Step 1: System Definition

Almost every system in this course treats what users send as data. It stores a like, routes an email, or ranks a post. A code judge takes something riskier: a program written by a stranger, which it then runs on the service's own machines. That one fact shapes most of the design.

The service works like LeetCode or Codeforces. A user writes a solution to a coding problem and submits it. The system compiles the code if the language needs it and runs it against test cases the user never sees. It then returns a verdict such as Accepted, Wrong Answer on test 12, or Time Limit Exceeded