Migrating a legacy system to the cloud is like performing open-heart surgery on a running patient. The business cannot stop, data must stay consistent, and every cut carries risk. After leading over a dozen enterprise cloud migrations, here's our battle-tested playbook.
Phase 1: Assessment (4-6 weeks). Map every application, database, and dependency. Classify each into one of four categories: rehost (lift and shift), replatform (minor cloud optimizations), refactor (re-architect for cloud-native), or replace (buy instead of build). Most enterprises end up with 60% rehost, 20% replatform, 15% refactor, and 5% replace on the first pass.
Phase 2: Foundation (4-8 weeks). Set up the landing zone — networking, security groups, IAM roles, logging, monitoring. This is where most migrations fail: teams start moving workloads before the foundation is solid. Our rule is no workload migration until the landing zone passes a full security audit.
Phase 3: Wave planning (2 weeks). Group applications into migration waves. Each wave should contain 3-5 related applications. The first wave is always the lowest-risk, lowest-complexity applications — internal tools, read-only databases. Never start with the customer-facing monolith.
Phase 4: Migration execution (varies per wave). For rehost: use AWS Application Migration Service or Azure Migrate. For replatform: switch to managed databases, add auto-scaling, enable CDN. For refactor: break the monolith into services, containerize, adopt CI/CD.
The critical success metric is not "everything migrated" — it's "business as usual throughout." Our migrations maintain 99.9% availability during the move by using blue-green deployment patterns where both environments run in parallel until the new one is validated.
Post-migration: decommission old infrastructure immediately (saving 30-50% on hosting costs), implement cost monitoring (cloud bills typically spike before optimization), and celebrate each wave completion with the team.
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