# WRONG - VCN too small, cannot expand later (OCI limitation)
oci network vcn create --cidr-block "10.0.0.0/24"
# Only 256 IPs total, exhausted quickly

# WRONG - copying AWS habit (/16 VPC default)
# OCI supports larger: /16 to /30

# RIGHT - start with /16, plan for growth
oci network vcn create --cidr-block "10.0.0.0/16"
# 65,536 IPs, room for 256 /24 subnets

# CRITICAL: OCI VCNs CANNOT be resized after creation
# Must create new VCN and migrate if too small

# WRONG - subnet tied to single AD
oci network subnet create \
  --vcn-id <vcn-ocid> \
  --cidr-block "10.0.1.0/24" \
  --availability-domain "fMgC:US-ASHBURN-AD-1"  # AD-specific!

# Problem: Can't launch instances in other ADs, no HA

# RIGHT - regional subnet (works in all ADs)
oci network subnet create \
  --vcn-id <vcn-ocid> \
  --cidr-block "10.0.1.0/24"
  # No --availability-domain flag = regional
  # Instances can be in any AD in region

OCI has TWO network security mechanisms:

Security Lists (stateful, subnet-level):
- Applied to ALL resources in subnet
- Use for: Broad rules (internet egress, DNS)
- Limit: 5 per subnet
- Changes: Affect all instances in subnet

Network Security Groups (NSG, resource-level):
- Applied to specific resources
- Use for: Granular rules (app tier → DB tier)
- Limit: 5 per resource, 120 rules per NSG
- Changes: Affect only tagged resources

# WRONG - using Security Lists for app-specific rules
Security List: Allow app-tier → database (applies to ENTIRE subnet)

# RIGHT - use NSG for app-tier resources
NSG "app-tier": Allow egress to NSG "db-tier" on port 1521
# Only instances in app-tier NSG can reach DB

OCI Availability Domains (ADs):
- 3 ADs per region (most regions)
- Isolated fault domains
- <1ms latency between ADs

# WRONG - all resources in single AD
All instances in AD-1 → AD failure = complete outage

# RIGHT - distribute across ADs
Production instances: AD-1, AD-2, AD-3
Load balancer: Automatically multi-AD
Database: Autonomous (auto 3-AD) or RAC (2+ nodes)

SLA impact:
Single-AD: NO SLA (OCI doesn't guarantee)
Multi-AD: 99.95% SLA

# WRONG - AD names are tenant-specific, not portable
availability_domain = "fMgC:US-ASHBURN-AD-1"  # Only works in YOUR tenancy!

# Another tenant's AD name for same physical AD:
availability_domain = "xYzA:US-ASHBURN-AD-1"  # Different prefix!

# RIGHT - query AD names dynamically
data "oci_identity_availability_domains" "ads" {
  compartment_id = var.tenancy_ocid
}

resource "oci_core_instance" "web" {
  availability_domain = data.oci_identity_availability_domains.ads.availability_domains[0].name
}

Cloud Guard = OCI threat detection + auto-response

# DANGER - auto-remediation can break production
Detector: "Public bucket detected"
Auto-remediation: Make bucket private → breaks public website!

Detector: "Security list allows 0.0.0.0/0"
Auto-remediation: Remove rule → breaks internet access!

# SAFER approach:
1. Enable detectors (read-only mode first)
2. Review findings for 1-2 weeks
3. Tune responders to avoid false positives
4. Enable auto-remediation for trusted patterns only

OCI supports:
✓ Oracle Linux (free, optimized)
✓ Ubuntu, CentOS, Rocky Linux (free)
✓ Windows Server (BYOL or license-included)
✓ Custom images (import your own)

# WRONG assumption: "OCI = must use Oracle Linux"
Reality: Any Linux works, Ubuntu has larger community

# Cost: Oracle Linux is FREE (no license cost)
# But if team knows Ubuntu → use Ubuntu

# Gotcha 1: 2 ADB limit is TENANCY-wide, not per region
Can have: 1 ATP in Phoenix + 1 ADW in Ashburn = 2 (limit reached)
Cannot: Add 3rd ADB in any region

# Gotcha 2: Arm instances must be VM.Standard.A1.Flex only
Cannot: Use newer A2 shapes (paid only)

# Gotcha 3: Free tier != trial credits
Free tier: Permanent, no expiration
Trial: $300 credit for 30 days (separate)

# Gotcha 4: Stopped ADB counts toward 2 ADB limit
To free slot: Must DELETE ADB, not just STOP

OCI Regions with 3 ADs (most regions):
- US: Phoenix, Ashburn
- UK: London
- DE: Frankfurt
- AU: Sydney, Melbourne

Pattern: Distribute instances across all 3 ADs

AD-1: Web tier (2 instances) + DB primary
AD-2: Web tier (2 instances) + DB standby
AD-3: Web tier (2 instances) + DB standby

Load Balancer: Automatically spans ADs

# Check shape availability by AD
oci compute shape list \
  --compartment-id <ocid> \
  --availability-domain "fMgC:US-ASHBURN-AD-1"

Best practice: Spread instances across ADs AND FDs

AD-1:
  FD-1: Web instance 1
  FD-2: Web instance 2
  FD-3: Web instance 3

AD-2:
  FD-1: Web instance 4
  (repeat pattern)

Protection:
- AD failure: 2 ADs survive (66% capacity)
- FD failure: Only 1 instance affected (16% capacity)

Root Compartment (tenancy)
│
├─ SharedServices (networking, security)
│  ├─ Network (VCNs, DRGs)
│  └─ Security (Vault, KMS, Cloud Guard)
│
├─ Production
│  ├─ App1
│  │  ├─ Compute
│  │  ├─ Database
│  │  └─ Storage
│  └─ App2
│
├─ NonProduction
│  ├─ Development
│  ├─ Testing
│  └─ Staging
│
└─ Sandbox (developers, auto-cleanup)

Fixed shapes (legacy):
VM.Standard2.4: 4 OCPUs, 60 GB RAM, $218/month

Flex shapes (right-size RAM independently):
VM.Standard.E4.Flex: 4 OCPUs, 16 GB RAM, $109/month (50% savings)

Flex advantage: Pay only for RAM you need
- 1 OCPU = 1-64 GB RAM configurable
- Most apps don't need 15GB per OCPU (fixed ratio)

AMD instance: VM.Standard.E4.Flex (1 OCPU) = $0.03/hr
Arm instance: VM.Standard.A1.Flex (1 OCPU) = $0.01/hr (67% cheaper)

Always-Free Arm: 4 OCPUs free forever!

Use case: Web servers, CI/CD runners, dev environments
Limitation: ARM64 only (not all apps compatible)

Day 0-30: Standard ($0.0255/GB/mo)
Day 31-90: Infrequent ($0.0125/GB/mo)
Day 91+: Archive ($0.0024/GB/mo)

1 TB data for 1 year:
Without tiering: $0.0255 × 1000 × 12 = $306/year
With tiering: $0.0255 × 1000 × 1 + $0.0125 × 1000 × 2 + $0.0024 × 1000 × 9 = $72/year
Savings: $234/year (76%)

Security Zone enforces:
✓ All storage encrypted
✓ No public buckets
✓ No internet gateways in VCN
✓ All databases private endpoint only
✓ Cloud Guard enabled

Enforcement: API rejects violating requests (preventive, not detective)

Example:
oci os bucket create --public-access-type ObjectRead
→ FAILS if compartment is in Security Zone

Use case: Production, PCI-DSS, healthcare (mandatory controls)