Service-to-service authentication in Microservices - Architecture Diagram

System Overview - Service-to-service authentication

This system enables secure communication between microservices by verifying each service's identity before allowing access. It ensures that only authorized services can call other services, protecting sensitive data and operations.

Key requirements include strong authentication, token management, and minimal latency impact.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
+----------------+       +----------------+       +----------------+
| Service A      | <---> | Auth Service   |       | Service B      |
| (Client)      |       | (Token Issuer) |       | (Resource)     |
+----------------+       +----------------+       +----------------+
       |                          |                        |
       |                          |                        |
       +--------------------------+------------------------+
                                  |
                                  v
                            Token Store (Cache)
                                  |
                                  v
                            Database (for tokens)

Components

User

user

Initiates requests to Service A

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Entry point for all service requests, routes to appropriate services

Service A

service

Client service that requests resources from Service B

Auth Service

authentication_service

Issues and validates tokens for service authentication

Service B

service

Resource service that requires authentication for access

Token Store (Cache)

cache

Caches valid tokens for quick validation

Database

database

Stores token data and service credentials securely

Request Flow - 13 Hops

User → Load Balancer

Load Balancer → API Gateway

Service A → Auth Service

Auth Service → Database

Auth Service → Token Store (Cache)

Auth Service → Service A

Service A → API Gateway

API Gateway → Service B

Service B → Token Store (Cache)

Service B → Database

Service B → API Gateway

API Gateway → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Auth Service

Impact:New tokens cannot be issued, so Service A cannot authenticate to Service B. Existing tokens may still be valid if cached.

Mitigation:Deploy multiple Auth Service instances with load balancing and failover. Use token caching to allow short-term continued access.

Architecture Quiz - 3 Questions

Which component is responsible for issuing authentication tokens?

A Auth Service

B API Gateway

C Service B

D Load Balancer

Design Principle

This architecture uses a dedicated authentication service to centralize token issuance and validation, improving security and scalability. Caching tokens reduces latency for validation, and load balancing ensures high availability and even traffic distribution.

Practice

1. What is the main purpose of service-to-service authentication in microservices?

easy

A. To ensure that one service can securely verify the identity of another service

B. To speed up communication between services

C. To store data between services

D. To monitor the health of services

Solution

1. Step 1: Understand the role of authentication

   Authentication is about verifying identity to ensure trust between entities.

2. Step 2: Apply to microservices context

   In microservices, service-to-service authentication ensures one service knows it is talking to a trusted service.

3. Final Answer:

   To ensure that one service can securely verify the identity of another service -> Option A

4. Quick Check:

   Authentication means verifying identity = A [OK]

Hint: Authentication means verifying identity between services [OK]

Common Mistakes:

• Confusing authentication with data storage
• Thinking authentication speeds up communication
• Mixing authentication with monitoring

2. Which of the following is a common method used for service-to-service authentication?

easy

A. Using JWT tokens issued by an authentication server

B. Using SQL queries to verify service identity

C. Using CSS styles to secure communication

D. Using HTML forms for authentication

Solution

1. Step 1: Identify valid authentication methods

   JWT tokens are widely used for secure token-based authentication between services.

2. Step 2: Eliminate unrelated options

   SQL queries, CSS, and HTML forms are unrelated to service authentication.

3. Final Answer:

   Using JWT tokens issued by an authentication server -> Option A

4. Quick Check:

   JWT tokens = common authentication method [OK]

Hint: JWT tokens are standard for service authentication [OK]

Common Mistakes:

• Confusing UI technologies with authentication
• Thinking database queries authenticate services
• Mixing frontend and backend concepts

3. Consider this simplified code snippet for service-to-service authentication using JWT:

token = auth_server.issue_token(service_id="serviceA")
if auth_server.verify_token(token):
    print("Access granted")
else:
    print("Access denied")

What will be printed if the token is valid?

medium

A. Access denied

B. Error: token missing

C. Access granted

D. No output

Solution

1. Step 1: Understand token issuance and verification

   The token is issued by the auth server and then verified immediately.

2. Step 2: Check the conditional logic

   If the token is valid, verify_token returns True, so "Access granted" is printed.

3. Final Answer:

   Access granted -> Option C

4. Quick Check:

   Valid token means access granted [OK]

Hint: Valid token means verify_token returns True [OK]

Common Mistakes:

• Assuming token is invalid without checking
• Confusing print outputs
• Ignoring the if-else structure

4. A microservice uses mTLS for service-to-service authentication but fails to connect. Which is the most likely cause?

medium

A. The server service is down

B. The API key is expired

C. The database is unreachable

D. The client service does not have a valid client certificate

Solution

1. Step 1: Understand mTLS requirements

   mTLS requires both client and server to have valid certificates for mutual authentication.

2. Step 2: Identify the cause of failure

   If connection fails due to authentication, missing or invalid client certificate is the likely cause.

3. Final Answer:

   The client service does not have a valid client certificate -> Option D

4. Quick Check:

   mTLS needs valid client cert = B [OK]

Hint: mTLS needs valid client certificate on both sides [OK]

Common Mistakes:

• Blaming server downtime without checking certificates
• Confusing database issues with authentication
• Mixing API keys with mTLS

5. You design a system where multiple microservices authenticate each other using JWT tokens issued by a central auth server. To improve scalability and security, which approach is best?

hard

A. Each service calls the auth server to verify tokens on every request

B. Each service validates tokens locally using the auth server's public key without calling the auth server every time

C. Services share a single API key for all authentication

D. Services trust any token without verification to reduce latency

Solution

1. Step 1: Consider scalability of token verification

   Calling the auth server on every request creates a bottleneck and reduces scalability.

2. Step 2: Use public key verification locally

   JWT tokens can be verified locally using the auth server's public key, improving speed and security.

3. Final Answer:

   Each service validates tokens locally using the auth server's public key without calling the auth server every time -> Option B

4. Quick Check:

   Local JWT verification improves scalability = A [OK]

Hint: Verify JWT locally with public key for scalability [OK]

Common Mistakes:

• Calling auth server on every request causing bottlenecks
• Using shared API keys reduces security
• Skipping token verification breaks security