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docs: improve TechBooks scenario with context and abbreviations
- Add introductory paragraphs to tables explaining their purpose - Define abbreviations on first use (NACL, AMI, AZ, IOPS, SLA, etc.) - Clarify technical terms (OSI, ENI, LCU, TTL, OAC/OAI) - Improve readability with context for comparison tables
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‎docs/scenarios/techbooks/phases/phase-1-mvp-launch.md‎

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### WHY Custom VPC Instead of Default VPC?
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AWS provides a default VPC in every region, but understanding when to use a custom VPC is essential
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for production workloads and the exam:
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| Aspect | Default VPC | Custom VPC | WHY It Matters |
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| ------------------------ | ---------------------- | ------------------------------ | --------------------------------------------------------------- |
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| **CIDR Block** | Fixed 172.31.0.0/16 | You choose (e.g., 10.0.0.0/16) | Avoid IP conflicts when connecting to on-premises or other VPCs |
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**CIDR (Classless Inter-Domain Routing)** defines your IP address range using notation like
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`10.0.0.0/16`.
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The number after `/` tells you how many IP addresses you get:
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The number after `/` tells you how many IP addresses you get. This table shows common CIDR sizes
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you'll encounter on the exam:
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| CIDR | IP Addresses | Use Case |
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| ---- | ------------ | ---------------------------- |
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For Phase 1, we'll keep it simple but plan for growth:
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| Component | CIDR | Purpose |
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| --------------------- | ------------ | -------------------------- |
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| VPC | 10.0.0.0/16 | Room to grow |
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| Public Subnet (AZ-a) | 10.0.1.0/24 | Web servers, bastion hosts |
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| Public Subnet (AZ-b) | 10.0.2.0/24 | Future HA |
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| Private Subnet (AZ-a) | 10.0.10.0/24 | Databases (future) |
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| Private Subnet (AZ-b) | 10.0.20.0/24 | Future HA |
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| Component | CIDR | Purpose |
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| --------------------- | ------------ | ----------------------------- |
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| VPC | 10.0.0.0/16 | Room to grow |
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| Public Subnet (AZ-a) | 10.0.1.0/24 | Web servers, bastion hosts |
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| Public Subnet (AZ-b) | 10.0.2.0/24 | Future HA (High Availability) |
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| Private Subnet (AZ-a) | 10.0.10.0/24 | Databases (future) |
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| Private Subnet (AZ-b) | 10.0.20.0/24 | Future HA |
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> **Note:** AZ = Availability Zone (isolated data centers within a region).
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**WHY this design?**
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SG->>User: Response delivered
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```
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If security groups were **stateless** (like NACLs), you'd need to explicitly allow the response
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traffic on ephemeral ports (1024-65535).
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If security groups were **stateless** (like NACLs - Network Access Control Lists), you'd need to
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explicitly allow the response traffic on ephemeral ports (1024-65535).
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### Our TechBooks Security Group
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For our MVP:
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| Decision | Choice | WHY |
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| ----------------- | ----------------- | -------------------------------------------------------- |
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| **Instance Type** | t3.micro | Free tier eligible, burstable, enough for 100 users/day |
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| **AMI** | Amazon Linux 2023 | Free, optimized for AWS, long-term support |
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| **Storage** | 20GB gp3 | gp3 is cheaper than gp2 with better baseline performance |
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| Decision | Choice | WHY |
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| ----------------- | ----------------- | ------------------------------------------------------------------------------ |
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| **Instance Type** | t3.micro | Free tier eligible, burstable, enough for 100 users/day |
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| **AMI** | Amazon Linux 2023 | AMI (Amazon Machine Image) - Free, optimized for AWS, long-term support |
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| **Storage** | 20GB gp3 | gp3 (General Purpose SSD) is cheaper than gp2 with better baseline performance |
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### Instance Types Decoded
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### WHY Elastic IP Instead of Auto-Assigned Public IP?
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When you launch an EC2 instance, you can get a public IP two ways:
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When you launch an EC2 instance, you can get a public IP two ways. Understanding the difference is
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crucial for designing stable architectures:
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| Feature | Auto-Assigned Public IP | Elastic IP (EIP) |
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| ---------------- | ------------------------ | -------------------------- |
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**When NOT to use Elastic IP:**
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- Behind a Load Balancer (the LB has its own DNS name)
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- Behind a Load Balancer (the Load Balancer has its own DNS name)
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- Using Route 53 with health checks (can auto-failover)
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- Auto Scaling groups (instances are disposable)
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‎docs/scenarios/techbooks/phases/phase-2-database-separation.md‎

