Requirements

Imagine you're building a caching system to efficiently store and retrieve data. This system should be highly configurable, allowing users to select different eviction policies (like Least Recently Used - LRU, or Least Frequently Used - LFU) and storage mechanisms (in-memory, or persistent storage like disk). Furthermore, the system must be thread-safe, allowing concurrent access from multiple threads without data corruption or performance degradation. The goal is to design a robust and flexible caching mechanism that can adapt to various application needs. This isn't just about storing key-value pairs. We're crafting a foundational component that could be used in a wide range of applications. The system's ability to swap out different eviction strategies and storage backends is key to its long-term value. # Requirements ## Functional Requirements 1. **`put(key, value)`**: Inserts a key-value pair into the cache. If the key already exists, update its value. 2. **`get(key)`**: Retrieves the value associated with a given key. Returns `null` if the key is not found. 3. **`remove(key)`**: Removes the key-value pair associated with the given key. 4. **`clear()`**: Removes all entries from the cache. 5. **`getSize()`**: Returns the number of items currently in the cache. 6. **`getMaxSize()`**: Returns the maximum number of items the cache can hold. 7. **Eviction Policies**: * **LRU (Least Recently Used)**: Evicts the least recently accessed entry. * **LFU (Least Frequently Used)**: Evicts the least frequently accessed entry. * **FIFO (First-In-First-Out)**: Evicts the oldest entry. * The eviction policy should be configurable at cache creation time. 8. **Storage**: * **In-Memory**: Stores data in RAM (default). * **Disk-Persistent**: Stores data on disk, allowing the cache to persist across restarts. * The storage mechanism should be configurable at cache creation time. 9. **Expiration (TTL)**: Each cache entry can have a Time-To-Live (TTL) value. If the TTL expires, the entry should be automatically removed from the cache. 10. **Cache Loader**: A mechanism to load data into the cache if it's not already present (e.g., from a database). ## Non-Functional Requirements 1. **Thread Safety**: The cache must be thread-safe, allowing concurrent access from multiple threads. Utilize appropriate synchronization mechanisms (locks, concurrent data structures) to prevent race conditions and ensure data consistency. 2. **Extensibility**: The design should allow for easy addition of new eviction policies and storage mechanisms without requiring major code changes. Use design patterns like Strategy and Factory to achieve this. 3. **Modularity**: The code should be well-organized into modules (classes and interfaces) with clear responsibilities. This improves maintainability and testability. 4. **Testability**: The code should be designed to be easily testable. Use dependency injection to facilitate mocking dependencies for unit testing. 5. **Performance**: The cache should provide good performance for both `put` and `get` operations. Consider the time complexity of different data structures and algorithms used in the implementation. Aim for O(1) or O(log n) performance for these operations where possible. 6. **Error Handling**: The code should handle potential errors gracefully, such as exceptions during disk I/O. 7. **Eviction Listeners**: Allow users to register listeners that are notified when an item is evicted from the cache. 8. **Metrics and Monitoring**: Provide basic metrics (e.g., hit rate, miss rate, eviction count) that can be used to monitor the cache's performance. ## Core Entities 1. **`Cache ` Interface**: Defines the core operations of the cache (put, get, remove, clear, getSize, getMaxSize). 2. **`CacheEntry ` Class**: Represents a single entry in the cache, containing the key, value, and optionally, TTL information. 3. **`EvictionPolicy ` Interface**: Defines the interface for different eviction policies (LRU, LFU, FIFO). Implementations will handle the logic for choosing which entry to evict. 4. **`Storage ` Interface**: Defines the interface for different storage mechanisms (in-memory, disk-persistent). Implementations will handle the storage and retrieval of data. 5. **`CacheLoader ` Interface**: Defines a mechanism for loading data into the cache if it's not already present. 6. **`EvictionListener ` Interface**: Defines an interface for listeners that are notified when an item is evicted from the cache. 7. **`CacheConfiguration ` Class**: Encapsulates the configuration parameters for the cache, such as maximum size, eviction policy, storage mechanism, and TTL. 8. **`CacheStats` Class**: Holds metrics for the cache (hit rate, miss rate, eviction count). This may be implemented as a simple class with atomic counters.

Design a Cache Mechanism.

Requirements

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