Feature flags concept in Spring Boot - Step-by-Step Execution

Concept Flow - Feature flags concept

Start Application

↓

Check Feature Flag Status

↓

Run Code Based on Flag

↓

End

The app starts, checks if a feature flag is ON or OFF, then runs code accordingly.

Execution Sample

Spring Boot

boolean isFeatureEnabled = featureFlagService.isEnabled("newFeature");
if (isFeatureEnabled) {
    // run new feature code
} else {
    // run old feature code
}

This code checks if a feature flag is enabled and runs code based on that.

Execution Table

Step	Action	Feature Flag Checked	Flag Value	Code Path Taken
1	Start application	N/A	N/A	N/A
2	Check feature flag 'newFeature'	newFeature	true	Run new feature code
3	Execute new feature code	newFeature	true	New feature code runs
4	End execution	N/A	N/A	N/A

💡 Feature flag 'newFeature' is true, so new feature code runs and execution ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
isFeatureEnabled	undefined	true	true	true

Key Moments - 2 Insights

Why does the code run the new feature instead of the old one?

Because the feature flag 'newFeature' is true at step 2 in the execution_table, so the if condition is true and new feature code runs.

What happens if the feature flag is false?

If the flag was false, the else branch would run old feature code instead, as shown by the decision in step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'isFeatureEnabled' after step 2?

A true

B false

C undefined

D null

💡 Hint

Check the variable_tracker row for 'isFeatureEnabled' after step 2.

At which step does the application decide which code path to run?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Code Path Taken' column in execution_table for when the decision happens.

If the feature flag was OFF, how would the 'Code Path Taken' change at step 2?

A No code runs

B Run new feature code

C Run old feature code

D Application crashes

💡 Hint

Refer to the concept_flow where OFF leads to disabling the feature and running old code.

Concept Snapshot

Feature flags let you turn features ON or OFF without changing code.
Check flag status in code, then run new or old feature accordingly.
In Spring Boot, use a service to get flag value.
This helps test or release features safely.
Always check flag before running feature code.

Full Transcript

Feature flags are like switches in your app. When the app starts, it checks if a feature flag is ON or OFF. If ON, it runs the new feature code. If OFF, it runs the old code. This lets developers control features without changing the app code every time. In Spring Boot, you might call a service to check if a feature is enabled. Then, use an if statement to decide which code to run. This way, you can test new features safely or turn them off if needed.

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