Fundamental
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Fundamental
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IaaS for Lift-and-Shift Legacy Migrations
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An organization plans to migrate a legacy on-premises application to Microsoft Azure without modifying the application code. The IT team requires complete control over the operating system, disk storage, and virtual network configuration to replicate their existing environment.
Which cloud service model should the organization use for this migration?
Explanations
Software as a Service (SaaS)
Software as a Service (SaaS)
What Software as a Service (SaaS) Is
Software as a Service (SaaS) is a model where end-users consume centrally hosted, ready-to-use software applications over the internet. Examples include Microsoft 365, Microsoft Dynamics 365, and web-based email solutions. In this model, the cloud provider manages every layer of the technology stack, including physical hardware, networking, operating systems, application runtime, and application updates.
Why It Doesn't Meet the Requirement
- No Custom Application Hosting: SaaS delivers pre-built end-user applications rather than hosting custom proprietary or legacy software.
- Zero Infrastructure Control: Customers have no visibility or control over operating systems, compute sizing, virtual networking, or storage provisioning.
Key Characteristics and Limitations
- Complete Provider Management: The vendor handles all infrastructure, data backup, security updates, and feature rollouts.
- Subscription-Based Consumption: Licensed primarily per user, seat, or subscription tier rather than compute capacity.
- Limited Configuration: Customization is limited strictly to user-level preferences, tenant settings, and integrations.
This Answer Would Be Correct If
The organization wanted to replace an in-house email server or CRM system with an off-the-shelf, fully managed software application accessed directly via a web browser.
Additional Context
SaaS represents the highest tier of cloud abstraction and is intended for end-user software delivery rather than migrating customized corporate workloads.
Function as a Service (FaaS) / Serverless
Function as a Service (FaaS) / Serverless
What Serverless / FaaS Is
Function as a Service (FaaS), commonly referred to as serverless computing, is an execution model where developers deploy discrete event-driven blocks of code (such as Azure Functions). The cloud platform automatically handles provisioning, executing, scaling, and terminating compute instances strictly when triggered by events.
Why It Doesn't Meet the Requirement
- No Persistent Server Environment: FaaS executes short-lived, stateless functions rather than running continuous long-term operating system environments.
- No Operating System Control: Users have zero access to the underlying virtual machines, OS configuration, or low-level network adapter settings.
- Incompatible with Monolithic Legacy Systems: Migrating a legacy application to serverless requires breaking it down entirely into micro-functions and event triggers.
Key Characteristics and Limitations
- Event-Driven Execution: Compute resources run only in response to specific triggers (e.g., HTTP requests, queue messages, database updates).
- Sub-Second Billing: Users are billed solely for the exact compute execution duration and memory consumed while code runs.
- Stateless Architecture: By default, individual function executions do not maintain local persistent state.
This Answer Would Be Correct If
The development team needed to run event-triggered data processing routines or scheduled background tasks without managing any persistent server instances.
Additional Context
Serverless is ideal for event processing and modern microservice architectures, but it is not viable for hosting un-refactored legacy applications requiring direct OS and persistent disk access.
Infrastructure as a Service (IaaS)
Infrastructure as a Service (IaaS)
What Infrastructure as a Service (IaaS) Is
Infrastructure as a Service (IaaS) is a cloud computing category where a cloud vendor provides fundamental compute, network, and storage resources on demand over the internet. In an IaaS model, the cloud provider manages the physical hardware, data centers, virtualization layer, and physical networking, while the consumer retains direct administrative control over the operating system, middleware, storage disks, installed runtimes, and networking configurations.
How It Meets the Scenario Requirements
- OS and Application Control: Azure Virtual Machines provide administrators with complete root or administrative access to customize the guest operating system and install legacy software dependencies.
- Lift-and-Shift Support: IaaS allows workloads to be migrated directly from physical servers or on-premises hypervisors into the cloud without refactoring application code.
- Custom Storage and Networking: Organizations can allocate specific virtual hard disks (VHDs), configure Azure Virtual Networks, establish custom subnets, and assign private IP addresses to mirror on-premises topologies.
Key Advantages and Capabilities
- Maximum Flexibility: Offers total freedom to configure OS patches, security baselines, and background system services.
- Rapid Cloud Onboarding: Minimizes application refactoring and development overhead during lift-and-shift cloud migrations.
- Granular Resource Sizing: Allows teams to resize virtual machine SKUs and disk performance tiers as performance demands change.
- Cost Efficiency: Operates on a pay-as-you-go consumption model, billing only for compute seconds and allocated storage used.
Why This Is Optimal
Because the organization specifically requires full administrative control over the underlying operating system and must migrate legacy software without code refactoring, IaaS is the only service model that provides the necessary hardware abstraction while maintaining OS-level authority.
Platform as a Service (PaaS)
Platform as a Service (PaaS)
What Platform as a Service (PaaS) Is
Platform as a Service (PaaS) provides a fully managed environment for building, running, and managing applications. In this model, the cloud provider manages the physical hardware, virtualization, operating system maintenance, security patching, and core platform runtimes, allowing developers to focus solely on application code and configuration settings.
Why It Doesn't Meet the Requirement
- Lack of OS Access: PaaS abstractions, such as Azure App Service, deliberately restrict administrative access to the underlying operating system and virtual machine configuration.
- Application Modification Required: Legacy applications that depend on specific OS registry keys, custom system services, or unique file-system structures often require significant architectural refactoring before they can run in a PaaS environment.
Key Characteristics and Limitations
- Abstracted Infrastructure: Users cannot choose or maintain the underlying operating system kernel or install custom OS-level drivers.
- Automated Maintenance: Platform patching, scaling, and framework updates are managed automatically by the cloud provider.
- Optimized for Modern Workloads: Best suited for cloud-native applications, web APIs, and containerized microservices rather than legacy software.
This Answer Would Be Correct If
The organization wanted to deploy new cloud-native web applications without managing servers, operating system updates, or storage infrastructure.
Additional Context
While PaaS reduces operational overhead, attempting a direct lift-and-shift migration of a complex legacy system to PaaS usually requires substantial code rewrites, conflicting with the scenario constraints.
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