cost-optimization

name	cost-optimization
description	Guides systematic cloud and infrastructure cost reduction. Use when cloud bills are growing unpredictably, when rightsizing resources, when evaluating reserved capacity, or when building cost-aware architecture. Use when you need to optimize spend without sacrificing reliability or performance.

Cost Optimization

Overview

Cost optimization is not about being cheap — it's about spending money where it creates value and eliminating waste. In cloud environments, costs compound silently: oversized instances, forgotten dev environments, unoptimized storage tiers, and data transfer charges that nobody tracks. This skill provides a systematic approach to finding and eliminating waste while maintaining (or improving) system reliability.

The 80/20 rule of cloud costs: 80% of your bill comes from 20% of your resources. Focus on the big items first.

When to Use

• Monthly cloud bill has grown > 20% without corresponding traffic growth
• Setting up new infrastructure and want cost-aware defaults
• Reviewing architecture for a system that's been running > 6 months
• Preparing for scaling events (launch, Black Friday, viral growth)
• Evaluating multi-cloud vs single-cloud strategies
• After an audit reveals "surprise" charges (data transfer, API calls, storage)

NOT for:

• Systems where reliability is worth any cost (life-critical, financial trading)
• One-time cost reduction without ongoing monitoring (costs creep back)

The Cost Optimization Process

Step 1: Measure and Allocate

You can't optimize what you can't measure. Before changing anything:

COST ALLOCATION FRAMEWORK:
├── Tag everything (owner, environment, product, cost-center)
├── Enable detailed billing exports (hourly granularity)
├── Set up cost anomaly detection
├── Allocate shared costs (network, monitoring, platform)
└── Create per-team visibility dashboards

Key metrics to track:

Metric	Why It Matters	Target
Cost per request	Efficiency of serving traffic	Trending down
Cost per user	Unit economics	Trending down
Idle resource %	Waste identification	< 10%
Storage $/GB	Tier optimization	Match workload pattern
Data transfer	Often the "surprise" line item	< 10% of total

Step 2: Identify Waste

Run through this checklist systematically:

Compute Waste:

• Instances with average CPU < 20% over 7 days
• Instances with memory utilization < 40%
• Running instances in dev/staging outside business hours
• Over-provisioned container requests (K8s)
• Unused auto-scaling groups with min-size > 0
• Legacy instance types (newer generations are cheaper per performance unit)

Storage Waste:

• Unattached volumes > 30 days old
• Snapshots without retention policies
• Objects in hot storage with no access in 90 days
• Database instances with storage utilization < 30%
• Logs stored indefinitely without lifecycle policies

Network Waste:

• Cross-AZ traffic that could be co-located
• Data transfer to internet instead of CDN
• NAT Gateway traffic that could use VPC endpoints
• Cross-region replication that's not needed
• Egress charges from services that could use private connectivity

Licensing/Reserved Waste:

• On-demand instances that could use reserved capacity
• Reserved capacity for services that are being decommissioned
• Database licenses where open-source alternatives suffice
• Unused SaaS subscriptions (monitoring, CI/CD, etc.)

Step 3: Rightsize Resources

RIGHTSIZING WORKFLOW:
1. Collect metrics (CPU, memory, disk I/O, network) for 14+ days
2. Identify peak vs average vs p99 usage patterns
3. Match instance/container size to actual needs + 20% headroom
4. Test the smaller size in staging under load
5. Deploy to production with monitoring
6. Observe for 48 hours, adjust if needed

Rightsizing examples:

# Before: c5.2xlarge (8 vCPU, 16 GB) — $0.34/hour
# Metrics show: 15% CPU avg, 45% memory avg, never bursts above 40%
# After: c5.large (2 vCPU, 4 GB) — $0.085/hour
# Savings: 75% for this instance

# Kubernetes: Adjust resource requests
# Before:
resources:
  requests:
    cpu: 2000m      # Never uses more than 500m
    memory: 8Gi     # Uses 2Gi consistently

# After:
resources:
  requests:
    cpu: 750m       # 50% overhead on observed peak
    memory: 3Gi     # 50% overhead on observed peak
  limits:
    cpu: 1500m
    memory: 6Gi

Step 4: Optimize Storage Tiers

Match storage to access patterns:

DATA ACCESS PATTERN → STORAGE TIER
─────────────────────────────────────────────────
Frequently accessed (< 7 days)    → Hot / Standard
Occasionally accessed (7-90 days) → Warm / Infrequent Access
Rarely accessed (90+ days)        → Cold / Archive
Never accessed (backup only)      → Glacier / Deep Archive

Storage optimization examples:

# S3 Lifecycle Policy
{
  "Rules": [
    {
      "ID": "MoveOldLogsToGlacier",
      "Status": "Enabled",
      "Filter": { "Prefix": "logs/" },
      "Transitions": [
        { "Days": 30, "StorageClass": "STANDARD_IA" },
        { "Days": 90, "StorageClass": "GLACIER" }
      ],
      "Expiration": { "Days": 2555 }  // 7 years retention
    }
  ]
}