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### Self-Managed vs RDS Comparison
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Understanding the operational differences helps you decide when RDS is worth the cost vs managing
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your own database:
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| Task | Self-Managed (EC2) | RDS |
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| ---------------- | ----------------------- | ---------------------------------- |
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| **Installation** | You do it | AWS handles |
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### NAT Gateway vs NAT Instance
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AWS offers two ways to provide outbound internet access from private subnets. This comparison helps
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you choose between the managed service and the self-managed approach:
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| Feature | NAT Gateway | NAT Instance |
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| ------------------- | ---------------------- | ------------------------ |
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| **Managed by** | AWS | You |
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### WHY Port 3306?
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Each database engine has its standard port. RDS uses the same ports because it runs the actual
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engine:
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engine. Knowing these ports is useful for Security Group configuration and troubleshooting:
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| RDS Engine | Default Port | WHY This Port |
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| -------------- | ------------ | -------------------------------- |
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└───────── Family: T = burstable
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```
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**Common RDS families:**
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**Common RDS families** (similar to EC2, but prefixed with "db."):
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| Family | Use Case |
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| ----------------- | --------------------------------- |
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### Storage
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RDS supports multiple storage types with different IOPS (Input/Output Operations Per Second)
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characteristics. Choose based on your performance and cost requirements:
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| Type | IOPS | Use Case | Cost |
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| ------------ | ----------------------- | ----------------------------- | ---- |
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| **gp2** | Burstable, 3 IOPS/GB | General purpose (legacy) | $ |
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OnDemand["On-demand<br>User initiated"]
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end
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subgraph Recovery["Point-in-Time Recovery"]
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subgraph Recovery["PITR (Point-in-Time Recovery)"]
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PITR["Restore to any second<br>within retention period"]
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end
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## See Also
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> **Related Learning:** For database migration strategies from on-premises SQL Server to RDS,
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> including DMS and SCT tooling, see
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> including DMS (Database Migration Service) and SCT (Schema Conversion Tool), see
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> [ShipFast Phase 4: Database Migration](/scenarios/shipfast/phases/phase-4-database-migration.md).
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---

‎docs/scenarios/techbooks/phases/phase-3-high-availability.md‎

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### Key Characteristics
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Availability Zones provide the isolation needed to survive infrastructure failures while maintaining
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low-latency connectivity for replication:
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| Property | Description | WHY It Matters |
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| ------------------------------ | ------------------------------------------- | ------------------------------------------- |
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| **Physical separation** | AZs are miles apart | Natural disaster in one won't affect others |
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### WHY Synchronous Replication?
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The choice between synchronous and asynchronous replication determines your data loss risk during
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failover:
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| Replication Type | How It Works | Data Loss Risk | Use Case |
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| ---------------- | ----------------------------------------------- | ------------------ | ------------- |
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| **Synchronous** | Write confirmed only after both copies saved | Zero (RPO = 0) | Multi-AZ (HA) |
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### The "Nines" of Availability
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Availability is commonly expressed in "nines" - each additional nine represents an order of
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magnitude less downtime. Understanding what each level means in real terms helps set realistic
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expectations:
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| Availability | Downtime/Year | Downtime/Month | Downtime/Week |
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| ----------------- | ------------- | -------------- | ------------- |
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| 99% (two 9s) | 3.65 days | 7.2 hours | 1.68 hours |
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### AWS RDS SLA
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AWS offers **99.95% availability** for Multi-AZ RDS deployments.
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AWS offers a SLA (Service Level Agreement) of **99.95% availability** for Multi-AZ RDS deployments.
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**What this means:**
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**Next decisions:**
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- Add Application Load Balancer (ALB)
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- Add ALB (Application Load Balancer)
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- Deploy EC2 instances in multiple AZs
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- Implement Auto Scaling
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- Achieve true fault tolerance for the web tier

