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Spring Cloud Config Server Interview Questions and Answers

Master Spring Cloud Config Server with interview questions covering centralized configuration, Git backend, Config Client, encryption, refresh scope, Spring Cloud Bus, configuration hierarchy, and production best practices.

Spring Cloud Config Server Interview Questions and Answers

Introduction

In a microservices architecture, dozens or even hundreds of services require configuration such as:

  • Database URLs
  • API Keys
  • Kafka Brokers
  • Redis Hosts
  • Feature Flags
  • Logging Levels
  • Third-party Endpoints

Managing these configurations separately for every service quickly becomes difficult.

Spring Cloud Config Server provides Centralized Configuration Management, allowing all services to retrieve their configuration from a single location.

Configuration is typically stored in:

  • Git Repository
  • Native File System
  • Vault
  • JDBC Database
  • AWS S3 (custom)
  • Kubernetes ConfigMaps (alternative approach)

Config Server Architecture

flowchart LR

GitRepository --> ConfigServer

ConfigServer --> CustomerService

ConfigServer --> PaymentService

ConfigServer --> LoanService

ConfigServer --> NotificationService

Q1. What is Spring Cloud Config Server?

Answer

Spring Cloud Config Server is a centralized configuration server that provides configuration properties to multiple Spring Boot microservices.

Instead of storing configuration inside each application,

all configuration is stored centrally.

Benefits

  • Centralized management
  • Version control
  • Environment separation
  • Dynamic refresh
  • Easier deployments

Q2. Why do we need Config Server?

Without Config Server

Every service contains its own

  • application.yml
  • application.properties

Updating a common property requires redeploying every service.

Without Config Server

flowchart LR

CustomerService --> LocalConfig

PaymentService --> LocalConfig

LoanService --> LocalConfig

With Config Server

flowchart LR

GitRepository --> ConfigServer

ConfigServer --> CustomerService

ConfigServer --> PaymentService

ConfigServer --> LoanService

One configuration change can serve multiple services.

Q3. How does Config Server work?

Startup process

  1. Config Server starts.
  2. Connects to Git Repository.
  3. Reads configuration files.
  4. Config Client requests configuration.
  5. Config Server returns configuration.
  6. Application starts.

Request Flow

sequenceDiagram
Customer Service->>Config Server: Request Configuration
Config Server->>Git Repository: Read application.yml
Git Repository-->>Config Server: Configuration
Config Server-->>Customer Service: Properties
Customer Service->>ApplicationContext: Initialize Beans

Q4. Where are configuration files stored?

Most commonly,

configuration is stored in Git.

Example

config-repository

|

|-- application.yml

|-- customer-service.yml

|-- payment-service.yml

|-- application-prod.yml

|-- application-dev.yml

Benefits

  • Version history
  • Pull requests
  • Rollback
  • Auditing
  • Team collaboration

Q5. How does Config Client connect to Config Server?

Dependency

<dependency>

<groupId>

org.springframework.cloud

</groupId>

<artifactId>

spring-cloud-starter-config

</artifactId>

</dependency>

Configuration

spring:

  config:

    import:

      configserver:

        http://localhost:8888

When the application starts,

it retrieves configuration from the Config Server.

Q6. How are environment-specific configurations managed?

Example

application.yml

application-dev.yml

application-test.yml

application-prod.yml

Client Request

/customer-service/prod

Response

Production-specific configuration.

Environment Flow

flowchart LR

Git --> application.yml

Git --> application-dev.yml

Git --> application-prod.yml

ConfigServer --> CustomerService

Q7. How does Configuration Refresh work?

Normally,

configuration changes require restarting the application.

Spring Cloud supports runtime refresh using

  • @RefreshScope
  • Spring Cloud Bus

Example

@RefreshScope

@RestController
public class ConfigController{

}

Refresh Endpoint

POST

/actuator/refresh

Q8. Can Config Server encrypt sensitive properties?

Yes.

Sensitive values such as passwords can be encrypted.

Example

{cipher}

AHSY782JSJ...

