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ADR-0001: Strangler Fig over big-bang rewrite

  • Status: Accepted
  • Date: 2026-06-22

Context

The legacy payments core is mission-critical (revenue-impacting, regulated). A big-bang rewrite is the highest-risk modernization strategy: it concentrates all risk into a single cutover, defers value delivery for months/years, and historically fails at enterprise scale.

Decision

Adopt the Strangler Fig pattern. Place a routing facade (strangler-gateway) in front of the legacy core. Build new capabilities as independent services behind the facade and migrate traffic endpoint-by-endpoint using weighted routing. Delete legacy routes only once the new path is proven.

Consequences

  • Positive: incremental value, reversible at every step (set weight back to 100% legacy), no shared database coupling, continuous production validation.
  • Negative: temporary duplication and the need for an Anti-Corruption Layer (see ADR-0002) and data synchronization (see ADR-0005) during the coexistence window.
  • The gateway becomes a critical component and must be highly available.