunk loading, module caching, and other non-HMR functionality.
+ */

 process.env.TURBOPACK = '1'


Unrelated, but should we really write to the global process env in our runtime code...

sokra · 2026-02-10T20:06:04Z

+  }
+}
+
+globalThis.__turbopack_server_hmr_apply__ = __turbopack_server_hmr_apply__


This doesn't need to be a global. We could expose that as export from the turbopack runtime chunk.

-> Follow-up PR

nextjs-bot · 2026-02-12T06:00:04Z

Allow CI Workflow Run

approve CI run for commit: 6ca6196

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

nextjs-bot · 2026-02-12T06:00:04Z

Allow CI Workflow Run

approve CI run for commit: 6ca6196

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

wbinnssmith mentioned this pull request Jan 27, 2026

Turbopack: Server HMR infrastructure #88870

Merged

wbinnssmith mentioned this pull request Jan 27, 2026

Turbopack: extract core hmr client logic into shared runtime #88912

Merged

vercel Bot reviewed Jan 27, 2026

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Comment thread turbopack/crates/turbopack-ecmascript-runtime/js/src/nodejs/runtime-base.ts Outdated

wbinnssmith force-pushed the wbinnssmith/server-hmr-runtime-refactor branch from 2019ed4 to 0bf0bbd Compare January 28, 2026 00:49

wbinnssmith force-pushed the wbinnssmith/server-hmr-runtime-nodejs branch from f46c1a1 to aecb159 Compare January 28, 2026 00:49

nextjs-bot added created-by: Turbopack team PRs by the Turbopack team. tests Turbopack Related to Turbopack with Next.js. type: next labels Jan 28, 2026

wbinnssmith changed the title ~~add test~~ Turbopack: Implement server hmr in nodejs dev runtime Jan 28, 2026

wbinnssmith force-pushed the wbinnssmith/server-hmr-runtime-refactor branch from 0bf0bbd to 0d41977 Compare January 28, 2026 10:25

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nextjs-bot added Documentation Related to Next.js' official documentation. examples Issue was opened via the examples template. labels Feb 4, 2026

wbinnssmith force-pushed the wbinnssmith/server-hmr-runtime-nodejs branch from 8d84640 to 2eddffe Compare February 4, 2026 01:25

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wbinnssmith mentioned this pull request Feb 4, 2026

Turbopack: Reorganize runtime files into shared/runtime directory #89461

Merged

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wbinnssmith mentioned this pull request Feb 5, 2026

Turbopack: make JavaScript runtimes moduleCache a Map #89515

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sokra reviewed Feb 5, 2026

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Comment thread turbopack/crates/turbopack-ecmascript-runtime/js/src/nodejs/dev/hmr-client.ts Outdated

sokra reviewed Feb 10, 2026

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sokra approved these changes Feb 10, 2026

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wbinnssmith added 2 commits February 11, 2026 11:11

add test

45ca5ba

Move browser HMR client to browser/runtime/base

6ca6196

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Stats from current PR

✅ No significant changes detected

⚡ Dev Server

📦 Dev Server (Webpack)

⚡ Production Builds

📦 Production Builds (Webpack)

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⚡ Turbopack

📦 Webpack

🔄 Shared (bundler-independent)

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Merging this PR will not alter performance

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