BAD - Flat compartments:
tenancy/
  ├─ app1-dev
  ├─ app1-test
  ├─ app1-prod
  ├─ app2-dev
  ├─ app2-test
  └─ app2-prod

Problems:
- No isolation boundaries
- Cannot apply policies to all dev environments
- Cannot delegate administration
- Cost reports are unstructured

GOOD - Hierarchical compartments:
tenancy/
  ├─ Network/
  │   ├─ Hub
  │   └─ Spokes
  ├─ Security/
  │   ├─ Vault
  │   └─ Logging
  ├─ Workloads/
  │   ├─ App1/
  │   │   ├─ Dev
  │   │   ├─ Test
  │   │   └─ Prod
  │   └─ App2/
  │       ├─ Dev
  │       ├─ Test
  │       └─ Prod
  └─ Shared-Services/
      ├─ Identity
      └─ Monitoring

BAD - Same CIDR in all environments:
Dev VCN: 10.0.0.0/16
Test VCN: 10.0.0.0/16  # Cannot peer with Dev!
Prod VCN: 10.0.0.0/16  # Cannot peer with Dev or Test!

Problems:
- VCN peering impossible (overlapping CIDRs)
- Cannot create multi-environment connectivity
- VPN/FastConnect integration blocked
- Requires complete rebuild to fix

GOOD - Non-overlapping CIDR allocation:
Dev VCN: 10.10.0.0/16
Test VCN: 10.20.0.0/16
Prod VCN: 10.30.0.0/16
Hub VCN: 10.0.0.0/16 (shared services)

Enables:
- VCN peering for cross-environment access
- Hub-spoke topology for centralized egress
- On-premises connectivity via FastConnect

# BAD - no security zone enforcement
oci iam compartment create \
  --compartment-id $PARENT_ID \
  --name "Prod" \
  --description "Production workloads"
# Result: No guardrails, resources can violate security policies

# GOOD - security zone enabled
# 1. Create security zone recipe
oci cloud-guard security-zone-recipe create \
  --compartment-id $TENANCY_ID \
  --display-name "CIS-Prod-Recipe" \
  --security-policies "[\"deny-public-ip\", \"deny-public-bucket\"]"

# 2. Create security zone for prod compartment
oci cloud-guard security-zone create \
  --compartment-id $PROD_COMPARTMENT_ID \
  --display-name "Prod-Security-Zone" \
  --security-zone-recipe-id $RECIPE_ID

# Enforces: No public IPs, no public buckets, encryption required

BAD - shared compartment:
App1/
  ├─ vm-dev-1 (development instance)
  ├─ vm-prod-1 (production instance)
  └─ db-prod (CRITICAL DATABASE)

Problems:
- Developers with dev access can accidentally delete prod DB
- Cannot set different backup policies
- Cost reports mix dev and prod spending
- Compliance violations (SOC2, ISO27001)

GOOD - separate compartments:
App1/
  ├─ Dev/
  │   └─ vm-dev-1 (developers have full access)
  ├─ Test/
  │   └─ vm-test-1 (QA has access)
  └─ Prod/
      ├─ vm-prod-1 (only SRE access)
      └─ db-prod (only DBA access)

Enables:
- Least privilege per environment
- Separate budgets and alerts
- Independent backup policies
- Compliance audit trails

BAD - resources in root:
tenancy (root)/
  ├─ vcn-1 (WRONG - in root)
  ├─ instance-1 (WRONG - in root)
  └─ database-1 (WRONG - in root)

Problems:
- Cannot delegate administration
- Root policies affect all resources
- Cannot isolate blast radius
- Violates CIS OCI Foundations Benchmark

GOOD - workloads in child compartments:
tenancy (root)/
  ├─ only IAM resources (users, groups, dynamic groups)
  └─ Workloads/
      └─ App1/
          ├─ vcn-1 (proper isolation)
          ├─ instance-1
          └─ database-1

Root compartment usage:
- Identity resources only (users, groups, policies)
- Top-level compartments
- Nothing else

BAD - no tags:
Resource created with no tags
Cost report shows: "oci.compute.instance: $5,234/month"
Question: Which team? Which project? Which environment?
Answer: Unknown - requires manual investigation

Result: Cannot chargeback costs, cannot optimize

GOOD - defined tag namespace + mandatory tags:
# 1. Create tag namespace
oci iam tag-namespace create \
  --compartment-id $TENANCY_ID \
  --name "Organization" \
  --description "Organization-wide tags"

# 2. Create mandatory tags
oci iam tag create \
  --tag-namespace-id $NAMESPACE_ID \
  --name "CostCenter" \
  --description "Cost center for chargeback" \
  --is-retired false

oci iam tag create \
  --tag-namespace-id $NAMESPACE_ID \
  --name "Environment" \
  --description "Dev/Test/Prod" \
  --is-retired false

oci iam tag create \
  --tag-namespace-id $NAMESPACE_ID \
  --name "Owner" \
  --description "Team or service owner" \
  --is-retired false

# 3. Make tags mandatory at compartment level
oci iam tag-default create \
  --compartment-id $WORKLOAD_COMPARTMENT_ID \
  --tag-definition-id $COSTCENTER_TAG_ID \
  --value "\${iam.principal.name}"

Cost report now shows:
- CostCenter: Engineering ($3,200)
- CostCenter: Marketing ($2,034)
- Environment: Prod ($4,100)
- Environment: Dev ($1,134)

BAD - single region:
All resources in us-ashburn-1
RTO: Hours-days (rebuild in new region)
RPO: Last backup (data loss)

Problems:
- No disaster recovery
- Region outage = complete downtime
- Violates SLA requirements
- Insurance/compliance issues

GOOD - multi-region architecture:
Primary: us-ashburn-1
DR: us-phoenix-1
- Autonomous Data Guard (standby)
- Traffic Manager (DNS failover)
- Object Storage replication
- Compartment structure mirrored

RTO: 15 minutes (automated failover)
RPO: Near-zero (Data Guard sync)

BAD - direct internet gateway:
DMZ Subnet → Internet Gateway → Internet
No egress filtering, all outbound traffic allowed

Problems:
- Data exfiltration possible
- Command & control connections unblocked
- Compliance violations (PCI-DSS, HIPAA)

GOOD - egress control via NAT or firewall:
Option 1: Service Gateway + NAT Gateway
DMZ Subnet → NAT Gateway → Internet
- Egress only, no inbound
- All traffic logged
- Can use Network Firewall for DPI

Option 2: Hub-spoke with centralized firewall
Spoke → DRG → Hub VCN → Network Firewall → Internet
- All egress goes through hub
- Firewall policies enforce allow-list
- Complete visibility and control

oci iam compartment create \
  --compartment-id $TENANCY_ID \
  --name "Workloads" \
  --description "Application workloads"