Decommissioning
Actually switching the old system off, and proving nothing depended on it.
3 to work through
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intermediate
A migration is complete and the old system is still running a year later. Why does decommissioning fail, and what makes it happen?
2 min answer -
intermediate
A replaced system is still running two years after its replacement went live. Why does decommissioning fail, and what makes it happen?
2 min answer -
advanced Multiple choice
A replaced system was made read-only for 30 days. No complaints came in and it was switched off, with its data exported to an archive. Three weeks later the annual regulatory extract fails, because the job read directly from the retired database. Which design decision made this possible?
3 min answer
5 terms in this topic
Archive-and-Shutdown
Retiring an application by exporting its data to durable archival storage in a readable format and shutting the application down - separating the obl…
practiceBusiness-Cycle Observation Window
Setting the period over which you watch for remaining consumers from the system's longest business cycle rather than from a round number of days, bec…
practiceDecommissioning
Actually switching off and removing a replaced system, which is where the savings in every modernisation business case live.
practiceDecommissioning Discipline
The work of actually switching a legacy system off, which is where migration savings are realised and which is routinely left undone.
practiceProgressive Shutdown
Retiring a system through reversible stages - read-only, then blocked, then stopped, then removed - so each stage surfaces remaining consumers safely…
Neighbouring topics
Legacy Modernization
General material on modernising existing systems.
Legacy Assessment
Understanding what a system does before deciding what to do with it.
The Six Rs
Rehost, replatform, refactor, repurchase, retire, retain — per application.
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.