A cloud engineer needs to migrate an application from on premises to a public cloud. Due to timing constraints, the application cannot be changed prior to migration. Which of the following migration strategies is best approach for this use case?

Rehosting, commonly known as "lift and shift," is the migration strategy of moving an application from its on-premises environment to a cloud IaaS platform with minimal or no modification. This approach is the fastest way to migrate because it does not require changes to the application's architecture or code. Given the strict constraints that the application cannot be changed and the migration must happen quickly, rehosting is the only suitable strategy. It allows the organization to meet its deadline by essentially replicating the existing environment in the cloud.

A. Retire: This strategy involves decommissioning the application entirely, which is contrary to the goal of migrating it.
B. Rearchitect: This involves fundamentally redesigning the application for cloud-native features, a time-intensive process that violates the "cannot be changed" constraint.
C. Refactor: This strategy requires modifying the application's code to better leverage cloud services, which is explicitly forbidden by the "cannot be changed" requirement.

1. Amazon Web Services (AWS) Documentation. In "6 Strategies for Migrating Applications to the Cloud," AWS defines Rehosting ("lift-and-shift") as moving applications without changes. It states, "Most rehosting can be automated with tools... but it can be challenging to capture all the dependencies." This aligns with a fast migration where the application itself is not altered. (Source: AWS Prescriptive Guidance, "6 Strategies for Migrating Applications to the Cloud," aws.amazon.com/cloud-migration/strategies/).

2. Microsoft Azure Cloud Adoption Framework. The framework details the "5 Rs of rationalization." It describes Rehost as: "Also known as a 'lift and shift' migration, a rehost option moves your current-state workload to the cloud with minimal changes to the application's code or architecture." This directly supports rehosting as the correct choice for a no-change, time-constrained scenario. (Source: Microsoft Docs, Azure Cloud Adoption Framework, "The 5 Rs of rationalization," docs.microsoft.com/en-us/azure/cloud-adoption-framework/digital-estate/5-rs-of-rationalization).

3. Gartner. In "Use the 5 'Rs' of Rationalization to Evaluate Your Application Portfolio," Gartner defines rehosting as redeploying an application component to a different infrastructure (physical, virtual, or cloud) without changing its code, features, or functions. This definition emphasizes the "no change" aspect critical to the question. (Source: Gartner Research, "Use the 5 'Rs' of Rationalization to Evaluate Your Application Portfolio," Document ID G00257333, Section: "The Five Treatment Options").