On-Prem to Cloud Migration: Running the Systems That Can't Move Yet

Most on-prem to cloud migration plans assume a single clean cut over, but the reality is hybrid setups that persist for months or years. Deciding how to connect the systems that remain on-prem with those in the cloud, managing user authentication, and anticipating failures between the two environments are critical to a successful migration. Factors like network connectivity, shared identity, latency, and ownership of issues across the boundary significantly impact the stability of the hybrid period.

Several systems stay behind due to contractual obligations, regulatory requirements, or support agreements tied to specific hardware or locations. Others are left behind because their inner workings are unclear, such as applications with hard-coded IP addresses or shared databases accessed by multiple tools. Bridging these lingering systems safely often requires untangling them, which takes longer than anticipated.

The hybrid setup does not indicate a failed plan; rather, it is a common outcome of migration. Mistakes in this stage often lead to prolonged, confusing incidents rather than an immediate outage. Mistakes may include slow, confusing incidents caused by oversized VPN tunnels, multiple logins for different domains, and unresolved gaps in system ownership. These issues can degrade performance, confuse users, and obstruct critical operations between the on-prem and cloud systems.

When migrating, the approach differs based on the volume of traffic crossing the boundary and its impact. For customer-facing operations or applications with frequent calls, a dedicated link such as AWS Direct Connect or Azure ExpressRoute is preferable, despite the longer setup time. For lighter traffic like admin tools or nightly batch jobs, standard VPN tunnels are sufficient. Measuring cross-boundary traffic over a couple of weeks can help determine the appropriate connection type.

Using a single identity system is recommended to provide consistent user access to both old and new systems. Active Directory on-prem can serve as the master list of users and sync with cloud directories like Microsoft Entra ID or Okta to ensure seamless user login and role management. Similarly, managing system-to-system logins requires similar care to ensure consistent access controls and credential management.

Addressing latency between the old and new systems is crucial. Treat every cross-boundary call as slower and less reliable than internal calls. Strategies include keeping frequently communicating systems together, batching and caching data transfers, and employing message queues for asynchronous operations. These strategies help reduce the impact of latency and improve overall system performance during the transition period.

Proper planning and early identification of critical components are essential. Addressing overlapping IP address ranges and DNS configurations upfront can prevent future complications. Planning for one identity system and consistent management of system-to-system logins are key to maintaining a seamless user experience. With careful consideration of network, identity, latency, and ownership, the transition from on-prem to cloud can be managed efficiently, minimizing disruptions and ensuring a smooth hybrid operation.