Legacy API Renewal for a Regulated Enterprise Platform

This case study is anonymized. No client identification, real metrics, or confidential information is published here. Bracketed placeholders mark values that are filled in for prospect conversations under NDA.

Context

A regulated enterprise organization operated a revenue-critical platform exposing several public and partner APIs. The APIs had grown organically over many years and lacked explicit contracts, consistent error semantics, and uniform authentication. Each release required heavy manual regression.

Problem

• Partner integrations broke quietly after internal refactors.
• Authentication and authorization behaviour varied by endpoint.
• Documentation drifted from runtime behaviour.
• No safe way to deprecate an endpoint without surprising consumers.

Constraints

• Zero tolerance for partner-visible downtime.
• Regulatory obligations on auditability of changes.
• Existing internal team had to retain ownership after handover.

Approach

1. Inventory the surface — every endpoint, every consumer, every authentication path.
2. Establish explicit contracts (OpenAPI) and contract tests, baselined against the live behaviour.
3. Introduce a thin compatibility layer to absorb future changes safely.
4. Renew endpoints in priority order, behind feature flags, with shadow traffic comparison.
5. Hand over to the internal team with documented patterns and tests.

Deliverables

• Full API inventory and dependency map
• Versioned OpenAPI contracts for every public endpoint
• Contract test suite running in CI
• Compatibility layer with telemetry on every divergence
• Internal modernization playbook for future endpoints

Measurable outcomes

• Partner-visible incidents: [Replace with verified metric]
• Mean time to diagnose integration regressions: [Replace with verified metric]
• Endpoints migrated under feature flag: [Replace with verified metric]
• Test coverage on the API layer: [Replace with verified metric]

Technologies

.NET, REST, OpenAPI, PostgreSQL, feature flags, contract testing.

Lessons learned

The hardest part of API renewal is not the new code — it is making the existing, undocumented behaviour visible enough that change becomes safe. Once the contracts and the comparison telemetry are in place, the actual renewal is almost mechanical.