10  Transition & Rollout Plan

10.1 Phased Migration Strategy

The transition follows incremental phases to minimize business disruption and maintain confidence in production stability.

10.1.1 Phase 0: Foundations

• Confirm target integration architecture and governance standards.
• Establish identity, observability, and contract management baselines.
• Align teams on operational readiness and ownership model.

10.1.2 Phase 1: Pilot Domain Enablement

• Select low-risk, high-learning business domain.
• Enable event publication and consumer read-through via GraphQL.
• Validate synchronization, traceability, and incident response runbooks.

10.1.3 Phase 2: Controlled Expansion

• Add additional domains in priority order.
• Introduce REST write-back capabilities for approved external consumers.
• Track drift, performance, and adoption KPIs against defined thresholds.

10.1.4 Phase 3: Coexistence Optimization

• Harden dual-run operations and reduce manual reconciliation.
• Retire obsolete point-to-point dependencies.
• Prepare domain-level cutover readiness sign-off.

10.1.5 Phase 4: Cutover and Decommission

• Transition eligible domains fully to the decoupled model.
• Decommission superseded legacy integration mechanisms.
• Complete post-cutover validation and operational handover.

10.2 Milestone Timeline (Planning Baseline)

Use this as a planning template and refine dates during delivery governance.

1. Foundation complete: architecture controls and telemetry in place.
2. Pilot go-live: first domain synchronized in production.
3. Multi-domain expansion: additional domains onboarded with stable KPIs.
4. Write-back readiness: REST command flows validated for external integration.
5. Domain cutover waves: progressive retirement of legacy pathways.
6. Program closeout: transition objectives met and support model stabilized.

10.3 Risk Register and Mitigation

10.3.1 Key Risks

• Data synchronization drift between legacy and new consumers.
• Contract change misalignment across producer and consumer teams.
• Skills and capacity constraints for operating dual-run architecture.
• Security gaps during staged exposure of integration endpoints.
• Performance bottlenecks in high-volume event/query scenarios.

10.3.2 Mitigation Measures

• Enforce contract versioning and compatibility review gates.
• Implement replay, reconciliation, and automated drift detection.
• Use phased onboarding and domain readiness criteria for each rollout wave.
• Conduct security threat modeling and pre-release control validation.
• Run load and soak testing before each expansion milestone.

10.4 Rollback and Contingency Procedures

Rollback procedures are defined per migration phase and domain.

10.4.1 Operational Rollback Principles

• Preserve legacy execution path until new path stability is demonstrated.
• Support feature toggles to disable new integration paths quickly.
• Maintain clear criteria for rollback triggers based on SLO breaches or data integrity exceptions.

10.4.2 Rollback Steps (Per Domain)

1. Pause new event-driven processing for affected domain.
2. Redirect writes to validated legacy pathway.
3. Execute reconciliation checks to identify and resolve drift.
4. Restore service baselines and communicate status.
5. Conduct post-incident review and update controls before reattempt.

10.5 Governance and Communication

• Weekly architecture and delivery governance forum.
• Release readiness checkpoints per phase.
• Business and operations stakeholder updates at each milestone.
• Decision log maintained for scope, risk, and sequencing changes.