Data centres are buildings made for one job: housing thousands of servers and keeping them running all day, every day. Every app on your phone depends on them. Each app has a part that you see, on your phone, and a backend, the part that stores the data and runs the app's logic on servers. The backend runs in a data centre.
Consider what happens when you pay your electricity bill from your phone at 11 p.m. in Indore. Your request travels over the mobile network and a fibre cable to a data centre, perhaps in Navi Mumbai, about 600 km away. There, a server checks your account, a storage system records the payment, and the reply reaches your phone in under a second. You never see the building. But if its power failed for even a minute, thousands of payments like yours would fail with it.
A data centre can be understood in three layers.
The first layer is the building and its facilities: space, power, cooling, security and cables. Many data centres rent out space to other companies by the rack, by a locked cage of racks or by a whole room. This business is called colocation. A rack is a tall metal frame that holds dozens of servers, one above another; the page on server racks explains it. The building's power and cooling equipment carries electricity to every rack, keeps batteries and diesel generators ready, and removes the heat. Short cables across the hall, called cross-connects, join each tenant to the cloud providers and telecom operators in the same building.
The second layer is the servers and storage that fill the racks. A server is built from the same parts as a laptop, but with many more of each, and it keeps running when one part fails. Storage systems keep extra copies of the data, so that nothing is lost when a disk fails. AI clusters join thousands of GPU servers so that they work on one job.
The third layer is the network inside the building. Data centre switches and routers carry traffic between the servers and out to the internet.
Every layer is built with redundancy, which means a spare for every part that can fail. Each server has two power supplies. The building has several fibre links. Every piece of data has extra copies.
AI is changing data centres. A rack of AI accelerators, the chips that train and run AI models, uses many times the electricity of an ordinary rack. It also gives off so much heat that air cannot cool it, so new data centres pipe cold liquid to the chips. Builders now choose sites first for their supply of electricity, not for their fibre. India is building many new data centres, most of them around Mumbai and Chennai.
Owning a data centre is a different business from the cloud, which rents out servers by the hour. The cloud providers keep their own servers in data centres too.