AWS, Azure & GCP · Cloud across providers
What are the main cloud computing models?
Short answer
Cloud computing is on-demand delivery of compute, storage, networking and managed services over the internet, billed by consumption. The main models are IaaS, PaaS and SaaS, plus serverless abstractions.
Cloud computing rests on a few properties: on-demand self-service, broad network access, resource pooling, rapid elasticity and measured service. You request capacity through an API, the provider pools hardware across customers, and you pay for what you consume.
The three classic models
- IaaS: virtual machines, disks and networks you configure yourself, such as EC2, Azure Virtual Machines or Compute Engine
- PaaS: a managed runtime where you deploy code and the platform handles servers, such as App Service, Elastic Beanstalk or Cloud Run
- SaaS: a finished product you consume, such as email, CRM or collaboration tools
- Serverless and FaaS sit above PaaS for event-driven code and managed services
The models differ mainly in where the shared responsibility line sits. With IaaS you patch the guest OS and runtime. With PaaS the provider patches the platform and you own the application and its data. With SaaS you own mostly configuration, identity and data.
Follow-ups you should expect
Interviewers use these to check whether the first answer was understanding or memorisation.
- 1Where does the shared responsibility line move between IaaS and PaaS?
- 2What makes a service serverless rather than just managed?
- 3When would you deliberately choose IaaS over PaaS?
- 4How does measured service affect architecture decisions?
Say it in the interview
Cloud computing is on-demand, elastic infrastructure and services billed by consumption. I usually describe three models: IaaS, where I manage virtual machines and networks; PaaS, where I deploy code to a managed runtime; and SaaS, where I consume a finished application. The real difference is where the shared responsibility line sits, so I choose based on control, operational capacity and cost rather than a generic preference.
Use this as a model, not a script. Saying it in your own words is what makes it survive follow-up pressure.
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