Config Server decrypts values before sending them to clients.

Alternative solutions

  • HashiCorp Vault
  • AWS Secrets Manager
  • Azure Key Vault

Q9. What happens if Config Server is unavailable?

Possible approaches

  • Fail Fast
  • Retry
  • Local Cache
  • High Availability Config Servers

Configuration

spring:

  cloud:

    config:

      fail-fast: true

Typical production setup

flowchart LR

GitRepository --> ConfigServer1

GitRepository --> ConfigServer2

ConfigServer1 --> Services

ConfigServer2 --> Services

Q10. Config Server Best Practices

Store Configuration in Git

Maintain version history.

Separate DEV/TEST/PROD

Use profile-specific configuration.

Encrypt Sensitive Values

Avoid plain-text passwords.

Enable Refresh

Use @RefreshScope where dynamic updates are required.

Secure Config Server

Protect it using OAuth2, JWT, or mTLS.

Banking Example

flowchart TD

GitRepository --> ConfigServer

ConfigServer --> CustomerService

ConfigServer --> PaymentService

ConfigServer --> LoanService

CustomerService --> PostgreSQL

PaymentService --> Kafka

LoanService --> MongoDB

Every service retrieves its configuration from a centralized source.

Common Interview Questions

  • What is Config Server?
  • Why use centralized configuration?
  • How does Config Server work?
  • What is Config Client?
  • Where are configuration files stored?
  • How does profile-based configuration work?
  • What is @RefreshScope?
  • How do you encrypt configuration?
  • What happens if Config Server fails?
  • Config Server best practices?

Quick Revision

Topic Summary
Config Server Centralized configuration
Config Client Retrieves configuration
Git Most common backend
Profiles Environment-specific configuration
@RefreshScope Runtime configuration refresh
Spring Cloud Bus Distributed refresh
Encryption Secure sensitive properties
Fail Fast Startup failure on config issues
Retry Recover from temporary failures
High Availability Multiple Config Server instances

Config Retrieval Lifecycle

sequenceDiagram
Spring Boot App->>Config Server: Request Configuration
Config Server->>Git Repository: Read Config
Git Repository-->>Config Server: application-prod.yml
Config Server-->>Spring Boot App: Configuration
Spring Boot App->>ApplicationContext: Initialize Beans
ApplicationContext-->>Application: Ready

Production Example – Banking Platform

A banking platform consists of:

  • Customer Service
  • Account Service
  • Payment Service
  • Loan Service
  • Notification Service

Configuration Strategy

  • Configuration stored in a Git repository.
  • Separate files for DEV, TEST, UAT, and PROD.
  • Database credentials encrypted.
  • Config Server deployed with two replicas behind a load balancer.
  • Services fetch configuration during startup.
  • Runtime configuration changes propagated using Spring Cloud Bus with Kafka.
flowchart LR

GitRepository --> ConfigServerCluster

ConfigServerCluster --> CustomerService

ConfigServerCluster --> PaymentService

ConfigServerCluster --> LoanService

ConfigServerCluster --> NotificationService

SpringCloudBus --> Kafka

Kafka --> CustomerService

Kafka --> PaymentService

Kafka --> LoanService

This architecture provides centralized configuration management, secure secret handling, high availability, and dynamic configuration updates across all microservices.

Key Takeaways

  • Spring Cloud Config Server centralizes configuration for multiple microservices, eliminating duplicated configuration files.
  • Configuration is typically stored in Git, providing version control, auditing, rollback capability, and collaborative management.
  • Config Clients retrieve configuration during application startup using the Config Server.
  • Environment-specific configuration is managed through profile-specific files such as application-dev.yml and application-prod.yml.
  • @RefreshScope and Spring Cloud Bus enable runtime configuration updates without restarting services.
  • Sensitive properties should be encrypted or managed using dedicated secret management solutions such as Vault or AWS Secrets Manager.
  • Production deployments should use multiple Config Server instances for high availability and resilience.
  • Centralized configuration improves consistency, operational efficiency, and maintainability across enterprise Spring Cloud applications.