Strangling a 20-Year-Old Insurance Mainframe
Incremental strangler-fig migration cut a monolith into 14 services with zero downtime and a 5x deploy rate.
The challenge
Sentinel’s policy administration ran on a mainframe that no one fully understood. Changes took six months, deploys happened quarterly, and the vendor had raised licensing 40% year over year. A rewrite was estimated at three years and $20M — and leadership didn’t believe it would finish.
The solution
We applied the strangler-fig pattern: a gateway routed traffic by capability, new microservices were built in Go and Java behind contract tests, and Debezium streamed change data from the mainframe into Kafka for dual-writes. We cut over tenant by tenant, retiring the mainframe in 18 months without a single big-bang release.
Tech stack
They told us a rewrite would take three years. Fractal Coders retired the mainframe in eighteen months without ever taking us offline.
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