‎docs/scenarios/techbooks/phases/phase-4-auto-scaling.md‎

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### Comparison
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Understanding these two scaling approaches is fundamental - the exam frequently tests when to use
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each:
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| Aspect | Vertical (Scale Up) | Horizontal (Scale Out) |
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| ------------------- | -------------------------- | ---------------------------------- |
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| **How** | Bigger instance | More instances |
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```mermaid
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flowchart TB
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subgraph OSI["OSI Model"]
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subgraph OSI["OSI Model (Open Systems Interconnection)"]
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L7["Layer 7: Application<br>HTTP headers, paths, hosts"]
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L4["Layer 4: Transport<br>TCP/UDP ports"]
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L3["Layer 3: Network<br>IP addresses"]
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### Health Check Types
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Multiple health check mechanisms work together. Understanding which one does what is important for
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troubleshooting:
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| Type | Performed By | Checks | Action on Failure |
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| ----------------------- | ------------- | ------------------------- | ---------------------- |
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| **ELB Health Check** | Load Balancer | HTTP request to `/health` | Stop sending traffic |
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### TechBooks Scaling Configuration
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For TechBooks, we combine multiple policies for comprehensive coverage:
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| Policy | Type | Configuration |
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| ------------ | --------------- | ------------------------------ |
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| **Primary** | Target Tracking | CPU at 50% |
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**Changes:**
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| Aspect | Before (EIP) | After (ALB) |
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| ------------------------ | ----------------- | ---------------- |
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| **DNS points to** | Elastic IP | ALB DNS name |
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| **SSL termination** | EC2 instance | ALB |
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| **Instance replacement** | Must reassign EIP | Automatic |
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| **Cost** | Free if attached | ALB hourly + LCU |
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| Aspect | Before (EIP) | After (ALB) |
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| ------------------------ | ----------------- | ---------------------------------------------- |
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| **DNS points to** | Elastic IP | ALB DNS name |
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| **SSL termination** | EC2 instance | ALB |
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| **Instance replacement** | Must reassign EIP | Automatic |
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| **Cost** | Free if attached | ALB hourly + LCU (Load Balancer Capacity Unit) |
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### Updating DNS
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‎docs/scenarios/techbooks/phases/phase-5-going-global.md‎

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**The founder asks:** "Can we make the site fast for everyone, everywhere?"
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**Your decision:** Implement CloudFront CDN and optimize DNS with Route 53.
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**Your decision:** Implement CloudFront CDN (Content Delivery Network) and optimize DNS with
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Route 53.
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---
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### Edge Locations vs Regional Edge Caches
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CloudFront uses a two-tier caching architecture to balance speed and storage capacity:
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| Component | Count | Purpose |
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| ------------------------ | ----- | -------------------------- |
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| **Edge Locations** | 450+ | Serve content to end users |
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### Origin Types
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CloudFront can pull content from various origin types. Choose based on where your content lives:
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| Origin Type | Use Case | Example |
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| ----------------- | --------------- | ------------------------- |
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| **S3 Bucket** | Static files | Images, CSS, JS |
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### S3 vs EC2 for Static Content
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This comparison shows why offloading static content to S3 is a best practice:
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| Aspect | EC2 | S3 + CloudFront |
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| ------------------- | ------------------- | --------------------- |
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| **Cost (storage)** | EBS: $0.10/GB | S3: $0.023/GB |
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```
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> **SAA Exam Tip:** OAC (Origin Access Control) is the modern replacement for OAI (Origin Access
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> Identity). OAC supports more features including SSE-KMS encryption. Always use OAC for new
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> distributions.
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> Identity - legacy method). OAC supports more features including SSE-KMS encryption. Always use OAC
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> for new distributions.
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## Step 7: Cache Behaviors and TTL
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### Time To Live (TTL)
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### TTL (Time To Live)
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**TTL** determines how long CloudFront caches content before checking the origin.
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**TTL (Time To Live)** determines how long CloudFront caches content before checking the origin.
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```mermaid
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### TTL Settings
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CloudFront uses three TTL values that work together to control caching behavior:
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| Setting | Description | Default |
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| --------------- | ------------------------------ | ----------------- |
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| **Minimum TTL** | CloudFront won't cache shorter | 0 seconds |
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### Routing Policies
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Route 53 offers multiple routing policies - knowing when to use each is important for the exam:
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| Policy | Use Case | Example |
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| ---------------- | ---------------------------- | ----------------------- |
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| **Simple** | Single resource | One ALB |
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### Additional Security Options
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| Feature | What It Does | Use Case |
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| -------------------------- | ------------------------- | ------------------------ |
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| **AWS WAF** | Web Application Firewall | Block SQL injection, XSS |
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| **Geo Restriction** | Block/allow by country | Compliance, licensing |
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| **Signed URLs** | Time-limited, secure URLs | Paid content, downloads |
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| **Signed Cookies** | Multiple file access | Premium subscriptions |
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| **Field-Level Encryption** | Encrypt specific fields | Credit cards, PII |
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| Feature | What It Does | Use Case |
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| -------------------------- | ------------------------- | ----------------------------------------------- |
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| **AWS WAF** | Web Application Firewall | Block SQL injection, XSS (Cross-Site Scripting) |
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| **Geo Restriction** | Block/allow by country | Compliance, licensing |
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| **Signed URLs** | Time-limited, secure URLs | Paid content, downloads |
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| **Signed Cookies** | Multiple file access | Premium subscriptions |
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| **Field-Level Encryption** | Encrypt specific fields | Credit cards, PII |
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```mermaid
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flowchart TB

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