Legacy Integration Patterns
Anti-corruption layers, adapters and CDC against systems that cannot change.
4 to work through
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beginner
A new notifications service needs customer contact data that only a twelve-year-old billing system holds. The team points it at a read replica of the billing database and ships in a week. Why does this work, and why is it still the most expensive integration they could have chosen?
3 min answer -
intermediate
A fourteen-year-old policy system sustains roughly 350 transactions per second at its measured peak and the business peak already consumes about 260 of them. Four new services want to call it directly. Roughly how much of that system can you sell them, and what does the number decide?
3 min answer -
intermediate
New services must interact with a legacy system that cannot handle their load or their pace of change. Which integration patterns apply?
2 min answer -
advanced
A new platform must integrate with legacy systems that only support batch file exchange. How should the boundary be designed so the new system is not constrained by it?
2 min answer
3 terms in this topic
Anti-Corruption Layer in Integration
A translation boundary that keeps a legacy system's model, vocabulary and quirks from leaking into a new system that must integrate with it.
practiceLegacy Headroom Budget
The measured spare throughput of a legacy system after retry amplification, batch overlap and organic growth are deducted, allocated to new consumers…
patternLegacy Wrapping
Placing a modern interface in front of a legacy system so that consumers integrate with the wrapper rather than with the legacy system directly.
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.
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.