fix: direct ReadableStream perf blow-up (issue #1741) by aymaneallaoui · Pull Request #1772 · elysiajs/elysia

aymaneallaoui · 2026-03-01T23:31:50Z

Fixes 1741

Direct ReadableStream responses were super slow cuz binary chunks were getting JSON.stringify instead of streamed as bytes.

Changes

treat binary chunks (Blob/ArrayBuffer/typed views) as raw bytes
keep SSE + object stream behavior the same
add regression tests for binary stream cases

Result

Direct ReadableStream perf is now in the same range as new Response(body) on large real-world streams, without breaking existing stream behavior.

some perf metrics

before: direct 100MB stream could take ~26s (100% CPU vibe mentioned on issue), while wrapped response was ~0.2s
after: direct and wrapped are now in the same ballpark (~0.14s vs ~0.13s in local checks)

Also did randomized binary fuzz-style checks and concurrent large-stream stress runs to make sure this doesn’t regress under load.

ps: i tested all of it on same machine so yup

Summary by CodeRabbit

Release Notes

Improvements
- Enhanced binary data streaming support with improved handling of various binary formats (Uint8Array, Blob, ArrayBuffer, and buffer views).
- Optimized stream initialization and chunk processing for better reliability and consistency across SSE, binary, and JSON data types.
Tests
- Expanded streaming test coverage with validation of binary data handling and mixed binary source scenarios.

coderabbitai · 2026-03-01T23:32:11Z

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ℹ️ Recent review info

Configuration used: Organization UI

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Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 2c4c117 and 437729e.

📒 Files selected for processing (2)

src/adapter/utils.ts
test/response/stream.test.ts

Walkthrough

The PR refactors stream handling in adapter utilities by introducing a centralized enqueueChunk function that standardizes processing of different chunk types (binary, SSE, JSON/string). Helpers for Promise detection and binary data typing are added, along with three new test cases validating binary stream handling across various data types.

Changes

Cohort / File(s)	Summary
Stream handling refactorization `src/adapter/utils.ts`	Introduces `enqueueChunk` helper to centralize chunk processing logic. Adds utilities for Promise-like detection and binary chunk typing. Reworks stream initialization and generator item processing to use unified enqueue flow with conditional promise awaiting. Extends support for Blob, ArrayBuffer, Uint8Array, and ArrayBuffer views while maintaining SSE formatting.
Binary stream test coverage `test/response/stream.test.ts`	Adds three test cases: streaming large Uint8Array in subchunks, mixed binary views (Uint8Array slice, DataView, Blob), and generator-yielded Uint8Array chunks. All tests validate exact byte-for-byte correctness of binary data handling.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

Fix SSE double-wrapping bug when returning pre-formatted Response (#1661) #1663: Modifies SSE formatting logic in src/adapter/utils.ts with a skipFormat flag approach, overlapping the stream handling refactorization area.

Poem

🐰 Hopping through streams with organized care,
Chunks enqueued in a unified flow,
Binary data now knows where to go,
Promises await, and tests verify so,
A refactored path—efficient and fair! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly addresses the main change: fixing a performance issue with direct ReadableStream responses by preventing unnecessary JSON stringification of binary chunks.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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pkg-pr-new · 2026-03-01T23:32:44Z

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npm i https://pkg.pr.new/elysia@1772

commit: 3bf7566

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Pull request overview

This PR fixes a severe performance regression (issue #1741) where returning a ReadableStream directly from a route handler caused 100% CPU usage and 100x+ latency compared to wrapping the stream in new Response(body). The root cause was that binary chunks (Uint8Array, ArrayBuffer, typed views) were being passed through JSON.stringify before being enqueued, instead of being streamed as raw bytes.

Changes:

Adds isBinaryChunk, enqueueBinaryChunk, and isPromiseLike helpers to src/adapter/utils.ts for fast-path binary chunk handling in createStreamHandler
Refactors the stream loop's chunk dispatch into an enqueueChunk closure that routes binary types as raw bytes and text/SSE types through the existing JSON serialization path
Adds three regression tests covering large binary payloads, mixed typed-view/Blob chunks, and generator-function binary yields

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
`src/adapter/utils.ts`	Adds binary chunk helpers and refactors `createStreamHandler`'s enqueue logic to bypass JSON serialization for binary data
`test/response/stream.test.ts`	Adds three tests covering `ReadableStream` binary chunks, mixed view/Blob chunks, and generator binary output

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Copilot · 2026-03-01T23:37:09Z

src/adapter/utils.ts

+						if (!isSSE && isBinaryChunk(chunk))
+							return enqueueBinaryChunk(controller, chunk)


When a generator function yields a binary type (Uint8Array, ArrayBuffer, Blob, etc.) as its first value, init.value is that binary object. Since typeof Uint8Array === 'object' (and similarly for all ArrayBufferView/ArrayBuffer/Blob), the contentType determination (which reads init?.value && typeof init?.value === 'object') evaluates to 'application/json' — even though enqueueChunk now correctly emits raw bytes for that chunk. This means the response will advertise Content-Type: application/json while delivering binary bytes, which is incorrect.

The isBinaryChunk predicate should be used in the contentType conditional to detect binary init values and return 'application/octet-stream' instead.

Copilot · 2026-03-01T23:37:09Z

test/response/stream.test.ts

+
+		const response = await app.handle(req('/'))
+		const result = new Uint8Array(await response.arrayBuffer())
+


The test verifies that binary bytes from a generator are transmitted correctly but does not assert the Content-Type header. Because Uint8Array is an object, the existing contentType logic (lines 227-231 in src/adapter/utils.ts) sets Content-Type: application/json for a generator that yields a Uint8Array as its first chunk, even though the body now contains raw bytes. A content-type assertion would catch this regression.

Suggested change

expect(response.headers.get('content-type')).toMatch(

/^application\/octet-stream\b/i

)

SaltyAom · 2026-03-16T22:06:43Z

due to a merge conflict from #1803, I have rewrite your code to merge with backpressure-based stream and include the test cases and mentioned in CHANGELOG.md

Thanks for your contribution

fix: pass through binary ReadableStream chunks without JSON stringifying

437729e

Copilot AI review requested due to automatic review settings March 1, 2026 23:31

Copilot started reviewing on behalf of aymaneallaoui March 1, 2026 23:32 View session

Copilot AI reviewed Mar 1, 2026

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SaltyAom closed this in ccbd9e4 Mar 16, 2026

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fix: direct ReadableStream perf blow-up (issue #1741)#1772

fix: direct ReadableStream perf blow-up (issue #1741)#1772
aymaneallaoui wants to merge 1 commit intoelysiajs:mainfrom
aymaneallaoui:fix/readable-stream-cpu-latency

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		if (!isSSE && isBinaryChunk(chunk))
		return enqueueBinaryChunk(controller, chunk)


		const response = await app.handle(req('/'))
		const result = new Uint8Array(await response.arrayBuffer())

+		expect(response.headers.get('content-type')).toMatch(
+			/^application\/octet-stream\b/i
+		)

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Changes

Result

some perf metrics

Summary by CodeRabbit

Release Notes

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Changes

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aymaneallaoui commented Mar 1, 2026 •

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