The Six Rs
Rehost, replatform, refactor, repurchase, retire, retain — per application.
4 to work through
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beginner Multiple choice
A data-centre lease ends in seven months and 900 virtual machines on an on-premises hypervisor cluster have to leave. The portfolio review assigned every one of them "rehost" — rebuild each machine on native cloud instances. The team moves about 25 machines a week. Which disposition fits the constraint?
3 min answer -
intermediate
A company must modernise a portfolio of applications. How should each be assigned a modernisation approach, and what is the most common misassignment?
2 min answer -
intermediate
A travel platform must move a portfolio of applications off ageing infrastructure. How should each application be assigned among rehost, replatform, refactor, rebuild, replace and retire?
2 min answer -
intermediate Multiple choice
Of the six migration strategies, which is most often skipped and why does that matter?
1 min answer
5 terms in this topic
Disposition Decision
Choosing one of the standard migration treatments for each application, on the basis of its value, condition and the constraint that is actually binding.
patternRelocate Disposition
The migration treatment that moves a hypervisor-managed estate to a hypervisor-compatible target with the application and operating system unchanged,…
conceptRepurchase Displacement Cost
The data migration, process change, integration rebuild and customisation retirement that replacing a bespoke application with a purchased product cr…
practiceRetire-First Portfolio Bias
Assigning every application the cheapest modernisation option that satisfies the business driver, with a written justification required for anything …
practiceSix Rs Decision
Classifying each application into rehost, replatform, refactor, repurchase, retire or retain, based on value and effort rather than on preference.
Neighbouring topics
Legacy Modernization
General material on modernising existing systems.
Legacy Assessment
Understanding what a system does before deciding what to do with it.
Rehosting
Lift and shift: fastest, cheapest, and capturing none of the benefits.
Replatforming
Targeted changes that capture operational wins without a rewrite.
Refactoring & Re-architecting
Changing structure incrementally while the system keeps running.
Rebuild vs Re-architect
Why greenfield replacement fails, and the narrow cases where it does not.
Strangler Fig
Routing capabilities to new implementations behind a facade, one at a time.
Application Decomposition
Finding seams in a monolith, starting from the data.
Database Migration
Moving engines, versions and schemas without losing data or uptime.
Data Migration Strategies
Backfill, dual-write, reconciliation and verification.
Zero-Downtime Migration
Expand, migrate, contract — and the contract phase that never happens.
Parallel Run
Shadowing the old system to discover the rules nobody documented.
Cutover Planning
Rehearsals, go/no-go criteria and a rollback that has been executed.
Legacy Integration Patterns
Anti-corruption layers, adapters and CDC against systems that cannot change.
Mainframe Modernization
Batch windows, COBOL, and the risk profile of core banking systems.
Migration Risk
Bounding blast radius, staging by cohort, and honest readiness reporting.
Modernisation Business Case
Pricing tail risk so deferred maintenance becomes fundable.
Coexistence Patterns
Running old and new together for years without corrupting either.
Decommissioning
Actually switching the old system off, and proving nothing depended on it.