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Replacing a legacy system without a day of downtime
A big-bang cutover concentrates all the risk into one weekend. Phased migration spreads it into small, reversible steps — slower on paper, faster in practice.
Outcomes
- Route traffic through a facade both systems sit behind
- Move one capability at a time, starting with low risk
- Dual-run and reconcile before switching the source of truth
- Keep a rollback for every phase, and rehearse it
- Decommission only after a full reporting cycle has passed
Problems this solves
Big-bang cut-over risks the business
Replacing in slices keeps the old system live until each piece is proven in production.
Everything reads the old database
Downstream consumers are inventoried and given a stable interface during transition.
Rules exist only in the old code
Recovered, documented and covered by tests before the slice is replaced.
Architecture and integration
- Routing layer in front of the legacy system so traffic can be moved per slice.
- Change-data-capture or dual writes to keep data consistent during transition.
- Comparison harness logging differences between old and new behaviour.
- Read-only archive of the legacy system retained after decommissioning.
How delivery runs
- 01
Inventory and slice plan
Identify seams where a piece can be replaced independently.
- 02
Route traffic gradually
New implementation runs behind a routing layer, with a percentage of traffic and instant fallback.
- 03
Run in parallel
Both systems process, results compared, differences investigated before switching fully.
- 04
Decommission
Old slice switched off only after a defined period of clean operation.
What moves the price
Number of slices
Each slice carries its own parallel run and verification effort.
Data synchronisation
Keeping two systems consistent during transition is real engineering, not overhead.
Consumer count
Every downstream system reading the legacy data must be handled.
Where projects go wrong
Parallel running forever
Each slice gets a decommission date agreed up front.
Silent behaviour differences
The comparison harness makes differences visible before the switch, not after.
FAQ
Questions buyers ask
Is phasing more expensive?+
Slightly more engineering, considerably less business risk. For systems that cannot stop, it is the cheaper option overall.
How long does a full migration take?+
Typically six to eighteen months depending on module count and data quality — with value delivered throughout.
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