Explain the 6R cloud migration framework. What are the six migration strategies, and when is each one most appropriate?

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The 6R framework, originally proposed by Gartner and popularized by AWS, provides six distinct strategies for migrating applications to the cloud. Each strategy represents a different level of effort, risk, and potential benefit, and the right choice depends on the specific application's characteristics and business context.

Rehosting, often called lift and shift, involves moving an application to the cloud with minimal or no changes. The application is migrated as-is to cloud infrastructure, typically from a physical server to a virtual machine. This is the fastest and lowest-risk migration approach, ideal for applications that need to move quickly due to data center lease expiration or when the organization wants to start benefiting from cloud economics immediately. While rehosting does not optimize for cloud-native features, it provides a foundation for future optimization once the application is running in the cloud.

Replatforming, sometimes called lift, tinker, and shift, makes a few targeted optimizations during migration without changing the core application architecture. Examples include migrating a self-managed database to a managed service like RDS, replacing a local file system with object storage like S3, or updating the application to use a cloud-based load balancer. Replatforming balances speed with moderate improvements in operations and cost.

Repurchasing involves replacing an existing application with a different product, typically a SaaS solution. Migrating from a self-hosted CRM to Salesforce, or from a custom HR system to Workday, are examples. This strategy makes sense when commercial solutions better meet business needs and the organization wants to stop maintaining custom software.

Refactoring, also called re-architecting, involves redesigning the application to take full advantage of cloud-native features. This might mean breaking a monolith into microservices, adopting serverless architectures, or implementing event-driven patterns. Refactoring offers the greatest long-term benefits in scalability, performance, and cost efficiency but requires the most time, effort, and technical skill.

Retiring means identifying applications that are no longer needed and decommissioning them. Organizations often discover during migration assessment that 10 to 20 percent of their application portfolio is redundant, obsolete, or rarely used. Retiring these applications saves migration effort and reduces ongoing maintenance costs.

Retaining, sometimes called revisit, means keeping an application in its current environment, at least for now. Some applications may have regulatory requirements that prevent cloud migration, deep dependencies on on-premises hardware, or an approaching end-of-life that makes migration investment unwarranted. Retained applications are typically revisited in future migration waves.

In practice, a large migration involves a portfolio approach where different applications use different strategies. Quick wins are rehosted to build cloud skills and demonstrate progress, operationally expensive applications are replatformed, strategically important applications are refactored, and unnecessary applications are retired.