Paraphrased summary of Google's guidance on Cloud migration for the real world -
The popular “R” migration strategies (Gartner's 5 R's, AWS's 6 R's, Citrix’s 7 R’s, Infosys’ 8 R’s) aren’t really strategies at all.
An IT organization's policies and capabilities do more to determine the migration path than anything else and ultimately override any architect’s prior top-down planning. Almost no application falls squarely into any one of the migration strategies across all of its layers. To develop a real migration strategy, first answer why your organization is adopting the cloud. How you approach it follows from that.
To take a more holistic approach to migration according to the Google Cloud Adoption Framework, determine whether your cloud migration needs are tactical, strategic or transformational.
If your objective is to reduce cost with minimal change to your applications (i.e., tactical) then take the migration factory approach.
If your objective is to maximize the benefits you get from your software (i.e., strategic) then take the modernization factory or the greenfield software development approach.
If your objective is to innovate with net new applications that solve new business problems (i.e., transformational) then take the greenfield software development approach.
Migration factory approach works well if the applications are simple and similar enough and if the team handling the migration can execute it autonomously without needing to coordinate much with individual application teams. It is suited for initiatives that are infrastructure-led rather than application-led.
Greenfield software development approach - Rather than migrate an existing application at all, develop a new “greenfield” or “cloud-native” application instead, following agile software development practices. Treat newly developed application as a product and not as a project, meaning the team does not abandon its work after the last milestone has been reached. In this regard, the approach is fundamentally different from any kind of migration.
Modernization factory approach - A modernization factory requires a hybrid team construct, composed of a small cross-functional app team that is familiar with the individual application plus a “factory” team that is familiar with the portfolio of target services and modernization tactics to be employed across all applications. Customers usually realize their true TCO savings through incremental modernizations. Improving their DevOps capabilities goes hand in hand with cloud adoption. Provisioning consistent project environments with the help of infrastructure as code constitutes a noteworthy modernization.
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