Parallel Run
Shadowing the old system to discover the rules nobody documented.
4 to work through
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intermediate
An enterprise runs old and new systems in parallel to validate the replacement. What makes a parallel run effective, and what makes it expensive theatre?
2 min answer -
advanced
A financial calculation engine is being replaced and the results must be proven equivalent before cutover. How should a parallel run be designed, and how are discrepancies triaged?
3 min answer -
advanced
A logistics platform is replacing its core routing system. Should it run the old and new systems in parallel, and for how long?
2 min answer -
advanced
You are reimplementing a legacy capability. What does running old and new in parallel find that testing cannot?
2 min answer
4 terms in this topic
Explained-Difference Criterion
Ending a shadow or parallel run when every observed difference has been explained, rather than when differences reach zero - because zero is unachiev…
practiceParallel Run
Operating the old and new systems simultaneously on the same inputs and comparing outputs, to build evidence that the replacement is correct before r…
patternShadow Comparison
Running a new implementation alongside the old on real traffic, comparing outputs without the new system affecting users.
patternTraffic Shadowing
Sending a copy of real production traffic to a new implementation while the existing one continues to serve users, comparing results without exposing…
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.
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.