What's a data center?

Servers need a home

When you watch a video or open a website, your message goes to a server: a computer whose job is to answer other computers. A video server sends videos; a web server sends web pages. Servers never sleep: someone, somewhere, always wants a video.

Think of a giant hotel, but for computers instead of people. Every room is full of servers, the lights never go off, and there’s staff on duty day and night. That’s a data center: a building (or a big room in one) full of servers, with everything they need to keep running all the time.

in one sentence A data center is a building full of servers, kept running day and night.

What’s inside

• Racks: tall metal cupboards, about two metres high, with servers stacked inside like books on shelves. Rack space is measured in U (one unit, 4.45 cm); a normal rack holds 42U.
• Power: lots of it, from two separate supplies, plus big batteries and diesel generators, so the servers keep running even if the electricity goes off.
• Cooling: servers get hot, like a laptop on your knees, times thousands. Rows of racks face each other so cold air goes in at the front and hot air comes out of the back, where it’s pulled away.
• Security: guards, cameras, locked cages and ID cards. You can’t just walk in.
• The network: switches and routers (from lesson 2.1) joining every server to every other server, and to the internet.

in one sentence Inside: racks of servers, lots of power, lots of cooling, tight security, and a network joining it all.

Two of everything

What happens if a car gets a flat tyre? You use the spare. Data centers have a spare for everything: two power supplies, two cables, two switches, two internet connections. Having a spare ready like this is called redundancy. If one thing breaks, the other takes over and nobody notices.

in one sentence Redundancy means a spare for everything, so one broken part doesn’t stop anything.

Leaf and spine

Think of a school with long corridors of classrooms, and two main staircases. Every corridor joins both staircases. To get from any classroom to any other, you walk along your corridor, up or down a staircase, and along the other corridor: always the same short trip. And if one staircase is closed, you use the other.

Most data center networks are built just like that, and it’s called leaf-spine:

• a leaf switch sits at the top of each rack, and that rack’s servers plug into it (people also call it a top-of-rack switch). The leaves are the corridors.
• the spine switches sit in the middle. They’re the staircases.
• Every leaf connects to every spine. So any server reaches any other by going leaf → spine → leaf.

in one sentence In leaf-spine, every leaf connects to every spine, so any two servers are always leaf → spine → leaf apart.

The door and the guard

Where the internet comes into the building, there’s a router (the post office from lesson 2.1) called the edge router, because it sits at the edge. Next to it is a firewall: a security guard for messages. It checks every message coming in and only lets through the kinds that are allowed, like requests for a video.

in one sentence The edge router brings the internet in, and the firewall checks every message at the door.

North-south and east-west

Draw the data center like a map, with the internet at the top:

• messages going in and out, between the internet and the servers, travel up and down the map. They’re called north-south traffic.
• messages between servers inside the data center, like a web server asking a database (a server that stores information, like a giant filing cabinet) for something, travel across the map. That’s east-west traffic.

And the cloud? It’s just someone else’s data center. When a company uses the cloud, it rents servers in a data center that a cloud company runs.

in one sentence North-south is traffic in and out of the data center; east-west is servers talking to each other inside it.