Zero-Downtime Migration of Legacy Enterprise Systems to Cloud Infrastructure: A Framework for Saudi Arabia's Digital Transformation

Cloud migration is often presented as an infrastructure relocation problem, yet enterprise continuity depends on a more difficult requirement: moving intertwined applications, databases, interfaces, identities, and operating practices without a user-visible service interruption. This review develops an evidence-informed framework for zero-downtime migration of legacy enterprise systems to cloud infrastructure, with specific attention to Saudi Arabia's digital transformation context. An integrative review synthesizes peer-reviewed research on legacy modernization, application portfolio assessment, microservice decomposition, data migration, progressive delivery, infrastructure automation, cybersecurity, and Saudi cloud adoption. The evidence indicates that zero downtime is not produced by a single architecture pattern. It emerges from coordinated controls: dependency discovery, workload characterization, parallel environments, continuous data replication, backward-compatible schema evolution, progressive traffic shifting, observable service-level objectives, transactional reconciliation, and rapid rollback. The review also finds that indiscriminate microservice decomposition can increase migration risk when domain boundaries, data ownership, and operational capabilities are immature. Accordingly, the proposed framework separates continuity controls from modernization depth and uses stage gates to decide when rehosting, replatforming, containerization, or deeper refactoring is justified. For Saudi organizations, the framework adds governance emphasis on trust, cybersecurity, organizational readiness, service continuity, and locally appropriate cloud operating capabilities. The paper contributes a structured synthesis that links technical migration mechanisms with managerial assurance and identifies research priorities for empirical validation in Saudi enterprise and public-service environments.