AWS for DevOps — Spot Instances

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14.4 — Spot Instances

Spot Instances use unused EC2 capacity at up to 90% discount. The trade-off: AWS can reclaim them with a 2-minute warning.

Real-World Analogy

Spot Instances are like standby airline tickets. You get a seat at a massive discount, but if the flight fills up with full-fare passengers, you get bumped with 2 minutes notice. Perfect if you’re flexible; terrible if you absolutely must be on that flight.

Good Workloads for Spot

Workload Why It Works
CI/CD build agents Builds can restart; short-lived
Batch data processing Jobs are idempotent; can resume
Auto Scaling groups (mixed) Other instances absorb traffic
Dev/test environments Interruption is acceptable
Machine learning training Checkpointing supports resume
Containers (ECS/EKS) Orchestrator reschedules tasks

Bad Workloads for Spot

Workload Why It’s Risky
Single-instance databases Interruption = downtime
Stateful applications without checkpointing Data loss risk
Long-running jobs that can’t be restarted Wasted compute
Primary production web servers User-facing interruptions

Using Spot with Auto Scaling Groups

# Create a mixed instances policy (On-Demand + Spot)
$ aws autoscaling create-auto-scaling-group \
    --auto-scaling-group-name production-asg \
    --mixed-instances-policy '{
      "LaunchTemplate": {
        "LaunchTemplateSpecification": {
          "LaunchTemplateId": "lt-abc123",
          "Version": "$Latest"
        },
        "Overrides": [
          {"InstanceType": "m5.large"},
          {"InstanceType": "m5a.large"},
          {"InstanceType": "m4.large"},
          {"InstanceType": "m5d.large"}
        ]
      },
      "InstancesDistribution": {
        "OnDemandBaseCapacity": 2,
        "OnDemandPercentageAboveBaseCapacity": 20,
        "SpotAllocationStrategy": "capacity-optimized"
      }
    }' \
    --min-size 4 \
    --max-size 20 \
    --desired-capacity 10 \
    --vpc-zone-identifier "subnet-abc123,subnet-def456"

This creates:

  • 2 On-Demand instances (base capacity — always running)
  • Of the remaining 8: 20% On-Demand + 80% Spot
  • Result: 4 On-Demand + 6 Spot = 10 total

Spot Allocation Strategies

Strategy How It Works Best For
capacity-optimized Picks pools with most capacity Lowest interruption rate (recommended)
lowest-price Picks cheapest pools Maximum savings (higher interruption)
diversified Spreads across all pools Balanced approach

Handling Spot Interruptions

# Check for interruption notice (from within the instance)
$ curl -s http://169.254.169.254/latest/meta-data/spot/instance-action
{"action": "terminate", "time": "2026-03-30T12:00:00Z"}

Best practices for handling interruptions:

  1. Use multiple instance types (4+ types)
  2. Spread across multiple AZs
  3. Use capacity-optimized allocation strategy
  4. Implement graceful shutdown handlers
  5. Design applications to be stateless

Spot with ECS Fargate

# Use Fargate Spot for non-critical tasks
# In your ECS service, set the capacity provider strategy:
$ aws ecs create-service \
    --cluster production \
    --service-name batch-processor \
    --task-definition batch:1 \
    --capacity-provider-strategy '[
      {"capacityProvider": "FARGATE", "weight": 1, "base": 1},
      {"capacityProvider": "FARGATE_SPOT", "weight": 4}
    ]' \
    --desired-count 5
# Result: 1 Fargate (base) + 4 Fargate Spot

Common Mistakes

Mistake Why It Hurts Fix
Single instance type Higher interruption risk Use 4+ instance types
100% Spot for production Service disruption Keep On-Demand base capacity
No graceful shutdown Data loss on interruption Handle SIGTERM signal
Not using capacity-optimized More interruptions Switch from lowest-price

Quick Reference

Task Command
Check Spot price history aws ec2 describe-spot-price-history --instance-types m5.large --product-descriptions "Linux/UNIX"
Request Spot Instance aws ec2 request-spot-instances --instance-count 1 --type one-time --launch-specification '...'
Create mixed ASG aws autoscaling create-auto-scaling-group --mixed-instances-policy '...'
Check interruption notice curl http://169.254.169.254/latest/meta-data/spot/instance-action

Next → Tagging & Right-Sizing

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