How Memcached Works?

Memcached operates as a high-performance, distributed memory caching system that can significantly improve the speed and scalability of web applications. Here’s a detailed explanation of how Memcached works within a system design:

1. Basic Architecture

Memcached is based on a client-server architecture, where multiple clients interact with one or more Memcached servers.

2. Client-Side Operations

1. Cache Request Flow:

• Hashing Key: When a client wants to store or retrieve data, it hashes the key using a consistent hashing algorithm to determine which Memcached server should handle the request.
• Server Interaction: The client sends the request to the identified server. This server then processes the request and either stores or retrieves the data.

2. Key Operations:

• Set: Adds a new key-value pair to the cache or updates an existing key.
• Get: Retrieves the value associated with a key.
• Delete: Removes a key-value pair from the cache.
• Add: Adds a new key-value pair only if the key does not already exist.
• Replace: Updates an existing key-value pair only if the key already exists.
• Increment/Decrement: Atomically modifies the value of an existing key by incrementing or decrementing it.

3. Server-Side Operations

• Memory Allocation: Memcached servers use a slab allocator to manage memory efficiently. Memory is divided into chunks of varying sizes, which are grouped into slabs. Each slab contains chunks of a specific size to minimize fragmentation and optimize allocation.
• Item Storage: When a new item is stored, it is placed in an appropriately sized chunk within a slab. If the slab is full, the least recently used item within that slab is evicted to make room for the new item.

4. Consistent Hashing

Consistent hashing is used to distribute keys across multiple servers. This ensures that each key is always mapped to the same server, and the distribution remains balanced even when servers are added or removed. This minimizes cache misses and ensures efficient load distribution.

5. Cache Management

• LRU Eviction: Memcached uses a Least Recently Used (LRU) eviction policy to manage cache items. When the cache is full, the least recently used items are removed to make space for new items.
• Expiration: Items can have an optional expiration time set, after which they are automatically removed from the cache.

6. Handling Cache Misses

When a client requests a key that is not found in the cache (a cache miss), the application must fetch the data from the primary data store (e.g., a database). The fetched data can then be added to the cache to optimize future requests.

7. Scalability and Load Balancing

Memcached is designed to scale horizontally by adding more servers. Load balancing is achieved through consistent hashing, which ensures even distribution of keys across all servers. This makes it easy to scale the cache by simply adding or removing servers as needed.

8. Fault Tolerance and Data Consistency

• No Built-in Replication: Memcached does not inherently provide data replication or fault tolerance. If a server goes down, all data stored in that server’s memory is lost. Applications must handle fault tolerance by implementing mechanisms such as data redundancy, failover strategies, or by using multiple Memcached clusters.
• Data Consistency: Since Memcached is a volatile cache, data consistency between the cache and the primary data store is managed by the application. Typically, the application updates the cache whenever there are changes in the primary data store to ensure consistency.

9. Monitoring and Maintenance

• Metrics: Monitoring Memcached involves tracking key metrics like cache hit and miss ratios, memory usage, item counts, and network traffic.
• Tools: Tools like memcached-tool and integration with monitoring systems (e.g., Nagios, Munin) help administrators monitor the performance and health of Memcached servers.
• Optimization: Regular maintenance tasks include optimizing memory allocation, adjusting the number of slabs, and managing the eviction policy to ensure optimal performance.

10. Security Considerations

• Network Security: Since Memcached lacks built-in authentication and encryption, it should be deployed within a secure network environment. Measures such as IP whitelisting, network segmentation, and firewall rules can help secure access.
• Application-Level Security: Additional security can be implemented at the application level, such as encrypting sensitive data before storing it in the cache and using secure client-server communication protocols.

Memcached is a powerful tool used in system design to speed up web applications by storing data in memory. It works as a caching layer, reducing the time needed to access frequently requested information. This helps websites and services handle more traffic efficiently, making them faster and more responsive. Memcached is widely used in tech companies to improve performance and scalability.