Hybrid cloud

Hybrid cloud is a way of running a company's computing in which some of its systems run on servers it owns, and others run on servers it rents from a cloud provider, and the two are managed as one. A server is a computer that runs programs for other computers. The company's own servers are called on-premises servers, or on-prem, because they sit in its own building or in a data centre it chooses. Renting servers over the internet is called cloud computing.

Here is how it works in general. The company keeps some systems on its own servers. It may do this because the systems are old and hard to move, because the data must stay under its own control, or because the servers are already paid for. It rents other servers from a cloud provider, by the hour, for work that grows and shrinks. A private network link joins the two places, so that a program in one can reach data in the other without using the public internet. The company then uses the same tools in both places. The same software, such as a Kubernetes platform, starts the apps in either place. The same infrastructure monitoring watches both, and the same login rules and security checks apply in both. Software such as private cloud software can also make the company's own servers behave like a cloud: staff ask for a server on a screen and receive it in minutes.

The hard part is keeping the two places alike. If the security settings in the cloud are different from those on the company's own servers, an attacker can enter through the weaker place. If nobody knows which app runs where, costs and failures are hard to trace.

Take, for example, a bank whose core banking system runs on its own servers, as it has for fifteen years. Its new mobile-app backend runs in the cloud, and asks the core system for each balance over the private link.

Consider an online retailer before a festival sale. It runs its shop on its own servers all year, and rents 200 extra cloud servers for the five days of the sale.

Say a hospital chain keeps patients' scan images on its own storage, to control who can see them. It sends copies with the names removed to a cloud service that trains an AI model to read the scans.

How is hybrid cloud different from multi-cloud? Multi-cloud means using two or more cloud providers at the same time, for example to avoid depending on one. It need not include any servers of the company's own. Hybrid cloud always includes the company's own servers. Many large companies do both. Some also rent machines that sit in their own building but are billed like the cloud, called on-premises as a service.