Migration sequencing determines a feasible order for changing interconnected applications and services. It combines technical prerequisites with business dates, operational capacity, and the temporary states that exist between moves. A portfolio ranking indicates importance; a sequence explains how the work can actually proceed.
The intermediate states matter because a migration rarely changes every dependency simultaneously. Each wave must leave an operating arrangement whose owners can explain data authority, interfaces, failure handling, and the obligations remaining for later work.
Classify dependencies by their effect
Infrastructure dependencies include shared storage, networks, identity, and runtime services. Business dependencies include coordinated processes and reporting periods. Operational dependencies include release windows, support availability, and scarce expertise.
AWS’s portfolio wave-planning guidance records these categories and updates them as new dependencies emerge. A schedule should therefore remain connected to discovery rather than treating the first inventory as a complete and permanent graph.
Observed traffic helps identify candidates but cannot establish every business relationship or dormant operation. Source, configuration, operating procedures, and owner confirmation provide additional evidence about which dependencies constrain a move.
Shared data can bind otherwise separate applications
A shared database gives several consumers access to a common schema. The Enterprise Integration Patterns discussion identifies technical and organizational coordination in designing that unified schema.
Map readers and writers to tables, views, routines, and transaction boundaries. Assign ownership of the invariants that must survive a change, including reporting and overnight work that may not appear in interactive traces.
Suppose one application requires a field conversion while another still interprets the old representation. Moving either application independently needs a compatibility mechanism or coordinated schema change. Their source-code separation does not provide deployment independence.
A wave leaves a temporary architecture
Define what runs in each environment after the wave, how the parts communicate, and which store owns writes. Temporary connectivity and duplicate operations need owners and removal conditions.
Latency, shared credentials, and cross-system transactions can change during coexistence. A configuration that is acceptable at the final destination can be unsuitable in the intervening split arrangement.
Acceptance should evaluate that intermediate state explicitly. Otherwise a wave can pass its individual application tests while leaving the overall process dependent on untested coordination.
Incremental routing has prerequisites
A strangler migration shifts selected operations through a controllable facade while replacing their implementation. Its feasibility depends on interceptable calls and enough time and capacity to operate both environments.
The entry-point inventory must include batch work and direct database access. Routing browser traffic cannot move operations that bypass the facade.
Each accepted increment should remove a measurable responsibility from the old system. Reversing the route also has a limited scope: it does not reverse target-side data changes or reconcile transactions created after the switch.
Capacity is a dependency too
A sequence needs the people and environments required for testing, cutover, support, and business acceptance. Two technically independent applications can still compete for the same domain expert or recovery window.
Choose wave scope from these constraints instead of adopting a universal application count. Readiness gates should identify unresolved evidence and the owner who can accept or defer the move.
Use actual observations from each wave to revise later estimates, compatibility assumptions, and temporary-state plans. The executable sequence is the one whose dependencies, intermediate operations, and recovery boundaries remain explainable as the estate changes, rather than the one that merely preserves the order of an initial ranking.