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PR Cost Diff — "Bu PR ne kadara mal olacak?"#

"Developer PR açıyor: replica 3 → 10, instance type m5.large → r5.4xlarge. Cost impact bilinmiyor → merge → ay sonu bill +$5K. PR-time cost visibility = preventive."

Bu rehber CI'da PR'ın cost impact'ini hesaplama ve PR yorumuna ekleme pratiğini, Kubecost / Infracost tooling'ini anlatır.

🎯 Niye PR Cost Diff?#

Senaryo (kötü)#

Dev: "Memory artırıyorum 1Gi → 4Gi" (PR)
Reviewer: "LGTM" (cost bilgisi yok)
Merge → 30 gün sonra:
  AWS bill +$2K, ay sonu sürpriz
  Finance: "Bu artış nereden?"
  Mühendis: "Şu PR'dan..."

Çözüm (iyi)#

Dev: "Memory artırıyorum 1Gi → 4Gi" (PR)
CI bot: 💰 +$1.5K/ay
Reviewer: "Hmm, gerek var mı? VPA ne öneriyor?"
Dev: gerçekten gerekli, justified
Merge → bilinçli karar

🔑 PR-time visibility = bilinçli karar.

🛠️ Tooling#

1. Infracost (Terraform)#

# CI integration
- uses: infracost/actions/setup@<VERSION>
- run: |
    infracost breakdown --path . > infra.json
- uses: infracost/actions/comment@<VERSION>
  with:
    path: infra.json

→ PR comment: 

💰 Infracost
  Monthly cost change: +$320/mo

  - aws_instance.web (3x → 10x): +$245
  - aws_rds_instance.db (db.t3 → db.r5): +$75

2. Kubecost (K8s)#

- uses: kubecost/kubecost-cost-action@<VERSION>
  with:
    api-key: ${{ secrets.KUBECOST_API_KEY }}
    cluster: prod

→ PR comment: 

💰 Kubecost Impact
  Resource changes:
    payments-api: replica 3 → 5
    Memory request: 1Gi → 2Gi

  Estimated impact:
    CPU: +400m → +$30/ay
    Memory: +2Gi → +$8/ay
    Total: +$38/ay (+%2.1)

3. Custom (manuel script)#

# scripts/cost-diff.py
import yaml
from cost_calculator import calculate

def diff(before_yaml, after_yaml):
    before = yaml.safe_load(before_yaml)
    after = yaml.safe_load(after_yaml)

    cost_before = calculate(before)
    cost_after = calculate(after)

    return {
        "before": cost_before,
        "after": cost_after,
        "diff": cost_after - cost_before,
        "percent": (cost_after - cost_before) / cost_before * 100
    }

🔧 Setup: Infracost#

Install + config#

brew install infracost
infracost auth login   # API key (free tier)

CI workflow#

# .github/workflows/cost-diff.yml
name: PR Cost Diff

on:
  pull_request:
    paths: ['terraform/**']

jobs:
  cost:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@<VERSION>
        with:
          fetch-depth: 0   # base + head ihtiyacı

      - uses: infracost/actions/setup@<VERSION>
        with:
          api-key: ${{ secrets.INFRACOST_API_KEY }}

      - name: Generate base
        run: |
          git checkout ${{ github.event.pull_request.base.sha }}
          infracost breakdown --path terraform/ \
            --format json --out-file /tmp/infracost-base.json

      - name: Generate head
        run: |
          git checkout ${{ github.event.pull_request.head.sha }}
          infracost diff --path terraform/ \
            --compare-to /tmp/infracost-base.json \
            --format json --out-file /tmp/infracost-diff.json

      - uses: infracost/actions/comment@<VERSION>
        with:
          path: /tmp/infracost-diff.json
          behavior: update

📊 Cost Threshold Alarm#

# Eğer +%10+ artış varsa label ekle
- name: Check threshold
  run: |
    DIFF=$(jq '.totalMonthlyCost' /tmp/infracost-diff.json)
    BASE=$(jq '.totalMonthlyCost' /tmp/infracost-base.json)
    PERCENT=$(echo "scale=2; $DIFF / $BASE * 100" | bc)

    if (( $(echo "$PERCENT > 10" | bc -l) )); then
      gh pr edit --add-label "cost-impact-high"
      gh pr comment --body "⚠️ %$PERCENT cost increase. Manager review needed."
    fi

🌳 Karar Akışı#

[PR açıldı]
   │
   ▼
[CI: cost diff calculate]
   │
   ├── < %5 increase → auto-merge OK
   │
   ├── %5-15 → reviewer dikkat (dashboard görünür)
   │
   └── > %15 → manager approval gerekli

🌍 Multi-Cloud Cost Compare#

Senaryo#

PR: "AWS Frankfurt'tan AWS Stockholm'e migrate"

Cost diff:
  Before: eu-central-1 → $5,000/ay
  After:  eu-north-1   → $3,800/ay
  Saving: -$1,200/ay (-%24)
  Bonus: -%80 carbon emission

→ Region migration + cost + carbon birlikte gösterilir.

🚫 Anti-Pattern Tablosu#

Anti-pattern Niye kötü Doğru
Cost diff yok Ay sonu sürpriz PR-time check
Threshold yok Tüm PR'lar approval %5/%15 tiered
Sadece Terraform / sadece K8s Cross-stack visibility yok Infracost + Kubecost combo
Cost data PR'a yazılı değil Reviewer görmez Otomatik comment
Negative cost (saving) görünmez Optimizasyon takdir edilmez "±/=" işaretle
Carbon impact yok Yeşil yazılım atlanır Kubecost carbon module
Cost label yok Tracking imkansız High-cost label otomatik
Periodic cost retrospective yok Pattern keşfi yok Monthly cost change report

📋 PR Cost Diff Adoption Checklist#

[ ] Tool seçimi: Infracost (TF) + Kubecost (K8s)
[ ] CI workflow: PR'da cost diff calculate
[ ] PR comment otomasyon
[ ] Threshold: %5 normal, %15 manager approval
[ ] Label: cost-impact-low/medium/high
[ ] Carbon impact (sustainability integration)
[ ] Multi-cloud compare (region migration için)
[ ] Monthly retrospective: top cost-impacting PR'lar
[ ] Documentation: dev'lere cost-aware PR yazma rehberi
[ ] FinOps + Engineering shared dashboard

📚 Referanslar#

"PR-time cost diff = preventive FinOps. Ay sonu bill sürpriz olmaz; her merge bilinçli karar. CI'a 5 dk entegrasyon, kalıcı disiplin."