Cloud Concepts and Architecture
Essential characteristics
Cloud is defined by five attributes: on-demand self-service, broad network access, resource pooling (multi-tenancy), rapid elasticity, and measured/metered service. The classic trap: a description of a ticket-driven, single-tenant virtualised data centre is not a private cloud, because self-service and pooling are missing. "Private cloud" and "on-premises" are not synonyms; a private cloud can even be hosted by a third party for one organisation's exclusive use.
Service models and the responsibility line
IaaS hands you virtual compute, storage and network (EC2, Azure VMs, Compute Engine). PaaS manages OS and runtime (App Service, App Engine, managed Kubernetes). SaaS delivers a finished application. FaaS/serverless functions are an event-driven slice of PaaS. Responsibility moves up the stack: in IaaS the customer patches the guest OS, configures firewalls and manages identities; in SaaS the provider runs the application while the customer still owns data, access grants and endpoints.
Deployment models and resilience
Public, private, hybrid (workloads span both, joined by secure connectivity) and community. Multi-tenancy is the economic engine of public cloud and the source of its isolation questions. Distinguish scalability (planned capacity growth) from elasticity (automatic response to demand), horizontal/scale-out from vertical/scale-up, high availability (survive a component failure, using regions and availability zones/availability sets) from disaster recovery (survive loss of a site), and remember an SLA typically promises service credits, not business continuity.
Rule of thumb: assign any control by asking "could the provider fix this without my involvement?" No OS to patch in your console → provider's job. Anything you configured in your own account (IAM, data, network rules, keys) → yours, whatever the service model.