Assessment and the 7 Rs
Picking a migration strategy per application, building the business case, discovering servers and dependencies, and planning waves.
Exam tasks: 4.1 (select existing workloads and processes for potential migration, determine the optimal migration approach, evaluate application migration tooling)
The decision: for each application in the portfolio, which of the 7 Rs fits the business goal and deadline, and what data do you need to collect before you can say so with confidence?
The migration phases
AWS frames a large migration as three phases. Questions often describe an activity and expect you to know which phase it belongs to, or which tool supports it.
| Phase | What happens | Typical tools and outputs |
|---|---|---|
| Assess | Decide whether to migrate and why. Build a directional business case | Migration Evaluator (TCO), Migration Readiness Assessment, high-level inventory |
| Mobilize | Close the gaps the assessment found. Detailed discovery, dependency mapping, a 7R decision per app | Application Discovery Service, landing zone with Control Tower, a cloud center of excellence, pilot waves |
| Migrate and modernize | Move applications in waves, validate, cut over, then optimize | Application Migration Service, DMS, DataSync, wave runbooks |
Landing zone first
If a question says the company is about to migrate hundreds of servers into AWS and has no account structure, the right first step is a multi-account landing zone (Control Tower), not starting replication. That belongs in Mobilize. See multi-account governance.
The 7 Rs
- RetireTurn it off
The application has no users or is duplicated elsewhere. Decommission it and archive the data.
- RetainLeave it where it is
Not ready, recently upgraded, tied to hardware or a compliance boundary, or not worth moving yet. Revisit later.
- RehostLift and shift
Move servers as they are onto EC2, usually with Application Migration Service. Fastest path at scale.
- RelocateMove the hypervisor
Move VMs to a cloud version of the same platform, such as VMware Cloud Foundation on Amazon EVS. No change to the VMs.
- RepurchaseDrop and shop
Replace the application with a SaaS product or a newer licensed version, for example a self-hosted CRM with a SaaS CRM.
- ReplatformLift, tinker and shift
Keep the core code but swap a component for a managed service, such as a self-managed database for RDS.
- RefactorRe-architect
Rewrite for cloud-native features: microservices, containers, serverless, purpose-built databases.
A decision tree per application
| Strategy | Time to migrate | Code change | Ongoing ops benefit | Exam signal |
|---|---|---|---|---|
| Retire | None | None | Removes cost | "Nobody has logged in for a year" |
| Retain | None | None | None yet | "Contract ends in 18 months", "mainframe not in scope" |
| Rehost | Fast | None | Low at first | "Data center lease ends in 4 months", "hundreds of servers" |
| Relocate | Fastest for VMware estates | None | Low | "Keep vSphere tooling and skills", "no VM conversion" |
| Repurchase | Varies | Replaced | High | "Commodity function", "licensing model changing" |
| Replatform | Medium | Minimal | Medium | "Reduce DBA effort", "managed patching and backups" |
| Refactor | Slow | Significant | Highest | "Scale to 10x", "release weekly", "remove the monolith" |
Refactoring under a hard deadline
When a question pairs a tight exit date with a large estate, answers that refactor everything first are wrong. The usual pattern is rehost now to meet the deadline, then replatform or refactor once in AWS. Modernizing after the move is cheaper because you're no longer paying for two data centers.
Rehost is not 'no change at all'
Rehosted servers still need new IP addresses or DNS, security groups, backup policies and monitoring. Rehost means the application code and OS stay the same, not that operations stay the same.
Building the business case: Migration Evaluator
Migration Evaluator produces a directional TCO and a projected AWS cost for the discovered estate.
- It deploys a collector on premises that gathers utilization over a period of weeks, or it can use existing inventory exports.
- It right-sizes to EC2 based on actual use rather than provisioned size, and models licensing options such as bring your own license versus license included for Windows and SQL Server.
- Output is a business case report for executives. It's an Assess-phase tool, not a migration tool.
Discovery: Application Discovery Service
AWS Application Discovery Service (ADS) collects server inventory, utilization and dependencies, and stores it in the Migration Hub home Region.
| Agentless Collector | Discovery Agent | |
|---|---|---|
| Deployed as | One OVA virtual appliance in VMware vCenter | Software installed on every server |
| Works for | VMware VMs only | Physical servers, any hypervisor, VMs in other clouds, Windows and Linux |
| Collects | VM inventory, configuration, CPU, memory and disk utilization from vCenter | All of that, plus running processes and inbound and outbound network connections |
| Dependency mapping | Not from the VM layer | Yes: which servers talk to which, on which ports |
| Effort | Low: one appliance per vCenter | Higher: agent rollout through config management |
| Database inventory | Optional database and analytics module for supported engines | No |
- A CSV import is the fallback when you can't install anything. You get inventory but no utilization or dependency data.
- Agent data can be exported to S3 and queried with Athena for custom dependency analysis.
Dependencies mean agents
Whenever the question needs network dependencies between servers to group them into waves, or the estate isn't all VMware, the answer is the Discovery Agent. For a quick, low-touch inventory of a VMware estate, it's the Agentless Collector.
Tracking: Migration Hub
AWS Migration Hub is the single place to see discovered servers and track migrations across tools.
- Home Region. All discovery and tracking data lives in one Region you choose, regardless of where you migrate.
- Tracking. Application Migration Service and DMS report status into Migration Hub, grouped by application.
- Strategy Recommendations. Analyzes discovered applications, including source code and binaries through a collector, and suggests a 7R path and target service such as containers or managed databases.
- EC2 instance recommendations from discovered utilization, with purchasing options.
- Orchestrator runs templated migration workflows, and Journeys guides teams through the phases.
2025 availability change
AWS Migration Hub (including Strategy Recommendations, Orchestrator, Journeys and Refactor Spaces) and Application Discovery Service stopped accepting new customers on November 7, 2025. Existing customers can keep using them, and AWS Transform is the recommended successor for new projects. SAP-C02 still tests the concepts above, so learn what each one does.
Wave planning
A wave is a group of applications that migrate and cut over together.
Group by dependency. Servers that talk to each other constantly, such as an app tier and its database, go in the same wave so you don't create cross-site latency during the migration.
Start small. Pilot waves use low-risk, low-dependency applications to prove the tooling, runbooks and network.
Grow the waves. Later waves get bigger as the factory matures. Business-critical apps move once patterns are proven.
Plan cutover windows. Match each wave to a business-approved downtime window, with rollback criteria.
Scenarios
The requirement is dependency mapping across VMware and physical servers, which only the Discovery Agent provides. The Agentless Collector covers only VMware and doesn't capture process-level network connections. A CSV import has no utilization or dependency data. Migration Evaluator builds a business case, not a dependency map.
An unused application should be retired. The order service needs scale its current design can't provide, which calls for refactoring. Payroll is going to SaaS, so it should be retained until then and repurchased, not refactored. Rehosting the order service doesn't fix its scaling limits, and relocating an unused tool wastes money.
Migration Evaluator is built for this Assess-phase question: it discovers utilization, right-sizes and produces a business case. Pricing Calculator by hand is slow and sizes from provisioned rather than actual use. MGN is a migration tool, and Cost Explorer only knows about existing AWS spend.
Further reading
Domain 4 · Migration and modernization
20% of the exam. Deciding what to move, how to move it, and what it should become on AWS.
Data transfer
Choosing between DataSync, Transfer Family, Snowball Edge, S3 Transfer Acceleration, Direct Connect, VPN and Storage Gateway by data size, bandwidth, deadline and pattern.