Distributed Config — Spring Cloud Config Server

The Problem

Ten microservices, each with its own configuration. Changing a database URL means updating ten config files and redeploying ten services. Spring Cloud Config centralizes configuration in one place.

Step 1: Config Server Setup

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-config-server</artifactId>
</dependency>

@SpringBootApplication
@EnableConfigServer
public class ConfigServerApplication {
    public static void main(String[] args) {
        SpringApplication.run(ConfigServerApplication.class, args);
    }
}

# config server's application.yml
server:
  port: 8888

spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/example/config-repo
          default-label: main
          clone-on-start: true
          search-paths: '{application}'

Step 2: Config Repository Structure

config-repo/
  product-service/
    application.yml
    application-staging.yml
    application-prod.yml
  order-service/
    application.yml
    application-staging.yml
    application-prod.yml

# config-repo/product-service/application.yml
server:
  port: 8080

spring:
  datasource:
    url: ${DB_URL}
    username: ${DB_USER}
    password: ${DB_PASSWORD}

feature-flags:
  new-search: false
  bulk-export: true

# config-repo/product-service/application-prod.yml
spring:
  datasource:
    hikari:
      maximum-pool-size: 20
      minimum-idle: 5

feature-flags:
  new-search: true

Step 3: Config Client (Your Service)

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-config</artifactId>
</dependency>

# bootstrap.yml (required for config client)
spring:
  application:
    name: product-service
  profiles:
    active: ${SPRING_PROFILES_ACTIVE:default}
  cloud:
    config:
      uri: http://config-server:8888
      fail-fast: true
      retry:
        max-attempts: 6
        initial-interval: 1000
        multiplier: 1.5

The client fetches config from the server on startup. fail-fast: true with retry means it keeps trying if the config server is temporarily down.

Step 4: Encryption of Sensitive Values

Install the JCE unlimited strength extension and set an encryption key:

# Set the encrypt key as an environment variable
export ENCRYPT_KEY=my-super-secret-key

Encrypt values through the config server:

curl http://config-server:8888/encrypt -d 'my-db-password'
# Returns: AQCGw...

Use the encrypted value in your config files:

spring:
  datasource:
    password: '{cipher}AQCGw...'

The config server decrypts it before serving to clients.

Step 5: Dynamic Refresh Without Restart

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

@RestController
@RefreshScope
public class FeatureFlagController {
    @Value("${feature-flags.new-search}")
    private boolean newSearchEnabled;

    @GetMapping("/features")
    public Map<String, Boolean> getFeatures() {
        return Map.of("newSearch", newSearchEnabled);
    }
}

# Trigger refresh (pulls latest config from server)
curl -X POST http://product-service:8080/actuator/refresh

@RefreshScope tells Spring to recreate the bean with new config values. No restart needed.

For multiple services, use Spring Cloud Bus with RabbitMQ or Kafka to broadcast refresh events.

Key Points

• Centralized config in a Git repository — version controlled, auditable
• Profile-specific files handle environment differences
• Encrypt sensitive values with {cipher} prefix
• @RefreshScope enables config reload without restart
• Use Spring Cloud Bus to refresh all services at once