AZ-900 Describe cloud concepts Practice Question
A startup is building a mobile app backend and wants to use cloud services. They want to focus entirely on writing code and deploying features without worrying about server maintenance, operating system patches, or scaling infrastructure. Which cloud service model best fits this requirement?
⚠ Common exam trap
Many candidates confuse FaaS (serverless) with PaaS, but the AZ-900 exam tests the core NIST service models (IaaS, PaaS, SaaS) where FaaS is considered a subset of PaaS or a serverless compute option, not a primary model; the question explicitly asks for the 'best fit' among the listed options, and PaaS directly addresses the full requirement of focusing on code and deployment without infrastructure management.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Platform as a Service (PaaS)
Platform as a Service (PaaS) is the correct model because it abstracts the underlying infrastructure, including servers, operating systems, and scaling, allowing developers to focus solely on writing code and deploying features. Azure App Service is a prime example of PaaS, providing built-in load balancing, auto-scaling, and patching without developer intervention.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Infrastructure as a Service (IaaS)
Why it's wrong here
Infrastructure as a Service (IaaS) offers virtual machines, virtual networks, and storage resources, but it leaves the customer responsible for managing the operating system, middleware, runtime, and application patches. While an IaaS VM could technically host a mobile app backend, the team would incur significant operational overhead: they must configure and secure the VM, install and update the runtime, manage the database, and implement their own scaling solutions. This level of management contradicts the scenario's emphasis on using cloud services to simplify backend operations. PaaS is more appropriate because it abstracts these infrastructure responsibilities, enabling developers to focus solely on application code.
When this WOULD be correct
A company needs to migrate an on-premises application to the cloud with minimal changes, requiring full control over the OS, middleware, and runtime environment, while still benefiting from scalable virtual machines.
- ✓
Platform as a Service (PaaS)
Why this is correct
Platform as a Service (PaaS) is the correct choice for building a custom mobile app backend because it abstracts the underlying infrastructure, including servers, storage, and networking, while also providing a fully managed platform for application hosting. The cloud provider handles critical operational concerns such as automatic scaling, patching, and high availability, allowing the development team to concentrate on writing application code and business logic. PaaS also offers built-in features like database management, authentication, and app monitoring, which are directly relevant to backend development. This makes it the most efficient model for deploying a complete, custom application without the burden of infrastructure maintenance.
- ✗
Software as a Service (SaaS)
Why it's wrong here
Software as a Service (SaaS) delivers pre-built, ready-to-use applications that are accessed over the internet, such as Office 365, Salesforce, or Google Workspace. With SaaS, the customer does not write any code; they only configure and use the software, which means it cannot support the development of a custom mobile app backend. Since the backend must be tailored to the specific business logic and data requirements of the mobile app, SaaS lacks the flexibility, programmability, and ownership needed. Therefore, SaaS is incorrect because it solves a completely different problem—providing finished applications rather than an environment to build one.
When this WOULD be correct
A company wants to use a cloud-based email service, customer relationship management (CRM) tool, or office productivity suite without managing the underlying infrastructure or software. For example, 'A small business needs email and collaboration tools without IT maintenance.'
- ✗
Function as a Service (FaaS)
Why it's wrong here
FaaS (e.g., Azure Functions) allows running small pieces of code in response to events. While it abstracts infrastructure, it is typically used for microservices or event-driven scenarios, not for a full backend application that might need a persistent server. The scenario describes a broader mobile app backend, which is better served by PaaS.
When this WOULD be correct
A question where the requirement is to run code in response to events (e.g., file uploads, database changes) without managing servers, and the application is composed of small, stateless functions. For example: 'A company wants to process images automatically when uploaded to cloud storage. Which service model minimizes operational overhead?'
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The AZ-900 exam frequently reuses these exact scenarios with slightly different constraints.
✓Platform as a Service (PaaS)Correct answer▾
Why this is correct
Platform as a Service (PaaS) is the correct choice for building a custom mobile app backend because it abstracts the underlying infrastructure, including servers, storage, and networking, while also providing a fully managed platform for application hosting. The cloud provider handles critical operational concerns such as automatic scaling, patching, and high availability, allowing the development team to concentrate on writing application code and business logic. PaaS also offers built-in features like database management, authentication, and app monitoring, which are directly relevant to backend development. This makes it the most efficient model for deploying a complete, custom application without the burden of infrastructure maintenance.
✗Infrastructure as a Service (IaaS)Wrong answer — click to see why▾
Why this is wrong here
IaaS provides virtualized servers and storage, but the startup would still need to manage operating systems, apply patches, and handle scaling, which contradicts their desire to focus solely on code and deployment.
★ When this WOULD be the correct answer
A company needs to migrate an on-premises application to the cloud with minimal changes, requiring full control over the OS, middleware, and runtime environment, while still benefiting from scalable virtual machines.
Why candidates choose this
Candidates may confuse IaaS with PaaS, thinking that cloud services automatically eliminate all maintenance, but IaaS still requires significant operational overhead for OS and infrastructure management.
✗Software as a Service (SaaS)Wrong answer — click to see why▾
Why this is wrong here
SaaS provides ready-to-use software applications, not a platform for building custom mobile app backends. The startup needs to write code and deploy features, which requires a development platform, not pre-built software.
★ When this WOULD be the correct answer
A company wants to use a cloud-based email service, customer relationship management (CRM) tool, or office productivity suite without managing the underlying infrastructure or software. For example, 'A small business needs email and collaboration tools without IT maintenance.'
Why candidates choose this
Candidates may confuse SaaS with PaaS because both are cloud services that reduce management overhead, but SaaS delivers finished applications, while PaaS provides a development environment.
✗Function as a Service (FaaS)Wrong answer — click to see why▾
Why this is wrong here
FaaS (Function as a Service) is a subset of serverless computing that runs individual functions in response to events, but it still requires developers to manage application logic and state, and does not provide a full platform for building and deploying a mobile app backend with features like databases and authentication.
★ When this WOULD be the correct answer
A question where the requirement is to run code in response to events (e.g., file uploads, database changes) without managing servers, and the application is composed of small, stateless functions. For example: 'A company wants to process images automatically when uploaded to cloud storage. Which service model minimizes operational overhead?'
Why candidates choose this
Candidates may confuse FaaS with PaaS because both abstract infrastructure, but FaaS is more granular and event-driven, leading them to think it fits a 'focus on code' scenario without realizing PaaS offers a more complete development platform.
Analysis generated from the official AZ-900blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Go deeper
Related to this question
Learn chapter
What is Cloud Computing?
Key term
Azure App Service
Azure App Service is a fully managed platform for building, deploying, and scaling web applications and APIs without managing the underlying infrastructure.
Key term
PaaS
Platform as a Service (PaaS) is a cloud computing model that provides a managed platform for developers to build, run, and manage applications without dealing with the underlying infrastructure.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This AZ-900 practice question is part of Courseiva's free Microsoft certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the AZ-900 exam.