Migrating Legacy to Microservices with Zero Downtime: An 8-Week Playbook
How we moved a 7-year-old monolith to a microservices architecture on AWS without stopping the service — including the strangler fig strategy, contract testing, and parallel rollback we ran for a fintech client.
Context
A fintech client ran a 7-year-old monolith that had become hard to evolve. Our goal: move to microservices on AWS with no meaningful downtime.
Strangler Fig
Rather than a big rewrite, we wrapped the monolith and moved endpoints one at a time to new services. Traffic shifted gradually via the API gateway.
Contract Testing
Every new service was validated with contract tests before taking production traffic, so integrations did not break on release.
Parallel Rollback
Old and new versions ran in parallel during the transition. If metrics regressed, traffic reverted instantly without waiting for a redeploy.
Result
The migration completed in 8 weeks with zero significant downtime, and the team's release velocity improved dramatically.