JWT Case Study: Building Authentication in Microservices

Let me tell you a story about one of the most fascinating technical challenges I tackled in one of my previous roles. This makes a great JWT case study for anyone just starting out or looking to learn from others’ experiences. Let’s dive right in!

The Auth Challenge: What We Faced

Our startup was hitting a critical growth stage, and our monolithic architecture was becoming a massive bottleneck. Deployments took forever, components were tightly coupled in all the wrong ways, and developer productivity was tanking hard. We absolutely needed to move to a microservice architecture—there was no question about it.

But here’s the thing – before we could split up our monolith, we had to solve one core problem: authentication and authorization in a distributed environment. Without this piece, nothing else would work.

As an ed-tech platform, we had specific components screaming to break free from the monolith:

• Payment processing
• Gradebook functionality
• Lecture delivery system

But how would we authenticate users across these separate services? And more importantly, how would we handle complex authorization rules?

The Business Problem Behind Our Technical Challenge

We weren’t just solving a technical problem – our product needs were evolving rapidly too. Our authorization system needed some serious upgrades:

1. Support for new roles: Beyond our basic Teacher/Student/Anonymous/Superuser roles, we needed to add:
   • Teaching Assistants (TAs)
   • School Administrators
   • And more roles down the line
2. Context-based roles: This was the real kicker. A user needed to have different roles in different contexts. For example:
   • A person could be a Student in Course A but a TA in Course B
   • Teachers sometimes needed to temporarily view their course as a Student

Our monolith was never designed for this kind of flexibility. The auth system we’d build needed to handle these complex scenarios flawlessly.

Choosing JWT: The Decision Process

When tackling authorization across microservices, we had several options:

1. Third-party auth providers: Auth0, Okta, AWS Cognito, etc.
2. Building our own dedicated auth service
3. A token-based solution that wouldn’t require a separate service

After thorough analysis, we decided on JWT (JSON Web Tokens). Here’s why:

• Cost efficiency: Third-party solutions were expensive for our startup budget
• Simplicity with effectiveness: We wanted something powerful yet straightforward to implement
• Reduced overhead: We weren’t ready to manage a high-availability auth service as a potential single point of failure

Pro tip: 💡 Always use a decision matrix when choosing between multiple technical approaches. It forces you to prioritize your requirements and make objective decisions.

Understanding JWT Authentication

If you’re new to JWT, here’s what makes it powerful: JSON Web Tokens are self-contained packages of information that can be verified because they’re digitally signed.

A JWT consists of three parts:

1. Header: Contains the token type and signing algorithm
2. Payload: The actual data (claims) you want to transmit
3. Signature: Used to verify the token’s authenticity

The beauty of JWT is that services can verify tokens independently without calling back to a central auth service for each request. This creates a truly distributed authentication system. Consider taking the Crash Course on JSON Web Tokens(JWT) to learn more in detail and with code examples.

Our JWT Implementation Approach

After selecting JWT as our solution, we needed to decide how to implement it effectively. Here are the key decisions we made:

1. Short-lived Tokens

Since JWTs can be inspected by third parties (though they’re base64 encoded), we made them short-lived. Each token was signed with a secret key shared across services to verify authenticity. This approach minimized the risk of token misuse if somehow compromised.

2. Client-side Token Refresh

We built client-side logic to handle token refreshing both when tokens expired normally and when user permissions changed. This was critical because information inside JWT would become outdated after certain actions (like enrolling in a new course).

3. Careful Token Size Management

Since we injected substantial authorization data into each token, we had to carefully manage token size to prevent performance impacts. This became especially important for users with many course enrollments.

Here’s a simplified example of how our JWT payload looked:

{
  "iss": "codesamplez.com",
  "iat": 1585529508,
  "exp": 1617065508,
  "sub": "[email protected]",
  "full_name": "John Doe",
  "roles": {
    "course": {
      "teacher": ["123", "345"],
      "student": ["567", "789"],
      "ta": ["246"]
    }
  }
}Code language: JSON / JSON with Comments (json)

Implementation Strategy: Our Execution Plan

This wasn’t a small project – it spanned multiple quarters with phased delivery. Here’s how we approached it:

Phase 1: Centralizing Authorization

First, we centralized all authorization logic in the monolith. This created a single source of truth and prepared us for the transition.

