A strangler migration is 70% complete and has stalled. The legacy system still runs. What happened and what do you do?

What the interviewer is testing

Whether you know the characteristic failure of incremental modernisation programmes.

What happened

The valuable 70% was migrated and the remaining 30% is unloved. The high-value, well-understood, frequently-changed functionality went first — correctly, because it delivered the benefit. What remains is the awkward part: rarely used, poorly understood, entangled, owned by nobody, and with no business sponsor.

Funding moved to the next priority once the visible benefit was delivered. The organisation now operates both systems permanently, which is more expensive than either alone — and that cost was never in the business case, which modelled the before and after states and omitted the middle.

The options

Finish it, which requires re-securing funding for the least attractive portion. The argument is the running cost of the legacy system plus the licence, infrastructure, support and scarce skills it consumes — expressed as an annual figure. That number is usually larger than anyone expects and is the only argument that works.

Retire the remaining functionality rather than migrating it. Check usage: a meaningful portion of what remains may not be used at all, and deleting it is far cheaper than porting it. This is the option most often overlooked and it frequently covers a third of the remainder.

Deliberately accept coexistence, with the legacy system placed in a maintained, contained state with a documented long-term plan. Legitimate if genuinely cheaper, and it must be a decision with an owner rather than the default outcome of inaction.

Buy or replace the remaining functionality if a product covers it.

The lesson for the next programme

Plan and fund decommissioning at the start, treat the final slice as part of the deal, and report retirements rather than migrations as the programme metric. Otherwise the estate only ever grows.

What a strong answer adds

Sequencing differently next time: identify what can be retired before migrating anything, and consider tackling a hard slice early while political capital and funding are strongest — rather than leaving every difficult thing to the end.

Common weak answers

Pushing to finish without securing funding. Accepting coexistence without making it an explicit, owned decision.