Enterprise · Stack migration

Moving a business off a legacy stack while it keeps running

We migrated a core system off an aging stack onto a modern one without a freeze, replacing it piece by piece behind a stable interface so the business kept shipping the whole way through.

Legacy migrationCharacterisation testingAnti-corruption layerData migration
Monolithno big-bang cutoverModular services
The challenge

The old system still ran the business, but the people who understood it were retiring, the platform was out of support, and every change took longer than the last. A rewrite from scratch had been tried before and stalled. They needed to modernise without betting the company on a single cutover.

What we built
  • A characterisation pass that captured the legacy system's real behaviour as tests, so the new code had a definition of correct to build against.
  • An anti-corruption layer that let old and new run side by side, with traffic shifted gradually behind a stable interface.
  • Incremental replacement of one capability at a time, each verified against the captured behaviour before the old path was retired.
  • A data migration that moved records online, reconciling continuously until the new store matched the old exactly.
The outcome
  • The business migrated capability by capability, with no freeze and no big-bang cutover.
  • Each replaced piece was proven against the old system's real behaviour before it went live.
  • The team came off an unsupported stack onto one they can hire for and change safely.
FAQ

Common questions

We don't rewrite from scratch. We replace the system piece by piece behind a stable interface, with old and new running side by side, so the business keeps shipping the whole way. There's no single cutover to bet the company on, which is what sinks big-bang rewrites.

A characterisation pass captures the legacy system's real behaviour as tests first, so the new code has a definition of correct to build against. Each replaced capability is verified against that captured behaviour before the old path retires, so undocumented quirks don't quietly disappear.

Yes. An anti-corruption layer lets old and new run together, with traffic shifted gradually behind a stable interface. The data migration moves records online and reconciles continuously, so there's no freeze and no customer-visible downtime while the move happens.

You come off an unsupported platform onto one you can hire for and change safely. Changes that took longer and longer on the legacy system become routine again, and the knowledge stops living only in the heads of people who are retiring.

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