diagnostics_channel: add BoundedChannel and scopes by Qard · Pull Request #61680 · nodejs/node · GitHub

This adds BoundedChannel, adds a using syntax equivalent to runStores, and modifies the internals of TracingChannel to use these to avoid closures in several places.

Why BoundedChannel?

Reviewers asked why a new class is needed instead of just using the existing channel() or tracingChannel() APIs. Here's the motivation:

vs channel(): A bare Channel has no built-in concept of start/end lifecycle events or store binding for async context propagation. BoundedChannel combines two channels with a withScope() / run() API that ties start → store entry → user code → end → store exit into a single unit, which a raw channel can't express.
vs tracingChannel(): TracingChannel is designed for tracing operations with async continuations (it has 5 events: start, end, asyncStart, asyncEnd, error). BoundedChannel is a simpler, synchronous-only primitive with just start and end. It is also used internally by TracingChannel to implement its call window and continuation window, making the code cleaner and removing several closure allocations.

The name "Bounded" reflects that the scope is bounded by the using block — start fires when the scope begins, end fires when it is disposed.

Depends on #61674

cc @nodejs/diagnostics

Review requested:

@nodejs/web-infra

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https://github.com/nodejs/node/actions/runs/21678728231

Could you please expand on PR description on why this is needed? We have channels, tracingChannels and now windowChannel, it would be great to know for which kind of situations we need each one of them.

Suggested change

      
            > Stability: 1 - Experimental
          
            > Stability: 1 - Experimental

Perhaps 1.1 Active Development instead?

The particular change is inherited from #61674. Do you want that made there? What about the new APIs in diagnostics_channel? Same status?

WindowChannel is mainly just to enable using syntax scopes, but is exposed as there are possible use cases where one may want to emit events around a block/scope without the greater complexity of TracingChannel.

It's also replacing most of the internals of TracingChannel to make it more coherent since there seemed to be a lot of misunderstanding of how it worked. Now TracingChannel is just a WindowChannel around start + end, a WindowChannel around asyncStart + asyncEnd, and an error channel.

Codecov Report

❌ Patch coverage is 98.24561% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.72%. Comparing base (5b6091c) to head (c3c4886).
⚠️ Report is 34 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/diagnostics_channel.js	98.24%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61680      +/-   ##
==========================================
+ Coverage   89.71%   89.72%   +0.01%     
==========================================
  Files         692      692              
  Lines      213986   214167     +181     
  Branches    41055    41077      +22     
==========================================
+ Hits       191969   192153     +184     
- Misses      14089    14102      +13     
+ Partials     7928     7912      -16

Files with missing lines	Coverage Δ
lib/diagnostics_channel.js	98.13% <98.24%> (-0.36%)	⬇️

... and 46 files with indirect coverage changes

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This name window channel is confusing. It's not related to the specific window API in JavaScript. It reads to me that it is a version of tracing channel that enables using support. I feel terms like scope could be a more intuitive name.

I feel "scope" is an equally problematic name in that it makes many think of lexical scope which is typically associated with closures, which is exactly what this is explicitly trying to not be. Naming is hard. 🤷🏻

I'm happy to pick a different name if you have a better idea for one, but I'm not convinced "scope" is much better. I had also considered "block" but that also has weird associations with "blocking" which is again misleading. 😐

I went with BoundedChannel. Hopefully that is clear and unique enough naming.

CI: https://ci.nodejs.org/job/node-test-pull-request/72224/

CI: https://ci.nodejs.org/job/node-test-pull-request/72314/

CI: https://ci.nodejs.org/job/node-test-pull-request/72320/

CI: https://ci.nodejs.org/job/node-test-pull-request/72322/

CI: https://ci.nodejs.org/job/node-test-pull-request/72326/

CI: https://ci.nodejs.org/job/node-test-pull-request/72351/

CI: https://ci.nodejs.org/job/node-test-pull-request/72357/

Landed in cb78a7f

That PR doesn't land cleanly on existing release lines and is using the using keyword, which is only available on main. I've added the labels

PR-URL: #61680 Reviewed-By: James M Snell <jasnell@gmail.com>

@Qard The BoundedChannel constructor is publicly exposed here, whereas the TracingChannel constructor is not. Which way round do we want these?

Qard self-assigned this Feb 4, 2026

Qard added the diagnostics_channel Issues and PRs related to diagnostics channel label Feb 4, 2026

nodejs-github-bot added lib / src Issues and PRs related to general changes in the lib or src directory. needs-ci PRs that need a full CI run. labels Feb 4, 2026

Qard added the request-ci Add this label to start a Jenkins CI on a PR. label Feb 4, 2026

Qard force-pushed the dc-windows branch from 9794b2b to 9218a73 Compare February 4, 2026 16:00

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Qard force-pushed the dc-windows branch 7 times, most recently from c764f06 to 1018c57 Compare February 4, 2026 17:50

RafaelGSS reviewed Feb 4, 2026

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Qard force-pushed the dc-windows branch from 1018c57 to 12f7bc9 Compare February 5, 2026 11:55

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Qard mentioned this pull request Feb 12, 2026

async_hooks: add using scopes to AsyncLocalStorage #61674

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Qard changed the title diagnostics_channel: add WindowChannel and scopes diagnostics_channel: add BoundedChannel and scopes Mar 17, 2026

Qard force-pushed the dc-windows branch 2 times, most recently from 47e089b to bf59a1f Compare March 17, 2026 20:21

Qard mentioned this pull request Mar 20, 2026

Session Proposal: Observability infrastructure openjs-foundation/summit#485

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Qard force-pushed the dc-windows branch 2 times, most recently from 3f4d7df to b4ce8e9 Compare March 28, 2026 21:02

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diagnostics_channel: add BoundedChannel and scopes

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Qard force-pushed the dc-windows branch from b4ce8e9 to c3c4886 Compare March 29, 2026 12:17

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2026-04-01, Version 25.9.0 (Current) #62522

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aduh95 added backport-requested-v24.x PRs awaiting manual backport to the v24.x-staging branch. dont-land-on-v25.x labels Mar 31, 2026

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diagnostics_channel: add BoundedChannel and scopes

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PR-URL: #61680 Reviewed-By: James M Snell <jasnell@gmail.com>

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2026-05-07, Version 26.1.0 #63137

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Renegade334 mentioned this pull request Jul 1, 2026

Segfaults with [await] using within node core #64230

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               });
               ```
+              #### `diagnostics_channel.windowChannel(nameOrChannels)`

diagnostics_channel: add BoundedChannel and scopes by Qard · Pull Request #61680 · nodejs/node · GitHub

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diagnostics_channel: add BoundedChannel and scopes#61680nodejs-github-bot merged 1 commit intonodejs:mainnodejs/node:mainfrom Qard:dc-windowsQard/node:dc-windowsCopy head branch name to clipboard

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diagnostics_channel: add BoundedChannel and scopes#61680
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