Feature Flag Enabled Spring PetClinic

This project is an extension of the official Spring PetClinic application with a custom-built Feature Flag service implemented from scratch (without using libraries like FF4J or Togglz).

The feature flag system allows enabling/disabling application features dynamically using database-driven flags that persist across restarts.

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How to Run the Application

Prerequisites

• Java 17+
• Maven 3.8+
• Docker & Docker Compose

Steps

1. Clone the repository

   git clone https://github.com/XTiNCT-7/spring-petclinic.git
   cd spring-petclinic
   

2. Start MySQL using Docker

   docker-compose up mysql
   

3. Build and run the application

   mvn clean spring-boot:run
   

4. Access the application:

   • Application UI: http://localhost:8080
   • Feature Flag APIs: http://localhost:8080/feature-flags

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Assumptions & Design Decisions

• Feature flags are stored in the database and persist across application restarts.
• No authentication is applied to feature flag management APIs (as per requirement).
• Feature flag evaluation is centralized in a helper service so it can be reused across controllers, services, and views.
• A custom annotation (@FeatureToggle) is used to guard controller endpoints.
• Thymeleaf views are conditionally rendered using model attributes derived from feature flag checks.
• Feature flag behavior supports more than boolean enable/disable:
  • Global enable/disable
  • Whitelist-based access
  • Blacklist-based restriction
  • Percentage rollout (future-safe design)

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Feature Flags Implemented

Feature Flag Key	Type	Controls	Implementation Location
ADD_NEW_PET	SIMPLE	Enables adding a new pet to an owner	PetController, ownerDetails.html
ADD_VISIT	SIMPLE	Enables adding a visit for a pet	VisitController, ownerDetails.html
OWNER_SEARCH	SIMPLE	Enables owner search functionality	OwnerController, findOwners.html

Example

• If ADD_NEW_PET is disabled:
  • The "Add New Pet" button is hidden in the UI
  • Direct access to /owners/{id}/pets/new is blocked using @FeatureToggle

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Feature Flag Management APIs

Base Path: /feature-flags

Create Feature Flag

POST /feature-flags

Request Body

{
  "flagKey": "ADD_NEW_PET",
  "description": "Controls whether users can add new pets",
  "enabled": true,
  "flagType": "SIMPLE"
}

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Get All Feature Flags

GET /feature-flags

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Get Feature Flag by Key

GET /feature-flags/{flagKey}

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Update Feature Flag

PUT /feature-flags/{id}

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Delete Feature Flag

DELETE /feature-flags/{id}

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Custom Annotation Usage

@FeatureToggle(
    key = "OWNER_SEARCH",
    disabledMessage = "Owner search is restricted",
    disabledRedirect = "/owners/find"
)

This annotation prevents access to the controller method when the feature is disabled and optionally redirects the user.

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References

• Spring PetClinic: https://github.com/spring-projects/spring-petclinic

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