Feature flags concept in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a feature flag in software development?

A feature flag is a switch in the code that lets developers turn features on or off without changing the code or redeploying the app. It helps control which features users see.

intermediate

How do feature flags help in continuous delivery?

Feature flags allow teams to release new features safely by enabling them only for some users or in testing environments, reducing risk and making deployment faster.

intermediate

In Spring Boot, where can feature flags be typically stored?

Feature flags in Spring Boot can be stored in configuration files (like application.properties), databases, or external services, allowing dynamic control without code changes.

advanced

What is a common risk of using feature flags if not managed properly?

If feature flags are not cleaned up after use, they can clutter the code and cause confusion, making maintenance harder and increasing the chance of bugs.

intermediate

Explain the difference between a permanent and a temporary feature flag.

Temporary feature flags are used for short-term testing or rollout and removed after. Permanent flags control features that may always be optional or configurable.

What is the main purpose of a feature flag?

A To improve database performance

B To store user passwords securely

C To turn features on or off without redeploying

D To manage user sessions

Feature flags let developers enable or disable features dynamically without changing the code or redeploying.

Where can feature flags be stored in a Spring Boot application?

A In configuration files, databases, or external services

B Only in Java code

C Only in environment variables

D Only in the user interface

Spring Boot supports storing feature flags in config files, databases, or external services for flexibility.

What is a risk of leaving old feature flags in the code?

A They automatically delete themselves

B They make the code easier to read

C They improve app speed

D They clutter the code and cause confusion

Old feature flags can clutter code and make maintenance harder if not removed.

Which of these is NOT a benefit of using feature flags?

A Instant bug fixes without code changes

B Controlling feature visibility

C Testing features in production

D Safe feature rollout

Feature flags control feature visibility but do not fix bugs instantly without code changes.

What type of feature flag is used for long-term optional features?

A Temporary feature flag

B Permanent feature flag

C Debug flag

D Security flag

Permanent feature flags control features that remain optional or configurable long-term.

Describe how feature flags improve the deployment process in Spring Boot applications.

Think about how you can control features without changing the app code.

Explain the best practices for managing feature flags to avoid technical debt.

Consider how to keep code clean and understandable.

Practice

1. What is the main purpose of using feature flags in a Spring Boot application?

easy

A. To improve database performance

B. To manage application logging levels

C. To handle user authentication

D. To enable or disable features without changing the code

Solution

1. Step 1: Understand feature flags concept

   Feature flags allow toggling features on or off dynamically without redeploying code.

2. Step 2: Identify the main use in Spring Boot

   In Spring Boot, feature flags help control feature availability safely during runtime.

3. Final Answer:

   To enable or disable features without changing the code -> Option D

4. Quick Check:

   Feature flags = toggle features dynamically [OK]

Hint: Feature flags toggle features without code changes [OK]

Common Mistakes:

• Confusing feature flags with database optimization
• Thinking feature flags manage authentication
• Assuming feature flags control logging

2. Which of the following is the correct way to check a feature flag in a Spring Boot controller method?

easy

A. if(featureFlagService.isFeatureEnabled("newFeature")) { ... }

B. if(featureFlagService.checkFlag("newFeature")) { ... }

C. if(featureFlagService.enableFeature("newFeature")) { ... }

D. if(featureFlagService.toggleFeature("newFeature")) { ... }

Solution

1. Step 1: Recall typical method naming for feature flags

   Common method to check if a feature is enabled is named like isFeatureEnabled.

2. Step 2: Match method name with options

   Only if(featureFlagService.isFeatureEnabled("newFeature")) { ... } uses isFeatureEnabled which clearly indicates checking status.

3. Final Answer:

   if(featureFlagService.isFeatureEnabled("newFeature")) { ... } -> Option A

4. Quick Check:

   Check feature flag with isFeatureEnabled() [OK]

Hint: Look for method named isFeatureEnabled() to check flags [OK]

Common Mistakes:

• Using methods that sound like they change state (enableFeature, toggleFeature)
• Using incorrect method names not standard in feature flag services

3. Given the following Spring Boot code snippet, what will be the output if the feature flag "betaFeature" is disabled?

if(featureFlagService.isFeatureEnabled("betaFeature")) {
    System.out.println("Beta feature is ON");
} else {
    System.out.println("Beta feature is OFF");
}

medium

A. Beta feature is ON

B. Beta feature is OFF

C. No output

D. Runtime error

Solution

1. Step 1: Understand the condition check

   The code prints "Beta feature is ON" only if the flag is enabled (true).

2. Step 2: Analyze the flag state given

   The flag "betaFeature" is disabled, so isFeatureEnabled returns false, triggering the else block.

3. Final Answer:

   Beta feature is OFF -> Option B

4. Quick Check:

   Flag disabled = else block runs [OK]

Hint: Disabled flag triggers else block output [OK]

Common Mistakes:

• Assuming disabled flag prints ON message
• Expecting no output when else block exists
• Thinking code throws error on false flag

4. Identify the error in this Spring Boot feature flag usage:

@GetMapping("/feature")
public String feature() {
    if(featureFlagService.isFeatureEnabled) {
        return "Feature ON";
    } else {
        return "Feature OFF";
    }
}

medium

A. Return type should be void

B. Incorrect HTTP method annotation

C. Missing parentheses when calling isFeatureEnabled method

D. Feature flag service not injected

Solution

1. Step 1: Check method call syntax

   isFeatureEnabled is a method and must be called with parentheses ().

2. Step 2: Verify other code parts

   HTTP annotation and return type are correct; no injection info given but not error here.

3. Final Answer:

   Missing parentheses when calling isFeatureEnabled method -> Option C

4. Quick Check:

   Method calls need () [OK]

Hint: Method calls always need parentheses () [OK]

Common Mistakes:

• Forgetting () after method names
• Confusing HTTP method annotations
• Assuming return type must be void

5. You want to implement a feature flag in Spring Boot that disables a new payment method only for users in a "betaTesters" group. Which approach correctly combines feature flags with user context?

hard

A. Check if feature flag is enabled and user belongs to betaTesters before allowing payment

B. Enable feature flag globally and ignore user groups

C. Disable feature flag and allow all users access

D. Use feature flag only in frontend without backend checks

Solution

1. Step 1: Understand combining feature flags with user context

   Feature flags can be combined with user group checks to control feature access precisely.

2. Step 2: Evaluate options for correct logic

   Check if feature flag is enabled and user belongs to betaTesters before allowing payment correctly checks both the flag and user group before enabling the feature.

3. Final Answer:

   Check if feature flag is enabled and user belongs to betaTesters before allowing payment -> Option A

4. Quick Check:

   Feature flag + user group check = controlled access [OK]

Hint: Combine flag check with user group for precise control [OK]

Common Mistakes:

• Ignoring user groups when using feature flags
• Relying only on frontend checks
• Enabling feature globally without restrictions