Use the strangler pattern for controlled cloud migration

Large migrations fail when “move the system” is treated as one indivisible project. The strangler pattern reduces risk by placing a controlled boundary around the existing system and moving capabilities incrementally while production continues to operate. Find a business seam Start with a capability that has a clear owner, manageable data dependencies, and a measurable user outcome. A low value component is not always the safest pilot if it teaches nothing about identity, data, traffic, and operations. Choose a slice representative enough to validate the migration platform without putting the entire business at risk. Document the current request paths, data ownership, batch jobs, operational procedures, and hidden integrations. Migration plans are often defeated by an unrecorded report, shared table, or manual recovery step rather than by compute provisioning. Introduce a routing boundary Use an API gateway, facade, event route, DNS layer, or application adapter to direct selected traffic to the new capability. The boundary must support gradual exposure, rollback, and observability. Avoid duplicating business logic inside the router. The migration should make ownership clearer. If both old and new systems can update the same record indefinitely, the architecture has created distributed ambiguity rather than decomposition. Move data deliberately Choose a source of truth for each phase. Options include one time migration with a cutover, change data capture, event replication, or temporary dual writes. Dual writes require reconciliation because partial failure is unavoidable. Define how lag, ordering, deletion, and correction are handled. AWS’s migration guidance describes the routing and incremental replacement pattern. The same principle applies beyond one cloud provider: isolate change, preserve reversibility, and retire the old path in measured steps. Set exit criteria Each slice needs functional parity, performance and reliability evidence, security review, support readiness, cost visibility, and a tested rollback. After cutover, remove obsolete routes, jobs, credentials, data copies, and monitoring. A migration is not complete while the old operating burden remains. Incremental modernization is not slow by definition. It creates frequent proof, exposes dependencies early, and lets teams stop or redirect before a single high risk cutover consumes the whole program. Choose the first slice with evidence Score candidate capabilities on business value, coupling, data ownership, traffic pattern, compliance, operational pain, and rollback difficulty. A useful first slice has visible value and exercises the shared migration path, but does not require solving the hardest dependency before the team has learned how the new environment behaves. Establish a baseline before moving it: latency, error rate, throughput, operating effort, cost, recovery time, and critical user outcomes. Without a baseline, a migration can be technically complete while performance or operability quietly worsens. Avoid the distributed monolith Moving code into separate services while retaining synchronous calls, shared tables, coordinated deployments, and central release approval preserves the original coupling with more failure modes. Give the extracted capability clear data ownership and a stable contract. Where immediate separation is impossible, document the temporary coupling and its removal milestone. Run the cutover Use progressive traffic exposure by tenant, region, operation, or percentage. Compare old and new outcomes, not only infrastructure metrics. Define automatic and manual rollback signals. Protect rollback from incompatible data written by the new system; sometimes traffic can return while reads must continue through a compatibility layer. Completion checklist A slice is finished when production traffic uses the new path, data ownership is unambiguous, objectives are met, on call teams can operate it, recovery is tested, costs are attributed, and the old path is removed. Track retired servers, licences, jobs, integrations, credentials, dashboards, and support procedures as first class migration outcomes. Otherwise the organisation pays for both architectures indefinitely.