Carving a core banking monolith into services
An incremental migration off a 15-year-old system, executed without a single cutover weekend.
- Big-bang cutovers
- 0
- Services extracted
- 11
- Faster release cadence
- 4×
The challenge
A single application handled accounts, payments, and reporting, with a release cadence of once a quarter and a test suite nobody trusted. Two previous rewrite attempts had been abandoned. Regulatory reporting deadlines meant the system could not be taken offline for any meaningful window.
Our approach
Started with a dependency audit to find genuine seams, then sequenced extraction by risk rather than by how easy each piece looked.
Put a facade in front of the monolith so callers could be moved service by service without coordinated client changes.
Backfilled characterization tests around each component before touching it, which caught three long-standing bugs that had been quietly producing wrong figures.
Ran old and new paths in parallel with output comparison until the results matched for a full reporting cycle.
The outcome
Eleven services extracted over 14 months with no scheduled downtime. Releases moved from quarterly to roughly weekly. The remaining monolith is now small enough that the team no longer treats replacing it as a project.
Working on something similar?
If this looks like your situation, we can probably tell you within one call whether the same approach applies.