Min Replicas Cost Surprise Lab¶

Lab Metadata¶

1. Question¶

Does min replicas cost surprise 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¶

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 Evidence (Live Azure Test — 2026-04-29)¶

[Observed] az containerapp update --min-replicas 5 succeeded; subsequent az containerapp show --query "properties.template.scale.minReplicas" returned 5.

[Observed] az containerapp update --min-replicas 0 succeeded; subsequent query returned 0.

[Correlated] az containerapp replica list after idle period with minReplicas=5 showed 5 running replicas; same command after minReplicas=0 and idle period showed 0 replicas.

[Inferred] The cost difference between minReplicas=5 and minReplicas=0 on the Consumption plan is purely the idle compute for 5 permanently-warm replicas — no traffic required to accumulate charges.

Environment: koreacentral, Consumption plan.

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¶

Min Replicas Cost Surprise 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.

Expected Evidence¶

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

Environment: rg-aca-lab-test6 / cae-lab6, koreacentral, Consumption plan. App: ca-min-replicas (0.5 vCPU / 1 Gi, minReplicas=5, maxReplicas=10).

[Observed] With zero traffic, az containerapp replica list returned 5 replicas running — all idle, each consuming 0.5 vCPU / 1 Gi.

[Measured] Idle cost at minReplicas=5: 5 × 0.5 vCPU × $0.000024/vCPU-s × 2,592,000 s/month ≈ $155.52/month (vCPU alone, pre-free-tier).

[Observed] After fix: az containerapp show --query "properties.template.scale.minReplicas" returned 1 — setting applied immediately.

[Observed] Post-fix scale verified: az containerapp show --query "properties.template.scale" returned {"minReplicas": 1, "maxReplicas": 10}.

[Inferred] Idle replicas run continuously regardless of traffic. Consumption plan charges per vCPU-second and memory-GiB-second for active replicas. minReplicas=5 with no traffic wastes 4 replicas worth of compute 24/7. Setting minReplicas=0 eliminates idle cost entirely but introduces cold start latency.

Fix: Set --min-replicas 1 for user-facing apps to balance cost and cold start. Set --min-replicas 0 only for background jobs where latency is acceptable.

Clean Up¶

Choose the final state that matches the intended production behavior.

az containerapp show \
    --name "$APP_NAME" \
    --resource-group "$RG" \
    --query "properties.template.scale" \
    --output json

Related Playbook¶

• Min Replicas Cost Surprise

See Also¶

• Cold Start and Scale-to-Zero Lab
• Cost-Aware Best Practices
• Workload Profiles

Sources¶

• Microsoft Learn: Azure Container Apps billing
• Microsoft Learn: Set scaling rules in Azure Container Apps
• Microsoft Learn: Workload profiles in Azure Container Apps