The codebase carried a decade of shortcuts, and every change touched three things nobody had written down, so estimates were fiction. I paid down the debt that actually blocked us — carved clean seams, wrote the missing tests first, and made the scary changes routine.

Added backpressure and at-least-once delivery with reconciliation; row loss under peak load went to zero.

The database had become the whole company's single point of failure, where one team's slow query was everyone else's outage. I put boundaries around it — read replicas, a caching layer with honest invalidation, and the query budgets that stopped one team from taking down the rest.

Added a caching layer with honest invalidation; 50% of reads stopped touching the primary.

A third-party dependency in the middle of a critical path went down and took us with it, because we had quietly trusted it never would. I made the system survive its dependencies — timeouts, fallbacks, and circuit breakers that turned an outage into a degradation nobody outside even noticed.

Introduced Redis and read replicas behind a clean interface; primary CPU dropped 60%.

Two teams were blocked on the same shared bottleneck and each was waiting for the other to move first. I designed the contract between them and the migration path to reach it, so both could ship on their own schedule instead of renegotiating scope at every release.

Decomposed the monolith behind a strangler proxy into 6 services — no maintenance window needed.