System design: Caching Strategies

Caching is one of the most fundamental techniques used in system design to improve application performance, reduce latency, and manage system load. The idea is simple: instead of fetching data from a slower backend system (e.g., a database) every time it is needed, you store frequently accessed data in a faster medium called a cache. This chapter explores various caching strategies, cache replacement policies, and practical considerations when implementing caching in your system.

Introduction to Caching and Cache Invalidation

Caching is a technique that stores copies of data or results in a high-speed storage layer, enabling faster access when the data is requested again. The cache can store the results of complex computations, database queries, or static files, reducing the number of times the backend system is accessed. The primary goal of caching is to improve system performance and reduce resource usage.

Benefits of Caching

• Reduced Latency: Since the cache is faster (e.g., in-memory storage), it provides quicker access to data compared to slower, persistent storage systems like databases or disk.
• Lower Load on Backend Systems: Caching reduces the number of requests that reach the database or API, allowing these systems to handle a larger load with less resource consumption.
• Cost Reduction: By reducing the frequency of expensive database queries or API calls, caching can lower the operational costs associated with cloud services or infrastructure.

Challenges with Caching

• Cache Invalidation: One of the most challenging aspects of caching is knowing when to invalidate (remove) outdated or stale data from the cache. If the cache holds data that no longer reflects the current state of the system, it can cause inconsistencies.
• Cache Coherence: In distributed systems, maintaining consistency between different caches or between cache and the original source of truth can be complex.

Cache Invalidation Strategies

To keep caches fresh and relevant, various invalidation strategies are employed:

1. Time-to-Live (TTL): Each cached entry has a TTL value, which defines how long the data should stay in the cache. Once the TTL expires, the data is considered stale and removed.

2. Write-Through Cache: Whenever data is written to the underlying storage (e.g., database), it is simultaneously written to the cache, ensuring that the cache remains up-to-date.

3. Write-Around Cache: Data is written directly to the backend storage without updating the cache. This avoids filling the cache with write-heavy data that might not be read often, but it can result in cache misses for newly written data.

4. Write-Back Cache: Data is written to the cache first and then asynchronously flushed to the backend system. This offers low-latency writes but increases the risk of data loss if the cache is not properly flushed to the persistent storage before a failure.

5. Cache Eviction Policies: These policies determine when to remove old or stale data from the cache when the cache becomes full.

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Client-Side vs. Server-Side Caching

Caching can occur at different layers in the architecture, and choosing the right caching layer depends on your application’s design and performance needs.

Client-Side Caching

• Definition: This involves caching data directly on the user's device, typically within the browser or the mobile app. Client-side caching is often used for caching static resources like HTML, JavaScript, CSS, and images.

• Examples:

  • Browser Cache: Web browsers cache static assets and resources locally to minimize the need to reload assets on every page visit.
  • HTTP Caching Headers: Developers can control browser caching behavior using HTTP headers like Cache-Control, Expires, and ETag.

• Advantages:

  • Reduced Latency: Data is stored directly on the client-side, making access instant.
  • Reduced Server Load: By offloading static resource delivery to the client, the server’s workload is reduced significantly.

• Challenges:

  • Data Staleness: Client-side caches can hold onto outdated data if the server changes and the cache is not invalidated properly.
  • Security: Sensitive data should not be cached on the client side, as it may be exposed to unauthorized users.

Server-Side Caching

• Definition: Server-side caching is when data is cached closer to the application or database server. This is commonly used for dynamic content or expensive computations, such as database queries or API results.

• Examples:

  • Memory-based Caching: Servers use memory-based storage systems like Redis or Memcached to store frequently requested data.
  • Application Cache: In frameworks like Spring Boot (Java) or Gin (Go), caching can be integrated at the application layer to store pre-processed data.

• Advantages:

  • Faster Data Access: Since caches are placed near the application, the data retrieval process is significantly faster.
  • Reduces Load on Databases: By caching database results, server-side caching reduces the number of database queries.

• Challenges:

  • Consistency: If the underlying data changes, the cache must be invalidated properly, otherwise, the system will serve stale data.
  • Memory Management: Server-side caches are often stored in memory (e.g., RAM), so careful memory management is essential to prevent overuse and slowdowns.

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Cache Replacement Policies

When the cache storage becomes full, a replacement policy must decide which data to evict to make room for new data. Different replacement policies fit different use cases, depending on the application’s requirements.

1. Least Recently Used (LRU)

• Description: The cache keeps track of the order in which data was accessed, evicting the data that hasn’t been used for the longest period.
• Use Case: LRU is widely used when data access patterns are predictable and data that was recently accessed is likely to be accessed again.
• Example: A product catalog website can use LRU to cache frequently viewed items since it is likely that a user will view these again soon.

2. Least Frequently Used (LFU)

• Description: LFU evicts the data that has been accessed the least frequently. Unlike LRU, which looks at recency, LFU focuses on access frequency.
• Use Case: LFU is useful for scenarios where some items are accessed much more often than others, such as frequently viewed products or top-selling items.
• Example: An online store might use LFU to keep items that are popular in the cache while evicting rarely accessed items.

3. First-In, First-Out (FIFO)

• Description: In FIFO, the oldest data in the cache is evicted first, regardless of how often it is accessed.
• Use Case: This policy is simple but not always optimal, as it doesn’t account for the recency or frequency of data access.
• Example: FIFO might be useful in systems where cached data expires after a predictable interval.

4. Random Replacement

• Description: A random item is evicted from the cache when new data needs to be added.
• Use Case: This is rarely used as it doesn’t optimize for data access patterns, but it is simple to implement.
• Example: A distributed caching system that doesn’t track access times or frequency might use random replacement to avoid overhead.

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Distributed Cache (Redis, Memcached)

Distributed caches allow data to be stored across multiple machines or nodes, making the caching layer scalable and fault-tolerant. These are particularly useful in large, distributed systems that require low-latency access to frequently used data.

1. Redis

Redis is an in-memory key-value store known for its speed and versatility. It supports various data structures such as strings, hashes, lists, sets, and sorted sets, which makes it more than just a simple key-value store.

• Advantages:

  • High Throughput: Redis can handle a massive number of read/write operations per second, making it ideal for high-traffic systems.
  • Persistence: Redis allows data persistence by saving data to disk periodically, which makes it more reliable compared to other purely in-memory caches.
  • Advanced Features: Redis supports data replication, transactions, and pub/sub messaging, making it suitable for distributed environments.

• Use Cases:

  • Session Store: Many web applications use Redis to store session data.
  • Leaderboard Management: Gaming platforms often use Redis for maintaining and retrieving real-time leaderboard data.

2. Memcached

Memcached is a simpler, high-performance, distributed memory object caching system. It’s primarily used to speed up dynamic web applications by reducing database load.

• Advantages:

  • Extremely Fast: Memcached is designed to be lightweight and simple, offering extremely fast data retrieval.
  • Horizontal Scalability: Memcached scales horizontally by adding more nodes, making it suitable for large-scale systems.
  • Ease of Use: Its API is simple, allowing developers to quickly integrate it with their applications.

• Use Cases:

  • Database Query Caching: Memcached is often used to cache expensive database queries, reducing database load.
  • Temporary Data Storage: Memcached is useful for storing short-lived data such as API request results or session data.

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Conclusion

Caching is a vital system design strategy that helps improve application performance, reduce latency, and minimize the load on backend systems. Understanding different caching strategies, policies, and distributed caching solutions is essential when designing scalable, high-performance systems. By carefully selecting where and how to implement caching, and choosing the

appropriate eviction policies, you can significantly enhance your system's efficiency and responsiveness.