Monitoring synthetic check proxy versus origin split response

When this happens

Use this when a cdn, load balancer, reverse proxy, or waf sits in front of the service. Do not stop at the screen message; validate whether the edge/proxy or the origin server creates the failure first.

Symptom checklist

• synthetic check appears repeatedly in the Monitoring UI or logs.
• metrics, log spikes, traces, SLO, synthetic checks differs between successful and failed requests.
• The issue appears only after separating alert noise from real incident signal.
• It often follows deploys, permission changes, configuration edits, or data refreshes.

Likely causes

• synthetic check specifically changes the investigation surface for Monitoring: verify the exact failing object, route, user, and timestamp before applying the broader pattern.
• The health check exercises a different dependency or path than real users.
• Metrics or logs are missing labels needed to separate real incidents from noise.
• Alert thresholds fire on normal batch or deploy behavior.
• A container restarts before readiness or dependency warmup completes.
• SLO burn is visible before any single log line looks severe.
• For the proxy versus origin split case, the first useful clue is whether the edge/proxy or the origin server creates the failure.

First 1-minute checks

1. Write down the first failure time, latest change, affected user, path, and object ID.
2. Compare metrics, log spikes, traces, SLO, synthetic checks for success and failure in the same window.
3. Test the hypothesis: operations failure caused by failing health checks, missing telemetry, noisy alert thresholds, deployment loop, or SLO burn.
4. Classify this as proxy versus origin split: whether the edge/proxy or the origin server creates the failure.
5. Capture current values before changing configuration.

First evidence

Treat synthetic check as a proxy versus origin split case. First collect evidence for whether the edge/proxy or the origin server creates the failure.

Output examples

Normal output

Direct origin and routed proxy return equivalent status and headers.

Failing output

Origin succeeds but proxy fails, or proxy succeeds while direct origin exposes the real backend error.

Output-to-action branches

• A CDN, load balancer, reverse proxy, or WAF sits in front of the service.
  Test direct origin and routed traffic separately, then change only the layer that differs.
• The working and failing outputs differ.
  Act on the differing layer first: For synthetic check, 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 assume the CDN error code is the application root cause.
• 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 "synthetic check" ./logs

curl -s https://status.example.com

grep -R "synthetic check" ./logs | tail

promtool query instant http_requests_total

date -u

kubectl get pods -A || true

kubectl describe pod POD || true

grep -R "readiness\|healthcheck\|alert\|latency\|SLO" ./logs

curl -Iv https://example.com --resolve example.com:443:ORIGIN_IP

Fix order

1. Record the full synthetic check message, failing URL, user, object ID, and latest change.
2. Collect issue-specific evidence for operations failure caused by failing health checks, missing telemetry, noisy alert thresholds, deployment loop, or SLO burn.
3. Compare the failing case with a successful case before editing settings.
4. If this is the proxy versus origin split branch, Test direct origin and routed traffic separately, then change only the layer that differs.
5. Re-check with the same command and URL, then record the normal output.

Actions by cause

• For synthetic check, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Separate liveness, readiness, synthetic checks, and user traffic signals.
• Add service, route, status, and deployment labels to metrics.
• Tune alert thresholds using recent normal traffic windows.
• Delay readiness until dependencies are actually available.
• Create a short incident note with query links for the same signal.
• For the proxy versus origin split branch, Test direct origin and routed traffic separately, then change only the layer that differs.

Evidence links

• Prometheus alerting rules official
• Grafana alerting 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 Monitoring synthetic check.

Environment-specific checks

• Shared hosting, proxies, VPNs, or CDN layers can change alert noise from real incident signal results.
• Do not trust only the Monitoring 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 metrics, log spikes, traces, SLO, synthetic checks.
• Add alert thresholds, burn rate, trace sampling, runbook 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

• Monitoring High latency fix note
• Monitoring High latency fix note 2
• Monitoring High latency fix note 3
• Monitoring High latency fix note 4
• Monitoring High latency fix note 5
• Monitoring High latency fix note 6
• Monitoring high latency first triage response
• Monitoring error budget first triage response