Container Apps Revision Mode: Cost Implications

Overview

Azure Container Apps supports two revision modes:

  1. Single Revision Mode (Current): Only one active revision at a time
  2. Multiple Revision Mode: Multiple active revisions with traffic splitting

Current Configuration

Engram uses Single Revision Mode by default. This means:

  • Only the latest revision is active
  • Previous revisions are automatically deactivated on new deployments
  • The same revision number is updated in place (e.g., 0000144 stays 0000144)

Cost Implications

Single Revision Mode (Current)

Advantages:

  • ✅ Lower Cost: Only one set of replicas running at any time
  • ✅ Simpler Management: No traffic splitting complexity
  • ✅ Scale-to-Zero Compatible: Idle cost = $0 (when minReplicas = 0)
  • ✅ Predictable Billing: Single revision = single resource consumption

Cost Structure:

  • Resource Consumption: Billed on vCPU-seconds and GiB-seconds
  • Free Tier: First 180,000 vCPU-seconds and 360,000 GiB-seconds/month per subscription
  • Idle Cost: $0 when scaled to zero replicas
  • Active Cost: Pay only for running replicas (based on actual usage)

Example Cost (Single Mode):

Scenario: 1,000 conversations/day
- Average: 2 replicas active
- vCPU-seconds: ~2M/month
- GiB-seconds: ~4M/month
- Cost: ~$5-10/month (after free tier)

Multiple Revision Mode

Disadvantages:

  • ❌ Higher Cost: Multiple revisions = multiple sets of replicas
  • ❌ Complex Traffic Management: Requires traffic splitting configuration
  • ❌ Increased Resource Usage: All active revisions consume resources
  • ❌ Operational Overhead: More monitoring and management needed

Cost Structure:

  • Resource Consumption: Each active revision consumes resources independently
  • Traffic Splitting: 50/50 split = 2x replicas (both revisions active)
  • Gradual Rollout: 10/90 split = 1.1x replicas (both revisions active)

Example Cost (Multiple Mode - 50/50 Traffic Split):

Scenario: 1,000 conversations/day, 50/50 traffic split
- Revision A: 2 replicas active
- Revision B: 2 replicas active
- Total: 4 replicas active
- vCPU-seconds: ~4M/month
- GiB-seconds: ~8M/month
- Cost: ~$10-20/month (after free tier)
- **Cost Increase: 2x compared to Single Mode**

Cost Comparison

Scenario Single Mode Multiple Mode (50/50) Cost Difference
Idle (0 replicas) $0 $0 Same
Light (1 replica) ~$2-5/month ~$4-10/month 2x
Medium (2 replicas) ~$5-10/month ~$10-20/month 2x
Heavy (5 replicas) ~$15-30/month ~$30-60/month 2x

When to Use Multiple Revision Mode

Use Multiple Mode Only If:

  • ✅ You need blue-green deployments (zero-downtime rollbacks)
  • ✅ You need gradual rollouts (A/B testing, canary deployments)
  • ✅ You need traffic splitting (testing new features with subset of users)
  • ✅ You have budget for 2x resource consumption

For Engram:

  • ✅ Single Mode is Optimal: We use scale-to-zero, so rollbacks are fast
  • ✅ Cost-Conscious: Single mode aligns with FinOps strategy
  • ✅ Simple Deployments: FastAPI deployments are lightweight
  • ✅ No Traffic Splitting Needed: Single active revision is sufficient

Recommendations

Current Setup (Single Mode) ✅

Keep Single Revision Mode because:

  1. Cost Efficiency: Only pay for one set of replicas
  2. Scale-to-Zero: Idle cost = $0 (minReplicas = 0)
  3. Fast Deployments: Container Apps deployments are quick (~2-5 minutes)
  4. Simple Rollbacks: Deploy previous image if needed (no traffic splitting)

If You Need Multiple Mode

Consider Multiple Mode Only If:

  • You need zero-downtime rollbacks (blue-green)
  • You need A/B testing capabilities
  • You have budget for 2x resource consumption
  • You need gradual feature rollouts

Cost Impact:

  • 2x resource consumption during traffic splits
  • Additional monitoring for multiple revisions
  • Operational complexity for traffic management

Cost Optimization Tips

Single Mode (Current)

  1. ✅ Scale-to-Zero: minReplicas = 0 (already configured)
  2. ✅ Right-Sizing: 0.5 vCPU, 1GB RAM per replica (already configured)
  3. ✅ Auto-Scaling: Max 10 replicas (prevents runaway costs)
  4. ✅ Idle Behavior: Scales to 0 after 30 minutes idle

Multiple Mode (If Needed)

  1. ⚠️ Minimize Traffic Splits: Keep splits short (minutes, not hours)
  2. ⚠️ Monitor Both Revisions: Track resource usage for both
  3. ⚠️ Quick Rollouts: Complete rollouts quickly to reduce overlap
  4. ⚠️ Budget Alerts: Set up alerts for 2x resource consumption

Monitoring

Key Metrics to Track

Single Mode:

  • Active replicas (should be 0 when idle)
  • vCPU-seconds per month
  • GiB-seconds per month
  • Deployment frequency

Multiple Mode:

  • Active revisions count
  • Traffic split percentages
  • Resource consumption per revision
  • Total resource consumption (sum of all revisions)

Cost Alerts

Set up Azure Cost Management alerts for:

  • Container Apps resource consumption > threshold
  • Multiple revisions active for > 1 hour
  • Resource consumption spikes

Conclusion

Single Revision Mode is the optimal choice for Engram:

✅ Cost-Efficient: Only one set of replicas ✅ Scale-to-Zero Compatible: $0 idle cost ✅ Simple Management: No traffic splitting complexity ✅ Fast Deployments: Quick rollouts and rollbacks ✅ FinOps Aligned: Matches our cost-conscious architecture

Cost Impact of Current Setup:

  • Idle: $0 (scales to zero)
  • Light Usage: ~$2-5/month
  • Medium Usage: ~$5-10/month
  • Heavy Usage: ~$15-30/month

If switching to Multiple Mode:

  • Cost Increase: 2x resource consumption during traffic splits
  • Use Case: Only if blue-green deployments or A/B testing is required
  • Recommendation: Keep Single Mode unless specific need arises