Several strategies exist for the migration of legacy applications and a complete greenfield rebuild from scratch is not always recommended. The talk presents the different 5R strategies (retire, replace, rehost, retain, reengineer) for legacy migration and makes a comparison with their advantages and drawbacks. Special characteristics of the strategies for a migration to the cloud (e.g. lift & shift) are also considered.

A strategy for itself will not be enough to master the challenges of legacy migration. Dependencies to other applications are sometimes ignored when the strategy is chosen and often it is necessary to prepare the application to meet the preconditions for the migration. At this point concrete tactics or patterns are used to convert the strategy to a successful project. Some tactics are the usage of bridging technologies or the layer-based approach for an iterative modernization of the complete application. The strangler fig pattern is quite popular for the stepwise migration of legacy applications. The talk presents concrete examples from successful migration projects for these and other tactics and patterns.