Cloud Concepts & Models
What is Cloud Computing?
On-demand access to compute, storage, and networking resources over the internet — pay only for what you use.
Five Key Characteristics
| Characteristic | Meaning |
|---|---|
| On-demand self-service | Provision instantly, no human interaction |
| Broad network access | Available from anywhere |
| Resource pooling | Shared infrastructure (multi-tenant) |
| Rapid elasticity | Scale up/down automatically |
| Measured service | Metered billing — pay per use |
Cloud Deployment Models
PUBLIC PRIVATE HYBRID
────────────── ────────────── ──────────────────────
Azure (cloud) Your datacenter Both, connected
Anyone can use Internal only Flexible split
Least control Full control Mixed control
Cheapest Most expensive Middle cost
When to Use Each
| Model | Use When |
|---|---|
| Public | Normal workloads, variable scale, no special compliance |
| Private | Regulatory, sensitive data, full control required |
| Hybrid | Some data must stay on-premises, burst to cloud for scale |
Exam: Hybrid cloud is for organisations that can’t fully move to public cloud (compliance, legacy systems).
Cloud Service Models
YOU MANAGE ←────────────────────────────────→ PROVIDER MANAGES
SaaS PaaS IaaS
(Use it) (Build on it) (Rent the hardware)
────────── ───────────── ──────────────────
Office 365 App Service Azure VMs
Dynamics SQL Database Azure Storage
Teams Functions Virtual Networks
Shared Responsibility
| Layer | IaaS | PaaS | SaaS |
|---|---|---|---|
| Application | You | You | Provider |
| Data | You | You | Provider |
| Runtime | You | Provider | Provider |
| OS | You | Provider | Provider |
| Virtualisation | Provider | Provider | Provider |
| Hardware / Physical | Provider | Provider | Provider |
Exam: With IaaS you’re responsible for OS patches. With PaaS and SaaS you’re not.
CapEx vs OpEx
| CapEx (Capital Expenditure) | OpEx (Operating Expenditure) | |
|---|---|---|
| What | Buy hardware upfront | Pay monthly/hourly |
| Example | On-premises server | Azure subscription |
| Cloud model | Private cloud (you own hardware) | Public cloud |
| Tax treatment | Depreciated over time | Deducted immediately |
Exam: Cloud = OpEx. On-premises = CapEx.
Key Benefits of Cloud
| Benefit | Meaning |
|---|---|
| High Availability | Stays up despite failures (SLA guarantees) |
| Scalability | Add resources as demand grows |
| Elasticity | Auto-scale up AND down with demand |
| Agility | Deploy in minutes, not months |
| Geo-distribution | Data centres worldwide |
| Disaster recovery | Backup and failover built in |
Scalability vs Elasticity:
- Scalability = can grow (add more VMs over time)
- Elasticity = automatically grows and shrinks in real time
Fault Tolerance & Disaster Recovery
| Term | Meaning |
|---|---|
| Fault tolerance | System keeps running when a component fails |
| High Availability | Minimal downtime — measured by SLA (e.g. 99.95%) |
| Disaster Recovery | Recover from major outage |
| RTO | Recovery Time Objective — max acceptable downtime |
| RPO | Recovery Point Objective — max acceptable data loss |
Exam Checklist
- Name the 3 cloud models and a use case for each
- Explain IaaS / PaaS / SaaS and who manages what
- Explain CapEx vs OpEx
- Distinguish scalability from elasticity
- Define HA, fault tolerance, RTO, RPO