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World A3: The Mall's Roads

Load balancers

You will learn: how a load balancer shares shoppers between healthy servers, and how to guard the servers behind it.

Checked with AWS 2026-11 on 1 Nov 2026

The problem

Tap each point on the timeline.

One counter, one crash, zero sales. Tap the story. What if two counters shared the queue?

Watch

A Load balancer: The mall's reception desk: it sends each shopper to a healthy counter. Billed while it runs. is the mall’s reception desk. It sends each shopper to a free, healthy counter. AWS has two common kinds. Sort the jobs.

Drag a card into a box. Or tap the card, then tap the box.

      Watch

      The counters sit in a Target group: The list of counters a load balancer may send shoppers to, with a health check for each.. The desk checks each one’s health. Here is the official example’s answer, cut short. Tap every healthy target.

      Tap every healthy target.

      0 of 2 found

      Predict

      One of two servers stops answering its health check. What does the load balancer do?

      Try it

      The app servers sit in back. Only the load balancer may reach them on port 80. Use its group, sg-lb, as the source.

      Set the door guard's rules. Only the right visitors get in.

      Visitors

      • Load balancershould get inTCP 80 from 10.0.0.10, a server in sg-lbIts replies go back to ports 1024–65535.
      • Strangerkeep outTCP 80 from 192.0.2.77Its replies go back to ports 32768–61000.
      • Another server in your VPCkeep outTCP 80 from 10.0.1.20Its replies go back to ports 32768–61000.

      Unit door · security group (the server)

      Allow rules only. It remembers who came in, so replies always go out.

      In

      No In rules yet: nothing gets in.

      Out: all traffic (a new group starts this way).

      Check

      Good load balancer habits, or risky? Swipe right for good.

      Swipe right for good, left for needs work. Or use the buttons.

      1 / 4

      The load balancer in public rows of two wings

      You did it!

      You can now put a reception desk in front of your servers, and guard them.

      Go deeper (optional)

      Auto Scaling groups can add counters at lunch and remove them at night. The load balancer finds the new ones itself. Remember the bill: a load balancer costs every hour, even with no shoppers.