Cloud migration and hybrid

Moving an estate while it keeps running

Cloud migration is the planned transfer of existing systems from servers an organisation owns or rents onto a cloud platform, usually in waves and rarely all at once. For a long stretch the estate is hybrid: some systems have moved, others wait, and the network, identity and data links between the two halves must behave as if nothing had changed. The job combines inventory work, architecture, project management and a good deal of negotiation with the people who own each application.

What the blueprints expect

AWS's professional architect exam, SAP-C02, gives migration and modernisation a domain of its own. Its task statements cover portfolio assessment, wave planning, total cost of ownership, choosing transfer tools for applications, databases and bulk data, and judging each application against seven common migration strategies before deciding where decoupling or serverless designs could replace the old shape. The associate exam is lighter but still asks when to move an application into containers, how to shift a database to the same or a different engine, and how to make legacy software more reliable when its code cannot change. Microsoft's AZ-305 folds migration into infrastructure design, from evaluating on-premises servers and data to recommending routes for workloads, databases and unstructured data, while AZ-900 asks only that candidates know what Azure Migrate and Data Box are for. Google's Professional Cloud Architect covers migration planning on its own platform.

• Hidden dependencies: an application moved without the file share, batch job or licence server it quietly relied on.
• Heavy data: large databases take longest to move and are hardest to keep in step during the cut-over.
• Network and identity: hybrid links, DNS and directory integration must exist before the first wave, not during it; see cloud networking.
• Cost surprise: a server rehosted at its old size keeps its old waste and adds a meter.
• Skills: operations staff asked to run a platform nobody has trained them on.

The beginner's error is to treat migration as a technical copy job. The hard questions are which systems should move at all, which should be switched off, and whether a managed hosting service would serve better than a cloud estate the organisation must run itself. At the small end, moving a site from a VPS or a dedicated server to cloud hosting follows the same logic in miniature, and our guide to site migration without downtime walks through it. Before migrating anything larger, settle the target structure in governance and landing zones.