OAuth 2.0 and OpenID Connect Explained — Flows, Tokens, and Production Pitfalls

Every time a user clicks 'Sign in with Google' or grants a fitness app access to their calendar, OAuth 2.0 is running the show. It's the protocol that powers delegated authorization for billions of API calls every day — and it's also one of the most misunderstood protocols in production systems. Teams routinely ship broken or insecure OAuth implementations because they treat it like a black box, copy-paste an authorization URL, and call it done. That's a recipe for token leakage, privilege escalation, and account takeover at scale.

What is OAuth 2.0 and OpenID Connect?

OAuth 2.0 and OpenID Connect is a core concept in System Design. Rather than starting with a dry definition, let's see it in action and understand why it exists.

// TheCodeForge — OAuth 2.0 and OpenID Connect example
// Always use meaningful names, not x or n
public class ForgeExample {
    public static void main(String[] args) {
        String topic = "OAuth 2.0 and OpenID Connect";
        System.out.println("Learning: " + topic + " 🔥");
    }
}

Frequently Asked Questions