Autoscale Group Capacity

Overview

Configure the autoscale group size parameters to define minimum, maximum, and desired instance counts for both BYOL and on-demand (PAYG) autoscale groups.

Configuration

# BYOL ASG capacity
asg_byol_asg_min_size         = 1
asg_byol_asg_max_size         = 2
asg_byol_asg_desired_size     = 1

# On-Demand (PAYG) ASG capacity
asg_ondemand_asg_min_size     = 0
asg_ondemand_asg_max_size     = 2
asg_ondemand_asg_desired_size = 0

Parameter Definitions

Parameter	Description	Recommendations
`min_size`	Minimum number of instances ASG maintains	Set to baseline capacity requirement
`max_size`	Maximum number of instances ASG can scale to	Set based on peak traffic projections + 20% buffer
`desired_size`	Target number of instances ASG attempts to maintain	Typically equals `min_size` for baseline capacity

Capacity Planning Strategies

Strategy 1: BYOL Baseline with PAYG Burst (Recommended)

Objective: Optimize costs by using BYOL for steady-state traffic and PAYG for unpredictable spikes

# BYOL handles baseline 24/7 traffic
asg_byol_asg_min_size = 2
asg_byol_asg_max_size = 4
asg_byol_asg_desired_size = 2

# PAYG handles burst traffic only
asg_ondemand_asg_min_size = 0
asg_ondemand_asg_max_size = 6
asg_ondemand_asg_desired_size = 0

Scaling behavior:

Normal operations: 2 BYOL instances handle traffic
Traffic increases: BYOL ASG scales up to 4 instances
Traffic continues increasing: PAYG ASG scales from 0 → 6 instances
Traffic decreases: PAYG ASG scales down to 0, then BYOL ASG scales down to 2

Strategy 2: All PAYG (Simplest)

Objective: Maximum flexibility with zero license management overhead

# No BYOL instances
asg_byol_asg_min_size = 0
asg_byol_asg_max_size = 0
asg_byol_asg_desired_size = 0

# All capacity is PAYG
asg_ondemand_asg_min_size = 2
asg_ondemand_asg_max_size = 8
asg_ondemand_asg_desired_size = 2

Use cases:

Proof of concept or testing
Short-term projects (< 6 months)
Extreme variability where license planning is impractical

Strategy 3: All BYOL (Lowest Cost)

Objective: Minimum operating costs for long-term, predictable workloads

# All capacity is BYOL
asg_byol_asg_min_size = 2
asg_byol_asg_max_size = 6
asg_byol_asg_desired_size = 2

# No PAYG instances
asg_ondemand_asg_min_size = 0
asg_ondemand_asg_max_size = 0
asg_ondemand_asg_desired_size = 0

Requirements:

Sufficient BYOL licenses for max_size (6 in this example)
Predictable traffic patterns that rarely exceed max capacity
Willingness to accept capacity ceiling (no burst beyond BYOL max)

CloudWatch Alarm Integration

Autoscale group scaling is triggered by CloudWatch alarms monitoring CPU utilization:

Default thresholds (set in underlying module):

Scale-out alarm: CPU > 70% for 2 consecutive periods (2 minutes)
Scale-in alarm: CPU < 30% for 2 consecutive periods (2 minutes)

Customization (requires editing underlying module):

# Located in module: fortinetdev/cloud-modules/aws
scale_out_threshold = 80  # Higher threshold = more aggressive cost optimization
scale_in_threshold  = 20  # Lower threshold = more aggressive cost optimization

Capacity Planning Calculator

Formula: Capacity Needed = (Peak Gbps Throughput) / (Per-Instance Gbps) × 1.2

Example:

Peak throughput requirement: 8 Gbps
c6i.xlarge (4 vCPU) with IPS enabled: ~2 Gbps per instance
Calculation: 8 / 2 × 1.2 = 4.8 → round up to 5 instances
Set max_size = 5 or higher for safety margin

Important Considerations

Tip

Testing Capacity Settings

For initial deployments and testing:

Start with min_size = 1 and max_size = 2 to verify traffic flows correctly
Test scaling by generating load and monitoring ASG behavior
Once validated, increase capacity to production values via AWS Console or Terraform update
No need to destroy/recreate stack just to change capacity settings

Next Steps

After configuring capacity, proceed to Primary Scale-In Protection to protect the primary instance from being terminated during scale-in events.