# Database: Use read replicas for read-heavy workloads
# Instead of scaling primary DB vertically ($$$), add read replicas ($)

Step 5: Reserved Capacity and Commitments

RESERVED CAPACITY DECISION MATRIX:

Usage Pattern          → Recommendation
─────────────────────────────────────────────────
24/7 steady state      → 1-year reserved (40-60% savings)
Predictable growth     → 3-year reserved (up to 70% savings)
Seasonal / burst       → Savings plans (flexible commitment)
Unpredictable / dev    → Stay on-demand (flexibility > savings)
Spot-capable workloads → Spot instances (up to 90% savings)

Spot / preemptible instance strategy:

// Stateful workloads: Use spot with fallback
const computeStrategy = {
  primary: 'spot',           // 90% cheaper
  fallback: 'on-demand',     // When spot is unavailable
  maxSpotPrice: 0.50,        // Don't pay more than 50% of on-demand
  interruptionBehavior: 'stop', // Save state, resume later
};

// Stateless workloads: Pure spot with auto-scaling
const workerPool = {
  instanceType: 'c5.xlarge',
  capacity: 'spot-only',
  minSize: 2,
  maxSize: 50,
  targetCPU: 70,
};

Step 6: Implement and Monitor

Every optimization must be measurable:

## Optimization: [Name]

### Baseline
- Monthly cost: $[X]
- Primary driver: [compute/storage/network]
- Waste identified: [specific resources]

### Changes Made
1. [Change 1 with justification]
2. [Change 2 with justification]

### Results
- New monthly cost: $[Y] ([Z]% reduction)
- Performance impact: [none / improved / degraded — quantify]
- Reliability impact: [none / improved / degraded — quantify]

### Ongoing Monitoring
- Alert if cost exceeds $[threshold]
- Review monthly on [date]
- Re-evaluate if traffic changes > 20%

Architecture Patterns for Cost Efficiency

Design for Scale-to-Zero

// Serverless functions for intermittent workloads
// Pay only for invocations, not idle time
export async function processWebhook(event: APIGatewayEvent) {
  // Cold start: ~200ms (acceptable for async processing)
  // Runtime: $0.20 per million requests vs $72/month for smallest VM
  await validateWebhook(event);
  await enqueueForProcessing(event.body);
  return { statusCode: 202 };
}

Caching to Reduce Compute

CACHE HIERARCHY (cheapest to most expensive):
1. Browser cache (free for you)
2. CDN edge cache (pennies per GB)
3. Application cache (memory — cheap but limited)
4. Database cache (query cache, materialized views)
5. Re-compute (most expensive)

Data Transfer Minimization

# BAD: Client → API → Database → API → Client (4 hops)
# GOOD: Client → CDN (cache hit) — 1 hop, $0.01/GB

# BAD: Cross-region replication for all data
# GOOD: Replicate metadata only, fetch data on-demand

# BAD: NAT Gateway for all outbound traffic ($0.045/GB)
# GOOD: VPC endpoints for AWS services ($0/GB)

Common Rationalizations

Rationalization	Reality
"We're too small for cost optimization to matter"	A single oversized database or forgotten dev environment can cost $500+/month. That's $6K/year — enough for a junior engineer's salary in many markets.
"Optimization will hurt performance"	Proper optimization improves performance. Rightsizing forces you to fix inefficiencies. Caching reduces latency.
"We'll just scale vertically"	Vertical scaling is the most expensive option. Horizontal scaling with smaller instances + auto-scaling is cheaper and more resilient.
"Reserved capacity locks us in"	Savings plans and convertible reserved instances provide flexibility. The "lock-in" cost is usually < 10% of the savings.
"Engineer time is more expensive than cloud costs"	True for small optimizations, but a single rightsizing exercise can save $10K+/year — that's more than the engineering time spent.

Red Flags

• Cloud bill is reviewed only when finance complains
• No tagging strategy (can't allocate costs to teams/products)
• "We'll optimize later" — later never comes
• All environments run 24/7 (dev/staging should auto-shutdown)
• Using latest instance types without comparing price/performance
• Data transfer costs > 15% of total bill
• No auto-scaling (manually sized for peak, paying for peak 24/7)

Verification

After optimization:

• Costs reduced by target amount (quantified)
• Performance metrics unchanged or improved
• Reliability metrics unchanged or improved
• Monitoring and alerts updated for new baseline
• Team trained on new cost-aware practices
• Review scheduled (monthly for first 3 months, then quarterly)

See Also

• For performance optimization alongside cost work, see performance-optimization
• For infrastructure decisions, see observability-and-instrumentation (you need metrics to optimize)