How to Implement Kill Switch Feature Flags in a Spring Boot Application

|

Feature flags (also known as feature toggles) allow you to control the availability of application features at runtime without deploying new code. Among the different types of feature flags, kill switch feature flags play a crucial role in production stability. They allow you to instantly disable a feature that is causing performance issues, security vulnerabilities, or unexpected errors.

In this article, we will walk through how to implement kill switch feature flags in a Spring Boot application using a step-by-step approach with code examples. We will also cover best practices to ensure your kill switches are robust and maintainable.

What is a Kill Switch Feature Flag?

A kill switch feature flag is a simple, boolean-controlled toggle that can immediately disable a problematic feature. Unlike other feature flags used for gradual rollouts or A/B testing, kill switches must be:

  • Fast to evaluate: No heavy computations or remote calls.
  • Easy to access: Any developer or operations team member can disable the feature quickly.
  • Fail-safe: The default state should be safe in case of configuration failure.

These properties make kill switches a safety net for applications running in production.

Create a Spring Boot Application

Start with a basic Spring Boot application using Maven or Gradle.

Maven dependencies (pom.xml):

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

The actuator dependency is optional but recommended to provide an operational interface for monitoring and management.

Define a Kill Switch Configuration

The simplest approach is to manage kill switches via application properties or an external configuration server (such as Spring Cloud Config).

application.properties:

feature.killSwitch.sampleFeature=false

This defines a property called feature.killSwitch.sampleFeature which defaults to false, meaning the feature is enabled by default.

Create a Kill Switch Service

We can encapsulate kill switch checks inside a dedicated service to ensure consistent behavior.

KillSwitchService.java:

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Service;
@Service
public class KillSwitchService {
@Value(“${feature.killSwitch.sampleFeature:false}”)
private boolean sampleFeatureDisabled;
public boolean isSampleFeatureDisabled() {
return sampleFeatureDisabled;
}
}

This service reads the flag from application properties. If the configuration source changes at runtime (using Spring Cloud Config or environment refresh), the flag can be updated without redeploying.

Integrate the Kill Switch in Business Logic

Now, modify your application logic to consult the kill switch before executing the feature code.

SampleFeatureController.java:

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class SampleFeatureController {
private final KillSwitchService killSwitchService;
public SampleFeatureController(KillSwitchService killSwitchService) {
this.killSwitchService = killSwitchService;
}
@GetMapping(“/sample-feature”)
public String sampleFeature() {
if (killSwitchService.isSampleFeatureDisabled()) {
return “Sample feature is currently disabled via kill switch.”;
}
// Normal feature logic
return “Sample feature executed successfully!”;
}
}

This ensures that whenever the kill switch is enabled, the feature immediately stops running without requiring a code change or redeployment.

Make the Kill Switch Dynamically Configurable

Hardcoding flags in properties works, but it’s not ideal for production. Using Spring Boot Actuator or Spring Cloud Config allows dynamic updates.

Using Spring Cloud Config with /actuator/refresh:

  • Store feature.killSwitch.sampleFeature in a Git repository managed by Spring Cloud Config.
  • Change the value remotely.
  • Call /actuator/refresh to reload properties at runtime.

Using a Custom Endpoint to Toggle Flags:

  • You can also create an endpoint to toggle the kill switch for faster manual control.

KillSwitchAdminController.java:

import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class KillSwitchAdminController {
private boolean sampleFeatureDisabled;
@PostMapping(“/admin/kill-switch”)
public String updateKillSwitch(@RequestParam boolean disable) {
this.sampleFeatureDisabled = disable;
return “Sample feature kill switch set to “ + disable;
}
public boolean isSampleFeatureDisabled() {
return sampleFeatureDisabled;
}
}

In this case, you could refactor the KillSwitchService to read from an in-memory value updated by this admin controller instead of application properties.

Best Practices for Kill Switch Implementation

  1. Centralize Flag Management: All kill switch logic should be in one service or component. This makes it easier to audit and modify.
  2. Use Descriptive Names: Always include the feature name in the flag for clarity.
  3. Default to Safe: Ensure the flag defaults to a safe state (disable risky features by default if uncertain).
  4. Secure Administrative Endpoints: If you expose an API for toggling kill switches, protect it with authentication and authorization.
  5. Monitor and Alert: Integrate flag changes into your monitoring system to track when a kill switch is used.
  6. Remove Deprecated Flags: Once a kill switch is no longer needed, remove it from the codebase to avoid clutter.

Advanced Integration with External Feature Flag Services

While property files and actuator endpoints work for basic needs, enterprise-grade solutions like LaunchDarkly, Unleash, or FF4J offer:

  • Dashboards to manage flags.
  • Audit logs.
  • SDKs for dynamic flag evaluation.
  • Multi-environment support.

For example, with LaunchDarkly, you would:

  • Configure an SDK client in Spring Boot.
  • Define a feature flag on LaunchDarkly’s dashboard.
  • Check the flag state dynamically in your controllers.

This approach provides robust control but adds external dependencies and cost.

Conclusion

Implementing kill switch feature flags in a Spring Boot application is an essential strategy to maintain stability and resilience in production systems. By integrating a simple boolean toggle, you gain the power to instantly disable problematic features without redeploying code, minimizing downtime and protecting user experience.

A well-designed kill switch should be centralized, fast to evaluate, easy to update, and secure against unauthorized changes. Using Spring Boot’s configuration system, actuator endpoints, or a feature flag management platform, you can create a flexible kill switch architecture that adapts to your team’s needs.

Remember:

  • Start simple with properties.
  • Add dynamic updates with actuator or config servers.
  • Scale to enterprise solutions if your application requires high governance.

By adhering to these best practices and maintaining clean code hygiene (removing unused flags), your kill switches will remain effective safety nets rather than hidden liabilities. In short, kill switch feature flags are not just toggles—they are critical circuit breakers that keep your application healthy under pressure.