Designing Simple Systems: The Login System

Let’s put everything we’ve learned together and design a simple, common system: The User Login System.

The Goal:

A user should be able to enter their username and password and be logged into the application.

Step 1: The Components

1. The Client: The browser where the user types their username and password.
2. The Server: The application running the “Login” code.
3. The Database: Where the user’s username and (hashed) password are stored.
4. The Load Balancer: Distributes traffic if we have many servers.

Step 2: The Flow

1. Request: The client sends a POST request to the Load Balancer with the username and password.
2. Routing: The Load Balancer sends the request to one of our servers.
3. Authentication: The server asks the database for the user’s record.
4. Verification: The server compares the password the user sent with the one in the database.
5. Response: If they match, the server sends back a “Success” message and a 200 OK status code.

Simple Diagram

       [ User ]
          |
          v
   [ Load Balancer ]
          |
          v
   [ Login Server ]
    /          \
   /            \
  v              v
[ Database ] [ Auth Cache ]
(Passwords)  (Sessions)

Considerations for Scale

• Is it fast?: We might use a Cache (like Redis) to store user sessions so we don’t have to check the database every time.
• Is it reliable?: If the database goes down, we need a Replica (a backup) ready to take over.
• Is it secure?: We never store passwords in “plain text”; we always “hash” them first.

Practice Problem

Try to draw a simple diagram for a “To-Do List” application. What components would you need?

Next: Common Design Mistakes