HTTP 409 cache or propagation drift response

When this happens

Use this when the panel says fixed but some users still see the old error. Do not stop at the screen message; validate browser cache, CDN cache, DNS TTL, panel propagation, and stale application cache first.

Symptom checklist

• 409 appears repeatedly in the HTTP UI or logs.
• status code, Location header, method, body size differs between successful and failed requests.
• The issue appears only after separating client request from server router decision.
• It often follows deploys, permission changes, configuration edits, or data refreshes.

Likely causes

• 409 specifically changes the investigation surface for HTTP: verify the exact failing object, route, user, and timestamp before applying the broader pattern.
• Two requests write the same logical record at the same time.
• A retry repeats an insert without an idempotency key.
• The unique index or remote branch is correct but the application does not handle conflict paths.
• Transaction lock order differs between workers.
• A batch job reprocesses already imported rows.
• For the cache or propagation drift case, the first useful clue is browser cache, CDN cache, DNS TTL, panel propagation, and stale application cache.

First 1-minute checks

1. Write down the first failure time, latest change, affected user, path, and object ID.
2. Compare status code, Location header, method, body size for success and failure in the same window.
3. Test the hypothesis: duplicate or lock conflict caused by concurrent writes, missing idempotency, unique keys, transaction order, or remote state drift.
4. Classify this as cache or propagation drift: browser cache, CDN cache, DNS TTL, panel propagation, and stale application cache.
5. Capture current values before changing configuration.

First evidence

Treat 409 as a cache or propagation drift case. First collect evidence for browser cache, CDN cache, DNS TTL, panel propagation, and stale application cache.

Output examples

Normal output

Fresh requests and DNS answers show the same intended value from multiple networks.

Failing output

Only one resolver, browser, CDN edge, or cached object still returns the old value.

Output-to-action branches

• The panel says fixed but some users still see the old error.
  Separate DNS, CDN, browser, and application cache, then purge only the stale layer.
• The working and failing outputs differ.
  Act on the differing layer first: For 409, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Command output is normal but users still fail.
  Separate browser cache, cookies, permissions, and network location before declaring it fixed.

Do not do this

• Do not repeatedly edit correct DNS records just because a stale resolver is still visible.
• Do not change multiple layers before identifying the failing layer.
• Do not delete production data, grant broad permissions, or disable security controls as a first response.

Evidence quality

Auto-generated operator draft: includes issue-specific causes, commands, output branches, and unsafe-action warnings. Official-source links and real incident validation are queued for enrichment.

Commands to run first

grep -R "409" ./logs

curl -i https://example.com/path

curl -X OPTIONS -i https://example.com/path

grep -R " 409 " ./logs

tail -n 200 /var/log/nginx/access.log

grep -R "duplicate\|deadlock\|conflict\|lock\|rejected" ./logs

git fetch --all --prune

git status --short --branch

curl -I https://example.com -H 'Cache-Control: no-cache' && dig +trace example.com

Fix order

1. Record the full 409 message, failing URL, user, object ID, and latest change.
2. Collect issue-specific evidence for duplicate or lock conflict caused by concurrent writes, missing idempotency, unique keys, transaction order, or remote state drift.
3. Compare the failing case with a successful case before editing settings.
4. If this is the cache or propagation drift branch, Separate DNS, CDN, browser, and application cache, then purge only the stale layer.
5. Re-check with the same command and URL, then record the normal output.

Actions by cause

• For 409, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Add an idempotency key or upsert path for repeated writes.
• Use unique constraints intentionally and handle duplicate exceptions.
• Sort lock acquisition order consistently.
• Split batch reprocessing from live writes.
• Fetch remote state before retrying rejected pushes or tags.
• For the cache or propagation drift branch, Separate DNS, CDN, browser, and application cache, then purge only the stale layer.

Evidence links

• MDN HTTP status codes official
• MDN CORS errors official

Verification metadata

• operator-draft
• official-reference-linked
• 2026-07-23

Update queue

• Review cadence
  weekly-source-review
• Next enrichment
  Add one official-source check and one real output example for HTTP 409.

Environment-specific checks

• Shared hosting, proxies, VPNs, or CDN layers can change client request from server router decision results.
• Do not trust only the HTTP UI; compare command output.
• Japanese hosting panels may show completion before DNS or SSL fully propagates.
• Test from both office and external networks.

Prevent it next time

• Store normal examples for status code, Location header, method, body size.
• Add rewrite, redirect, method allow, and upload limits to the release checklist.
• Keep recurring errors in the same note format.
• Split alerts by error rate, latency, certificates, disk, and permission changes.

Related notes

• Static Hosting 404 fix note
• API 429 fix note
• Browser API CORS fix note
• Static Hosting 404 fix note 2
• API 429 fix note 2
• Browser API CORS fix note 2
• Static Hosting 404 fix note 3
• API 429 fix note 3