The Fix
Adds support for switching the zlib implementation by allowing users to set the zlib backend using `aiohttp.set_zlib_backend()`.
Based on closed aio-libs/aiohttp issue #9798 · PR/commit linked
Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.
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+Allow user setting zlib compression backend -- by :user:`TimMenninger`
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+This change allows the user to call :func:`aiohttp.set_zlib_backend()` with the
Option A — Apply the official fix\nAdds support for switching the zlib implementation by allowing users to set the zlib backend using `aiohttp.set_zlib_backend()`.\nWhen NOT to use: This fix should not be used if the application relies on a specific zlib implementation behavior.\n\n
Why This Fix Works in Production
- Trigger: Users experienced performance degradation due to the inability to switch zlib backends.
- Mechanism: The library lacked support for user-defined zlib implementations, impacting performance
- If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.
Why This Breaks in Prod
- The library lacked support for user-defined zlib implementations, impacting performance
- Production symptom (often without a traceback): Users experienced performance degradation due to the inability to switch zlib backends.
Proof / Evidence
- GitHub issue: #9798
- Fix PR: https://github.com/aio-libs/aiohttp/pull/10700
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.70
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.81
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Yes, as I mentioned at that time, we should keep the binary tests as internal and add new tests which verify the decompressed results”
“I'd note that we do have downstreams that use zlib-ng as the default zlib library”
“I recall seeing that. Do you happen to remember the issue where it was reported. It would be good to make sure the tests are…”
“found https://github.com/aio-libs/aiohttp/issues/7255”
Failure Signature (Search String)
- Users experienced performance degradation due to the inability to switch zlib backends.
Copy-friendly signature
Failure Signature
-----------------
Users experienced performance degradation due to the inability to switch zlib backends.
Error Message
Signature-only (no traceback captured)
Error Message
-------------
Users experienced performance degradation due to the inability to switch zlib backends.
What Broke
Users experienced performance degradation due to the inability to switch zlib backends.
Why It Broke
The library lacked support for user-defined zlib implementations, impacting performance
Fix Options (Details)
Option A — Apply the official fix
Adds support for switching the zlib implementation by allowing users to set the zlib backend using `aiohttp.set_zlib_backend()`.
Fix reference: https://github.com/aio-libs/aiohttp/pull/10700
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be used if the application relies on a specific zlib implementation behavior.
Did This Fix Work in Your Case?
Quick signal helps us prioritize which fixes to verify and improve.
Prevention
- Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
- Pin production dependencies and upgrade only with a reproducible test that hits the failing path.
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.