When to Use IaaS, PaaS or SaaS – Making the Right Choice Every Time
Knowing the three cloud service models is theory. Knowing when to actually use each one is what makes you useful in a real IT project.
- The specific conditions that make IaaS the right choice for a workload
- When PaaS makes more sense than building everything from scratch on IaaS
- Which types of users and organisations are natural SaaS candidates
- How to read an AZ-900 scenario question and identify the correct service model quickly
What is When to Use IaaS, PaaS or SaaS?
Choosing between IaaS, PaaS, and SaaS is a practical decision that depends on three things: how much control you need over the environment, how much technical management capacity your team has, and what the workload actually requires.
There is no universal right answer. A software development team building a custom application has different needs from a sales team that just needs CRM software. A company migrating a legacy system has different needs from a startup launching a new web app. The skill is in matching the model to the need.
Why Does This Matter?
This is one of the most scenario-heavy topics in AZ-900. The exam describes a business situation and asks which service model is most appropriate. Getting this right requires understanding not just the definitions but the practical logic behind each choice. In real IT roles, this decision directly affects cost, delivery speed, and how much ongoing management work your team takes on.
The Real-World Story
Nandini runs a small software consultancy. Three different clients come to her in the same week, each needing a cloud solution. The first client runs a hospital that uses a highly specialised medical records application built fifteen years ago. It requires a specific version of Windows Server, a particular database engine, and a custom network configuration that no modern cloud platform supports out of the box. Nandini recommends IaaS — Azure Virtual Machines — so the client can replicate their exact environment in the cloud and keep full control over every layer of the stack. Anything less would not meet their technical requirements. The second client is a startup that wants to launch a customer-facing mobile app backend within two weeks. They have two developers and zero interest in managing servers, operating systems, or runtime environments. They just need to write their API code and deploy it. Nandini recommends PaaS — Azure App Service — so the developers can push their code and let Azure handle everything underneath. The team ships faster and spends zero time on infrastructure. The third client is a fifty-person logistics company that needs email, shared documents, and video calling for their team. They have no IT department and no desire to build any technology themselves. Nandini recommends SaaS — Microsoft 365 — because they do not need to build anything, they just need working software their team can use from day one. Three clients, three correct answers, all based entirely on what each client actually needed.
Going Deeper
IaaS is the right choice when you need maximum control over the operating environment. The most common IaaS scenarios involve migrating legacy applications that have specific OS or software version requirements that cannot be met by a managed platform. It also suits workloads that require custom network configurations, specific security setups, or software that is simply not available as a managed service. Development and test environments are another strong IaaS use case — teams often need the ability to spin up and destroy complete environments quickly, with full control over what is installed. The trade-off is always management overhead — choosing IaaS means your team owns everything above the virtual hardware layer.
PaaS is the right choice when your team wants to focus entirely on application development without managing infrastructure. If you are building a web application, an API, a mobile backend, or a data processing pipeline, PaaS platforms like Azure App Service, Azure Functions, or Azure SQL Database let you deploy your code and data without touching servers or operating systems. PaaS dramatically accelerates development because the platform handles patching, scaling, availability, and the runtime environment automatically. The trade-off is reduced flexibility — you work within the constraints the platform sets, which occasionally means you cannot do something highly specific that IaaS would allow.
SaaS is the right choice for business functions that do not require custom software. Email, document collaboration, video conferencing, accounting, customer relationship management, HR systems — these are all categories where mature, well-built SaaS products exist and building a custom alternative makes no financial or practical sense. SaaS also makes sense for small and medium organisations that have no dedicated IT team and need working software immediately with minimal setup. The trade-off is almost no customisation — you use the software largely as the provider designed it.
- Choose IaaS when you need full control over the environment, are migrating legacy applications, or have specific OS and configuration requirements.
- Choose PaaS when your team wants to focus on writing and deploying application code without managing servers, operating systems, or runtime environments.
- Choose SaaS when you need ready-to-use software for business functions like email, collaboration, or CRM and have no need to build a custom solution.
- The deciding factor is always the combination of control needed, team capacity for management, and what the workload specifically requires.
- For AZ-900 scenario questions, look for keywords — 'full control' and 'custom OS' point to IaaS, 'deploy code quickly' points to PaaS, 'ready to use' and 'no IT team' point to SaaS.
