Stateful vs Stateless Authentication

Authentication is at the core of securing applications. When you build APIs or web applications, you need to decide how to handle authentication.

Two popular approaches are stateful and stateless authentication. Let’s break down what they mean and how they differ technically.

1. What is Stateful Authentication?

Imagine you went to your regular Kirana (local grocery) shop. The shopkeeper knows you. Every time you walk in, he remembers your usual purchases and how you pay. He might even extend credit because he remembers your transaction history and trustworthiness. This is a stateful system. one that keeps track of your past interactions and uses that information to provide a better, more personalized experience.

How it Works:

• After a user logs in, the server generates a session and stores it in memory or a database.
• A session ID is stored in a cookie in the browser and sent to the server with every request.
• The server checks this session ID against its records to identify the user.

Example:

When you log into a website, the server creates a session on the backend (stateful). You receive a session cookie, and every time you make a request (like navigating to another page), the session ID from the cookie is sent. The server then checks the session and gives you access to protected resources.

Pros:

Since the server keeps track of the session, it’s easy to manage things like user-specific data (shopping carts, user profiles, etc.).

You can easily log out users by removing their session from the server.

Cons:

Scalability issues: Storing session data on the server consumes resources (RAM or DB).

In a distributed system with multiple servers, sessions must either be shared across all servers or stored centrally, adding complexity.

2. What is Stateless Authentication?

Stateless authentication is like going to a street vendor and purchasing something, he doesn't know what items do you like or what is use payment preference and you have to tell him/her what you need on every visit. This is the same way how Stateless authentication works.

In stateless authentication, the server doesn’t store any information about the user after they log in. Instead, all the necessary user information is stored in the token (usually a JWT—JSON Web Token) that the client holds.

How it Works:

• After successful login, the server generates a JWT token containing the user’s details and signs it.
• The token is sent to the client (browser or app), usually as part of the response.
• For each request, the client sends the token in the headers.
• The server decodes and validates the token, without needing to store any user session.

Example:

When using stateless authentication, after login, the server gives you a signed JWT token. This token is stored client-side (usually in local Storage or a cookie). For every API request, this token is included in the headers, and the server validates it on the fly.

Pros:

Highly scalable: Since no session is stored on the server, multiple servers can independently handle requests (no need for central session management).

Ideal for distributed or microservices architectures.

Cons:

Since the server doesn’t manage the session, logging out users is more complicated (you can't just remove a session from the server).

Tokens can grow in size depending on the amount of information they contain, making them slightly heavier to pass with each request.

Scenario Based Example

Stateful Authentication Example: Bank Portal

1. Login: You log in with your credentials.
2. Session Creation: After successful login, the server creates a session for you and stores session data (like user details and session ID) in a server-side database or in-memory store (like Redis).
3. Session Token: The server sends a session token (often stored in a cookie) back to your browser.
4. Making Requests: With each request you make (e.g., checking your balance or transferring funds), your browser sends the session token.
5. Session Lookup: The server uses the session token to look up the session data and verify your identity. If valid, it processes your request.
6. Session Expiry: If the session expires (due to inactivity or logout), you’ll need to log in again to create a new session.

This way bank can prohibit multiple sessions of a user and can invalidate the other session

Stateless Authentication Example: News Website

1. Login: You enter your username and password on the news website's login page.
2. Token Generation: After verifying your credentials, the server generates a JWT token containing your user information (like your user ID and any roles you have). This JWT is signed to ensure it hasn’t been altered.
3. Token Storage: The JWT is sent to your browser and stored in local storage or a cookie.
4. Accessing Articles: Every time you request an article or access a member-only section of the site, your browser sends the JWT with the request.
5. Token Verification: The server checks the JWT to confirm you are authorized to access the content. If the JWT is valid and hasn’t expired, you get access to the articles.
6. Expiration: JWTs have an expiration time. After this time, you’ll need to log in again or refresh the token to keep accessing member-only content.

Since controlling the user from the server is not necessary, JWT/Stateless mechanism can work perfectly.

Technical Differences:

Stateful Authentication:

1. Session Storage: The server remembers your session by storing data for each user.
2. Server Memory: Uses server memory to track active users.
3. Scalability: Harder to scale since sessions need to be shared across multiple servers.
4. Authentication Data: The client sends a session ID; the server retrieves the stored session data.
5. Session Expiry: Sessions can be easily ended by the server when needed.
6. Security: Risk of session hijacking but can be protected using secure cookies.
7. Server Restarts: Sessions may be lost if the server restarts unless they’re stored in a database.
8. Cross-Domain: Harder to handle across different websites or services.
9. Great for web apps where user sessions (like a shopping cart) need to be maintained.

Stateless Authentication:

1. No Session Storage: The server doesn’t remember anything; every request contains all needed info (like a token).
2. Server Memory: Doesn’t use server memory to track users.
3. Scalability: Easy to scale because requests don’t rely on shared session data.
4. Authentication Data: The client sends a token (like a JWT) with every request, and the server checks it.
5. Token Expiry: Tokens have expiration times, and revocation is harder.
6. Security: Tokens are signed to ensure they haven’t been tampered with, but need to be securely stored.
7. Server Restarts: No impact from server restarts since tokens are validated every time.
8. Cross-Domain: Works easily across multiple websites or services.
9. Perfect for APIs and microservices where each request should stand alone.

Conclusion:

Both stateful and stateless authentication have their use cases. Stateful authentication offers simplicity in having server side control over user sessions but can hit scaling issues. Stateless authentication, using tokens like JWT, is more scalable and ideal for modern, distributed systems.

Your choice should depend on your app’s architecture, security requirements, performance needs, and scalability goals.