Servers are computers built to run all day, every day, and to serve many users at once. A server is made of the same basic parts as a laptop: a processor, memory, disks and a network connection. But it has many more of each part, and it keeps working even when one of them fails. Most servers sit in racks in a data centre.
How is a server different from a laptop? Take a typical rack server that is 2U high, which means it fills two slots of a rack, about 9 cm. It may have two CPUs with 32 or more cores each, while a laptop has one CPU with 8 to 16 cores. It may have 512 GB of memory, against 16 GB in a laptop. Its memory uses error correction (ECC), which finds and corrects a wrong bit before it can damage data. It has two power supplies and several fans, and each of them can be pulled out and replaced while the server keeps running. This is called hot-swapping. It also has fast network ports of 25 or 100 gigabits per second.
Imagine an online coaching company in Kota on the morning it publishes mock-test results. At 10 a.m., 2 lakh students log in within a few minutes. A load balancer spreads their requests across 40 servers. That afternoon, one power supply in one server fails. The server keeps running on its second power supply, and no student notices. Its baseboard management controller sends an alert, and an engineer replaces the failed unit the next morning without switching the server off.
What runs on a server? First, the firmware starts the hardware. Then an operating system starts, often an enterprise Linux. Many servers run a hypervisor, software that splits one physical server into many virtual machines, each with its own operating system. A server keeps its data on its own disks or on enterprise storage systems. Data centre switches join the servers to each other and to the internet.
Servers come in a few shapes. Rack servers are the most common. Blade servers are thin boards that slide into a shared box, called a chassis, which supplies their power and network. GPU servers hold 4 or 8 GPUs each, for AI work.
A few large companies make most of the world's servers. Their engineering centres in Bengaluru and Pune work mostly on firmware and management software. The very largest cloud providers design their own servers and have them built by contract manufacturers.