How do you choose between IaaS, PaaS, and SaaS, and what are the key considerations for a hybrid cloud strategy?

Choosing between Infrastructure as a Service (IaaS), Platform as a Service (PaaS), and Software as a Service (SaaS) depends largely on the level of control required, the existing technology stack, and the operational overhead a team is willing to manage. IaaS, like AWS EC2 or Azure VMs, provides virtualized computing resources, storage, and networking. You manage the operating system, middleware, and applications, offering maximum flexibility but also requiring significant operational effort. PaaS, such as AWS Elastic Beanstalk or Azure App Service, abstracts away the underlying infrastructure, providing a ready-to-use platform for application deployment and scaling. This reduces operational burden but limits control over the OS and runtime environment. SaaS, like Salesforce or Google Workspace, is a fully managed application consumed directly by end-users, requiring no infrastructure management from your side at all.

Hybrid Cloud Strategy

A hybrid cloud strategy involves combining an on-premises private cloud with a public cloud service, allowing data and applications to be shared between them. Key considerations include seamless network connectivity with low latency and high bandwidth, consistent identity and access management across environments, and data governance policies for security and compliance. It also requires careful orchestration and management tools that can span both environments, ensuring consistent deployment and monitoring. Data locality, disaster recovery, and burst capacity for peak loads are common drivers for hybrid adoption, enabling organizations to leverage public cloud scalability while maintaining sensitive data on-premises.

Best practice

A best practice for selection is to conduct a thorough workload analysis. Evaluate each application’s requirements for customization, scalability, data sensitivity, compliance, and developer productivity. For legacy applications requiring specific OS or deep network control, IaaS is often suitable. For new, cloud-native applications focused on rapid development and deployment, PaaS offers significant advantages. SaaS is ideal for off-the-shelf business functions where customization is minimal. When considering hybrid, prioritize workloads that benefit most from public cloud elasticity or specific services, while keeping critical, latency-sensitive, or highly regulated data on-premises.

Edge case interviewers probe for

Interviewers might ask about situations where a seemingly obvious choice is suboptimal. For example, a company might initially lean towards PaaS for a microservices architecture for ease of deployment. However, if that architecture requires highly specialized, non-standard runtimes or extensive kernel-level tuning for performance, IaaS or even container orchestration on IaaS (like Kubernetes) might offer the necessary control that PaaS abstracts away. Another edge case is when data residency laws mandate specific geographic locations for data, making certain public cloud regions or a hybrid approach essential, even if full public cloud adoption would otherwise be simpler.

Common mistake

A common mistake is treating the public cloud simply as an extension of the on-premises data center without adopting cloud-native design principles. This can lead to “lift and shift” migrations that fail to realize the cost or scalability benefits of the cloud, often resulting in higher operational costs than anticipated. For hybrid setups, neglecting consistent security policies, identity management, and network architecture across both environments can create significant attack surfaces and operational complexity, undermining the perceived benefits of flexibility and control.

What the interviewer is checking

The interviewer is checking your fundamental understanding of cloud service models and their implications beyond marketing definitions. They want to see if you can analyze trade-offs between control, operational overhead, and flexibility, and apply this knowledge to real-world architectural decisions. For hybrid cloud, they are assessing your ability to design interconnected, secure, and manageable systems that span different environments, demonstrating an awareness of the complexities involved in integrating disparate infrastructure.