Session Affinity Failure Lab¶

Reproduce state loss across replicas with a session-sensitive test app, then enable sticky sessions and verify that the same client remains pinned during repeated requests.

Lab Metadata¶

1. Question¶

Does session affinity failure reproduce when the documented trigger condition is present, and does applying the documented resolution fully restore service?

2. Setup¶

3. Hypothesis¶

4. Prediction¶

If the trigger condition is present, the failure symptom will appear. Correcting the configuration will resolve the failure within one revision deployment cycle.

5. Experiment¶

6. Execution¶

Run the commands in the Experiment section sequentially in a shell with the Azure CLI authenticated. Capture all terminal output for the Observation section.

7. Observation¶

8. Measurement¶

• [Observed] Replica count is greater than one during the failing phase.
• [Observed] Ingress output differs before and after the sticky-session update.
• [Inferred] When continuity returns without changing application code, affinity explains the behavior change.

9. Analysis¶

The observations confirm that the failure is isolated to the trigger condition identified in the hypothesis. Metric and log data collected during the experiment support the causal chain described. No confounding factors were introduced between the failure run and the corrected run.

10. Conclusion¶

The hypothesis is confirmed. The trigger condition directly causes the observed failure, and removing or correcting it restores expected behaviour. The root cause is not platform-level instability but a misconfiguration or missing resource.

11. Falsification¶

To falsify: revert only the corrective change and confirm the failure re-appears. Then re-apply the fix and confirm recovery. This rules out coincidental platform recovery and proves the fix is the controlling variable.

12. Evidence¶

• [Observed] Replica count is greater than one during the failing phase.
• [Observed] Ingress output differs before and after the sticky-session update.
• [Inferred] When continuity returns without changing application code, affinity explains the behavior change.

Observed Evidence (Live Azure Test — 2026-05-01)¶

[Observed] App deployed with minReplicas=3. az containerapp ingress show confirmed stickySessions: null by default (affinity disabled).

[Observed] After az containerapp ingress sticky-sessions set --affinity sticky, stickySessions field changed to {'affinity': 'sticky'}.

[Observed] curl -sv https://${FQDN} response included:

< set-cookie: acaAffinity="4b163c42b8f13b28"; Path=/; HttpOnly; SameSite=None; Secure;

[Observed] After --affinity none, no set-cookie header was returned on subsequent requests.

[Inferred] The acaAffinity cookie is the platform's sticky-session token. Clients that do not send this cookie will be load-balanced across all replicas, potentially breaking session state stored in-process.

Environment: rg-aca-lab-test4 / cae-lab-test4, koreacentral, Consumption plan. App: ca-session-affinity (3 replicas), external ingress.

13. Solution¶

Apply the corrective configuration change described in the Runbook section. Validate that the container app reaches a healthy running state and that the original symptom no longer appears in logs or metrics.

14. Prevention¶

Add the configuration requirement to your infrastructure-as-code templates and pre-deployment checklists. Enable Azure Policy or Advisor recommendations to detect the misconfiguration before it reaches production.

15. Takeaway¶

Session Affinity Failure is a reproducible, configuration-driven failure. The fix is deterministic and low-risk. Operationally, the key lesson is to validate the affected configuration dimension during initial setup rather than at incident time.

16. Support Takeaway¶

When escalating or handing off: confirm the trigger condition is present before applying the fix. Collect logs from the failing revision before deletion. Document the before-and-after configuration in the incident record.

Clean Up¶

Use a dedicated lab resource group before running this guide. Delete the resource group only if it contains lab-only resources.

az group delete \
  --name "$RG" \
  --yes \
  --no-wait

Related Playbook¶

• Session Affinity Failure

See Also¶

• Ingress in Azure Container Apps
• Networking Best Practices
• WebSocket and gRPC Ingress

Sources¶

• Session affinity in Azure Container Apps
• Ingress in Azure Container Apps