Legacy Modernization
General material on modernising existing systems.
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A 15-year-old monolith runs the core of the business. Leadership wants microservices. How do you approach it, and what would make you refuse?
2 min answer -
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TSB's 2018 core banking migration moved 1.9 million customers in a single weekend and failed publicly. What would you have required before approving that cutover?
3 min answer
8 terms in this topic
Airbnb's Service-Oriented Migration
Airbnb decomposed a large Rails monolith by first extracting a unified data-access layer, so that services were built on owned data rather than on sh…
patternAnti-Corruption Layer
A translation layer that converts a legacy or external system's model into your own, so its concepts do not leak into your domain.
case-studyeBay's Architectural Generations
eBay rewrote its core platform several times across its first decade, each time because the previous generation had hit a limit that could not be tun…
practiceParallel Run
Running the old and new systems side by side on the same inputs and comparing outputs, before the new one is trusted.
conceptRe-architect vs Rebuild
Restructuring an existing system incrementally versus writing a replacement from scratch — and the strong evidence that incremental wins.
practiceReplatform
Migrating an application largely as-is while making targeted changes to exploit the target platform — usually the best return per unit of effort.
conceptSix Rs of Migration
The standard menu of options for each application in a migration — rehost, replatform, refactor, repurchase, retire, retain.
patternZero-Downtime Migration
Changing a schema or system while it stays in service, by adding the new alongside the old, migrating, then removing the old.
Neighbouring topics
Legacy Assessment
Understanding what a system does before deciding what to do with it.
No content yetThe Six Rs
Rehost, replatform, refactor, repurchase, retire, retain — per application.
No content yetRehosting
Lift and shift: fastest, cheapest, and capturing none of the benefits.
No content yetReplatforming
Targeted changes that capture operational wins without a rewrite.
No content yetRefactoring & Re-architecting
Changing structure incrementally while the system keeps running.
No content yetRebuild 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.
No content yetDatabase Migration
Moving engines, versions and schemas without losing data or uptime.
No content yetData Migration Strategies
Backfill, dual-write, reconciliation and verification.
Zero-Downtime Migration
Expand, migrate, contract — and the contract phase that never happens.
No content yetParallel Run
Shadowing the old system to discover the rules nobody documented.
No content yetCutover Planning
Rehearsals, go/no-go criteria and a rollback that has been executed.
No content yetLegacy Integration Patterns
Anti-corruption layers, adapters and CDC against systems that cannot change.
No content yetMainframe Modernization
Batch windows, COBOL, and the risk profile of core banking systems.
No content yetMigration Risk
Bounding blast radius, staging by cohort, and honest readiness reporting.
No content yetModernisation Business Case
Pricing tail risk so deferred maintenance becomes fundable.
No content yetCoexistence Patterns
Running old and new together for years without corrupting either.
No content yetDecommissioning
Actually switching the old system off, and proving nothing depended on it.
No content yet