Phase 2: Building the JWT System

We developed several key components:

• Token generation at login/registration
• Payload injection with user identity and authorization data
• Validation logic for JWT verification
• Authorization checks based on the token payload

Phase 3: Building a Reusable Authorization Library

We created a client library with a clean API that all services could use. Example:

• auth_lib.is_authenticated(JWT_token) – Simple validation check
• auth_lib.is_authorized(user, context, context_id, role) – Role-based permission check

This abstraction made it incredibly easy to implement consistent authorization across services.

Phase 4: Rolling Out Without Disruption

We took a cautious approach to deployment:

• On-demand migration of user authorization data (avoiding massive data migration)
• Feature flags to enable/disable the new system instantly if issues arose
• Gradual rollout to minimize user impact

The result? A smooth transition to our new architecture:

Fig: JWT Auth Architecture

Challenges We Faced (And How We Solved Them)

No major technical project comes without challenges. Here are the biggest ones we encountered:

1. JWT Payload Size

Problem: With context-based roles, users could enroll in hundreds of courses, potentially creating massive tokens that would slow down every request.

Solution: We implemented two strategies:

• Set a limit on how many contexts (courses) a user could enroll in (500)
• For users with many contexts, we included only the most recently accessed contexts in the token, ensuring the token size remained manageable

2. Token Refresh Timing

Problem: Since tokens lasted for several minutes, authorization changes wouldn’t be immediately reflected.

Solution: We identified specific actions that changed permissions and triggered forced token refreshes:

• Server responses included metadata indicating when tokens needed refreshing
• Client code would immediately request new tokens when critical changes occurred

3. Client-Side Implementation Complexity

Problem: We underestimated the work required to implement proper token handling on web and mobile clients.

Solution: We created detailed specifications and provided developer support during implementation. The investment was worth it, creating a more resilient system.

Lessons Learned: What We’d Do Differently

If I were implementing this system again, I’d make a few changes:

1. Better client-side support: The overhead of client-side token management was higher than expected. More thorough planning would have helped.
2. Limited granularity for permissions: JWT works beautifully for role-based authorization but doesn’t scale well for fine-grained permissions. If your system needs individual permission checks, JWT might not be ideal.

Interesting Security Findings

During implementation, we discovered some pre-existing security issues:

1. Missing context checks: Some code checked roles without verifying context-specific authorizations:

if(role == "teacher") { 
  //do stuff assuming this is a teacher
  //no check for specific context(course etc)
} else { 
  //do stuff assuming this is a student role
}Code language: JavaScript (javascript)

2. IDOR vulnerabilities: We found Insecure Direct Object Reference issues in the codebase from the early startup days before security was prioritized.

Identifying and fixing these vulnerabilities was an unexpected benefit of our authentication overhaul.

When Is JWT the Right Choice?

Despite the challenges, JWT was absolutely the right solution for our needs. However, it might not be right for every system. Consider JWT when:

• You need stateless, distributed authentication
• You have role-based (not permission-based) authorization needs
• You want to minimize backend service calls
• You’re building a microservice architecture

Remember: JWT doesn’t encrypt data—it only validates it through signature verification. Anyone can decode the base64 payload to see the data (but not modify it without the secret key).

Conclusion: JWT in Production

A couple of years after implementing our JWT-based auth system, I can confidently say it was the right choice for our specific use case. It allowed us to move from a monolith to microservices without creating a new single point of failure.

The system successfully handled our complex role requirements while keeping the performance impact minimal. Most importantly, it enabled our engineering organization to evolve our architecture while continuing to deliver new features.

If you’re considering JWT for your microservice authentication needs, I hope this case study provides valuable insights into both the benefits and challenges you might face.

Have you implemented JWT in your systems? What challenges did you encounter? Share your experiences in the comments below!