Cloud Computing is the on-demand delivery of IT resources—including compute power, storage databases, and networking—over the Internet with pay-as-you-go pricing. Rather than purchasing, racking, and maintaining physical datacenters and on-premises servers, organizations rent technological infrastructure from cloud hyperscalers like AWS, Microsoft Azure, and Google Cloud Platform.
The 5 Essential Characteristics (NIST SP 800-145)
The National Institute of Standards and Technology (NIST) defines cloud computing through five mandatory criteria:
- On-Demand Self-Service: Consumers can provision computing capabilities (such as server time or cloud storage) automatically through web portals or APIs without human intervention from the service provider.
- Broad Network Access: Capabilities are accessible over standard network protocols from diverse client platforms (laptops, mobile devices, branch offices).
- Resource Pooling: The provider's physical computing resources are pooled to serve multiple customers (multi-tenancy), dynamically allocating virtual resources according to consumer demand.
- Rapid Elasticity: Capabilities can be elastically provisioned and released—often automatically—to scale outward during peak traffic and scale inward during lulls to control costs.
- Measured Service: Resource usage is monitored, controlled, and billed transparently based on actual consumption (e.g., gigabyte-hours of storage, CPU-hours, egress bandwidth).
Financial Economics: CapEx vs OpEx
Cloud adoption fundamentally alters corporate IT financial modeling:
| Financial Model | Capital Expenditure (CapEx) | Operational Expenditure (OpEx) |
|---|---|---|
| Billing Nature | Substantial upfront capital investment | Recurring ongoing operational expenses |
| Asset Ownership | Company purchases physical servers, switches, and cooling units | Company rents compute capacity as an operating utility |
| Depreciation | Assets depreciate on accounting balance sheets over 3–5 years | Deducted as operational business expense in the current tax year |
| Capacity Risk | Risk of over-provisioning (idle hardware) or under-provisioning (outages) | Scale capacity dynamically; pay only for active resources consumed |
Cloud Deployment Models
Organizations deploy workloads across different architectural boundaries depending on regulatory and security requirements:
- Public Cloud: Services are owned and operated by a third-party cloud provider (AWS, Azure, GCP) and delivered over the public Internet. Maximum scalability and zero physical hardware maintenance.
- Private Cloud: Cloud infrastructure provisioned for exclusive use by a single organization, hosted either in an on-premises enterprise datacenter or managed by a dedicated hosting provider.
- Hybrid Cloud: A unified computing environment connecting private on-premises infrastructure with public cloud resources over encrypted ExpressRoute/DirectConnect or VPN tunnels, allowing workloads and data to shift seamlessly.
- Multi-Cloud: The intentional utilization of services across multiple distinct public cloud vendors (e.g., running workloads in AWS while leveraging Microsoft 365 and Entra ID in Azure) to avoid vendor lock-in.
The Shared Responsibility Model
Security in the cloud is a shared partnership between the cloud service provider (CSP) and the customer:
- Security OF the Cloud (Provider Responsibility): Physical datacenter security, power redundancy, hardware maintenance, host virtualization hypervisors, and global network backbone infrastructure.
- Security IN the Cloud (Customer Responsibility): Guest operating system patching, customer data encryption, firewall security group configurations, IAM user access policies, and application code.
Note: The boundary of the Shared Responsibility Model shifts depending on the service tier. In IaaS, you manage the OS, runtime, and data. In SaaS, the provider manages the entire infrastructure, leaving only data classification and user access under your control.
Summary
Cloud computing replaces rigid on-premises hardware procurement with elastic, on-demand infrastructure billed on an OpEx utility model. By internalizing the five essential NIST characteristics and the Shared Responsibility Model, IT engineers architect scalable, cost-efficient cloud solutions.