Legacy Assessment
Understanding what a system does before deciding what to do with it.
5 to work through
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intermediate
A replacement for a 16-year-old claims system passed every acceptance test and went live. Six weeks later the reinsurance return it produced was rejected by finance, and nothing in the test pack was wrong. An interviewer asks how you would have found that gap during the assessment. What do you say?
3 min answer -
intermediate
A team inherits a system nobody fully understands and is asked whether to rewrite it. What should be established before answering?
2 min answer -
intermediate
Before modernising, a team must assess a legacy system. What should the assessment measure, and what is usually skipped?
2 min answer -
intermediate
On Salesforce's Force.com platform, a customer's custom objects and fields are rows of metadata in shared tables rather than a schema of their own (Weissman and Bobrowski described the design at SIGMOD in 2009). What does that remove from the legacy problem, and what does it not remove?
3 min answer -
advanced
A twenty-year-old core system supports most of the business. It is on unsupported technology, three people understand it, and the last replacement attempt was abandoned after two years. Design the programme.
3 min answer
3 terms in this topic
Compensating Manual Control
A human step outside a legacy system that corrects its output before anyone consumes it, which means the real specification is the code plus the corr…
practiceLegacy Assessment
Establishing what a legacy system actually does, what depends on it, and what it costs, before deciding what to do with it.
practiceLegacy System Assessment
A structured evaluation of a system's business value, technical condition and risk, used to decide what to do with it rather than to describe it.
Neighbouring topics
Legacy Modernization
General material on modernising existing systems.
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.
Decommissioning
Actually switching the old system off, and proving nothing depended on it.