aiohttp DNSResolver objects churn on transient requests

The Fix

Fixes excessive DNSResolver object churn when using multiple sessions by implementing a shared resolver management system.

Based on closed aio-libs/aiohttp issue #10847 · 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%.

Jump to Verify Open PR/Commit

@@ -0,0 +1,5 @@
@@ -0,0 +1,5 @@
+Implemented shared DNS resolver management to fix excessive resolver object creation
+when using multiple client sessions. The new ``_DNSResolverManager`` singleton ensures
+only one ``DNSResolver`` object is created for default configurations, significantly

repro.py

/* Initialize channel to run queries, a single channel can accept unlimited
   * queries */
  if (ares_init_options(&channel, &options, optmask) != ARES_SUCCESS) {
    printf("c-ares initialization issue\n");
    return 1;
  }

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Apply the official fix\nFixes excessive DNSResolver object churn when using multiple sessions by implementing a shared resolver management system.\nWhen NOT to use: This fix is not applicable if custom resolver configurations are required for each session.\n\n

Why This Fix Works in Production

Trigger: ```python-traceback
Mechanism: Excessive DNSResolver object creation occurs when multiple client sessions are used without a shared resolver

Production impact:

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

Excessive DNSResolver object creation occurs when multiple client sessions are used without a shared resolver
Production symptom (often without a traceback): ```python-traceback

Proof / Evidence

GitHub issue: #10847
Fix PR: https://github.com/aio-libs/aiohttp/pull/10897
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.70
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.68

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“Also we don't benchmark requests without a session so we likely have a blindspot here”

@bdraco · 2025-05-09 · source

“https://github.com/aio-libs/aiohttp/pull/10848 As suspected <img width="658" alt="Image" src="https://github.com/user-attachments/assets/8054b766-12dc-4031-b86d-2a75c618cf1a" /> And its reading files in the event loop 🙈”

@bdraco · 2025-05-09 · source

“Too many DNSResolver instances is indeed a problem for Home Assistant Supervisor too”

@agners · 2025-05-13 · source

“Some failure modes appear to be quite severe See related discussion: https://github.com/home-assistant/core/issues/144802”

@bdraco · 2025-05-15 · source

Failure Signature (Search String)

```python-traceback
Some failure modes appear to be quite severe

Copy-friendly signature

signature.txt

Failure Signature
-----------------
```python-traceback
Some failure modes appear to be quite severe

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
```python-traceback
Some failure modes appear to be quite severe

Minimal Reproduction

repro.py

/* Initialize channel to run queries, a single channel can accept unlimited
   * queries */
  if (ares_init_options(&channel, &options, optmask) != ARES_SUCCESS) {
    printf("c-ares initialization issue\n");
    return 1;
  }

What Broke

Multiple DNSResolver instances lead to increased memory usage and potential performance degradation.

Why It Broke

Excessive DNSResolver object creation occurs when multiple client sessions are used without a shared resolver

Fix Options (Details)

Option A — Apply the official fix

Fixes excessive DNSResolver object churn when using multiple sessions by implementing a shared resolver management system.

When NOT to use: This fix is not applicable if custom resolver configurations are required for each session.

Fix reference: https://github.com/aio-libs/aiohttp/pull/10897

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix is not applicable if custom resolver configurations are required for each session.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

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.

Cluster: aiohttp:configuration-error All hubs All clusters

Related clusters: Data consistency Race condition Auth

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

aiohttp DNSResolver objects churn on transient requests without a session (Fix)