Feature flags in Kubernetes - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a feature flag in Kubernetes?

A feature flag is a way to turn features on or off in your application without changing the code. In Kubernetes, it helps control new features safely by toggling them using configuration.

beginner

How can ConfigMaps be used for feature flags in Kubernetes?

ConfigMaps store configuration data as key-value pairs. You can use them to hold feature flag values and mount them into pods so your app reads the flags and enables or disables features accordingly.

beginner

What is the benefit of using feature flags in Kubernetes deployments?

Feature flags let you release new features gradually, test them in production, and quickly disable them if problems occur, all without redeploying your app.

intermediate

Which Kubernetes resource is commonly used to manage feature flags dynamically?

ConfigMaps are commonly used because they can be updated without restarting pods, allowing feature flags to change dynamically.

intermediate

How does a pod access feature flags stored in a ConfigMap?

A pod can mount the ConfigMap as a volume or use environment variables from the ConfigMap. The application inside the pod reads these values to decide which features to enable.

What Kubernetes object is best suited to store feature flag settings?

A Service

B Pod

C Deployment

D ConfigMap

ConfigMaps store configuration data like feature flags, separate from application code.

How can feature flags be updated without redeploying an application in Kubernetes?

A By restarting the pod manually

B By updating the ConfigMap and letting pods reload it

C By changing the container image

D By deleting the deployment

Updating the ConfigMap allows pods to get new flag values without redeploying the app.

What is a key advantage of using feature flags in production?

A Enable or disable features instantly without code changes

B Increase pod memory automatically

C Scale the number of pods

D Change container images

Feature flags let you toggle features on or off instantly without changing code.

Which method can a pod use to access feature flags from a ConfigMap?

A Access Deployment specs

B Use a Service to fetch flags

C Mount ConfigMap as a volume or use environment variables

D Read from a Secret

Pods can mount ConfigMaps as files or environment variables to read feature flags.

Why is it useful to use feature flags during gradual rollouts?

A To control which users see new features safely

B To increase CPU limits

C To change pod labels

D To update container images

Feature flags help release features to some users first, reducing risk.

Explain how feature flags work in Kubernetes and why they are useful.

Think about how you can turn features on or off without changing code.

Describe the steps to implement a feature flag using a ConfigMap in Kubernetes.

Focus on ConfigMap creation, pod access, and dynamic updates.

Practice

1. What is the main purpose of feature flags in Kubernetes?

easy

A. To manage Kubernetes cluster nodes

B. To monitor cluster health

C. To schedule pods across nodes

D. To enable or disable application features without changing code

Solution

1. Step 1: Understand feature flags concept

   Feature flags allow toggling features on or off without modifying the application code.

2. Step 2: Relate to Kubernetes usage

   In Kubernetes, feature flags help control app behavior dynamically, often via ConfigMaps or environment variables.

3. Final Answer:

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

4. Quick Check:

   Feature flags = toggle features without code change [OK]

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

Common Mistakes:

• Confusing feature flags with cluster management
• Thinking feature flags manage pods or nodes
• Mixing feature flags with monitoring tools

2. Which Kubernetes resource is commonly used to store feature flags for an application?

easy

A. Service

B. Pod

C. ConfigMap

D. Ingress

Solution

1. Step 1: Identify resource types

   Pods run containers, Services expose them, Ingress manages external access, ConfigMaps store configuration data.

2. Step 2: Match feature flags storage

   Feature flags are configuration data, so ConfigMaps are the right resource to store them.

3. Final Answer:

   ConfigMap -> Option C

4. Quick Check:

   Feature flags stored in ConfigMap [OK]

Hint: ConfigMaps hold config data like feature flags [OK]

Common Mistakes:

• Choosing Pod instead of ConfigMap
• Confusing Service or Ingress with config storage
• Thinking feature flags are stored in Secrets

3. Given this ConfigMap YAML snippet for feature flags:

apiVersion: v1
kind: ConfigMap
metadata:
  name: feature-flags
data:
  FEATURE_X_ENABLED: "true"
  FEATURE_Y_ENABLED: "false"

What will be the value of FEATURE_Y_ENABLED when accessed as an environment variable in a pod?

medium

A. false

B. true

C. null

D. undefined

Solution

1. Step 1: Read ConfigMap data values

   The ConfigMap sets FEATURE_Y_ENABLED to the string "false" explicitly.

2. Step 2: Understand environment variable mapping

   When injected as env vars, values are strings exactly as in ConfigMap, so FEATURE_Y_ENABLED will be "false".

3. Final Answer:

   false -> Option A

4. Quick Check:

   ConfigMap value "false" = env var "false" [OK]

Hint: Env vars get exact string values from ConfigMap [OK]

Common Mistakes:

• Assuming boolean false instead of string "false"
• Thinking missing keys return null or undefined
• Confusing string values with boolean types

4. You have this environment variable setup in a pod spec:

- name: FEATURE_Z_ENABLED
  valueFrom:
    configMapKeyRef:
      name: feature-flags
      key: FEATURE_Z_ENABLED

If the ConfigMap 'feature-flags' does not have the key FEATURE_Z_ENABLED, what will happen when the pod starts?

medium

A. Pod will fail to start with an error

B. Pod will start with FEATURE_Z_ENABLED set to an empty string

C. Pod will start with FEATURE_Z_ENABLED set to null

D. Pod will ignore the environment variable

Solution

1. Step 1: Understand configMapKeyRef behavior

   If the specified key is missing in the ConfigMap, Kubernetes treats it as an error.

2. Step 2: Effect on pod startup

   The pod will fail to start because the environment variable cannot be resolved from the ConfigMap key.

3. Final Answer:

   Pod will fail to start with an error -> Option A

4. Quick Check:

   Missing ConfigMap key causes pod start failure [OK]

Hint: Missing ConfigMap key breaks pod start [OK]

Common Mistakes:

• Assuming empty string or null is set silently
• Thinking pod ignores missing keys
• Confusing with optional environment variables

5. You want to enable a new feature only for 10% of users using feature flags in Kubernetes. Which approach best supports this scenario?

hard

A. Use a ConfigMap with a boolean flag set to true or false

B. Store a percentage value in ConfigMap and let the app decide feature enablement per user

C. Use a Secret to store the feature flag and update it daily

D. Deploy two versions of the app and route 10% traffic to the new version

Solution

1. Step 1: Understand percentage-based feature flags

   To enable a feature for a subset of users, the flag must support partial enablement, not just true/false.

2. Step 2: Evaluate options

   Store a percentage value in ConfigMap and let the app decide feature enablement per user stores a percentage in ConfigMap; the app reads it and enables the feature for that percent of users dynamically.

3. Final Answer:

   Store a percentage value in ConfigMap and let the app decide feature enablement per user -> Option B

4. Quick Check:

   Percentage flags need app logic with ConfigMap value [OK]

Hint: Use percentage value in ConfigMap for partial rollout [OK]

Common Mistakes:

• Using boolean flags for partial user enablement
• Relying on Secrets for feature flags
• Thinking traffic routing replaces feature flags