Software Engineer

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Short, exam-focused notes on the three cloud service types: IaaS, PaaS, and SaaS. Read it once, then use the tables and the quiz at the end to review.

This post builds on my AZ-900 notes on cloud computing, cloud models, and Azure Stack.

1. The Big Idea in One Sentence

The service type decides how much you manage and how much Microsoft manages.

More control, more work  <---------------------------->  Less control, less work

   On-premises        IaaS              PaaS              SaaS
   (you do it all)    (you get VMs)     (you get a        (you get a
                                         platform)          finished app)

Memory trick: I → P → S = “I Prefer Simple.” Each step to the right is simpler for you.

2. The Pizza Analogy

Type Pizza version What you do
On-premises Make pizza at home Everything: dough, oven, table
IaaS Rent a kitchen You bring the ingredients and cook
PaaS Pizza delivery You just set the table and eat
SaaS Eat at a restaurant You just show up and eat

3. The Three Types

IaaS: Infrastructure as a Service

You rent the hardware (as virtual machines). You manage everything on top of it.

  • You manage: operating system, patches, runtime, apps, data
  • Microsoft manages: physical servers, network, datacenter
  • Most control, most work
  • Azure examples: Azure Virtual Machines, Azure Virtual Network, Azure Managed Disks
  • Use it for: lift-and-shift (move on-prem servers to Azure without changes), test and dev environments, needing full control of the OS

PaaS: Platform as a Service

You bring your code and data. Microsoft runs the platform (servers, OS, patching).

  • You manage: your application code and data
  • Microsoft manages: everything below your app, including the OS, patching, and runtime
  • Balanced control and work
  • Azure examples: Azure App Service, Azure SQL Database, Azure Functions, Azure Cosmos DB
  • Use it for: building and deploying apps fast without managing servers

SaaS: Software as a Service

You use a finished app. Microsoft runs all of it.

  • You manage: your data, users, and settings
  • Microsoft manages: the whole app and everything under it
  • Least control, least work
  • Examples: Microsoft 365 (Word, Excel, Outlook), Teams, Dynamics 365
  • Use it for: email, office work, messaging, CRM: ready-to-use software

4. Side-by-Side Comparison

  IaaS PaaS SaaS
You get Virtual machines A platform to run code A finished app
You manage OS + apps + data Apps + data Data + users
Who patches the OS? You Microsoft Microsoft
Control Highest Medium Lowest
Effort Highest Medium Lowest
Azure example Virtual Machines App Service, Azure SQL Database Microsoft 365
Typical user IT admins Developers End users

5. Shared Responsibility at a Glance

✅ = you manage it. ☁️ = Microsoft manages it. 🤝 = shared.

  IaaS PaaS SaaS
Data, devices, accounts and identities ✅ ✅ ✅
Identity and directory infrastructure ✅ 🤝 🤝
Applications ✅ 🤝 ☁️
Network controls ✅ 🤝 ☁️
Operating system ✅ ☁️ ☁️
Physical hosts, network, datacenter ☁️ ☁️ ☁️

The two rules that answer most questions:

  1. Your data, devices, and accounts are ALWAYS yours, even in SaaS.
  2. The physical datacenter is ALWAYS Microsoft’s, even in IaaS.

6. Where Does Serverless Fit?

Serverless = you write code (or a workflow), Microsoft runs it only when something triggers it, and you pay only when it runs. Servers still exist; you just never see them.

  • Azure Functions (run code on an event)
  • Azure Logic Apps (run a workflow)

On the exam, serverless is treated as part of PaaS.

7. Spot the Answer: Exam Keywords

If the question says… Answer
“full control of the OS”, “lift-and-shift”, “install custom software on the server”, “virtual machine” IaaS
“deploy code without managing servers”, “focus on development”, “Microsoft handles OS patching”, “managed database” PaaS
“ready-to-use”, “subscription software”, “email”, “Microsoft 365”, “no development needed” SaaS
“run code when an event happens”, “pay per execution” Serverless (PaaS)

8. Quick Quiz

1. You need to move 20 on-premises servers to Azure without changing them. Which type?

Show answer

IaaS. That’s lift-and-shift onto Azure Virtual Machines.

2. Your developers want to deploy a web app and never patch an OS. Which type?

Show answer

PaaS. For example, Azure App Service.

3. Your company switches its email to Microsoft 365. Which type?

Show answer

SaaS.

4. In IaaS, who patches the operating system?

Show answer

You, the customer. In PaaS and SaaS, Microsoft does.

5. In SaaS, who is responsible for your data?

Show answer

You, the customer. Data is always the customer’s responsibility.

6. Which type gives you the MOST control? The LEAST?

Show answer

Most: IaaS. Least: SaaS.

7. Is Azure SQL Database IaaS, PaaS, or SaaS?

Show answer

PaaS. Microsoft manages the database server and OS; you manage your data and schema. (SQL Server installed on an Azure VM would be IaaS.)

8. Azure Functions runs code only when an event happens. Which category?

Show answer

Serverless, which counts as PaaS on the exam.

9. Remember These 5 Things

  1. IaaS = rent VMs. You manage the OS. Most control. (Azure Virtual Machines)
  2. PaaS = bring your code. Microsoft manages the OS. (App Service, Azure SQL Database, Functions)
  3. SaaS = use the app. Microsoft manages everything but your data. (Microsoft 365)
  4. Data, devices, and identities are always yours. The physical datacenter is always Microsoft’s.
  5. I → P → S: control goes down, convenience goes up.

References

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