What is Memcached?

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.

Important Topics for Memcached?

  • What is Memcached?
  • Core Concepts of Memcached
  • How Memcached Works?
  • Benefits of Using Memcached
  • Use Cases of Memcached
  • Features of Memcached
  • Real-world Examples of Memcached Usage

What is Memcached?

Memcached is a distributed memory caching system used to enhance the performance and scalability of web applications by reducing the load on databases. It stores frequently accessed data in memory, allowing for faster retrieval compared to traditional storage methods like disk-based databases. Here’s a breakdown of its role in system design:

  • In-Memory Storage: Memcached stores data in RAM, which is much faster than accessing data from a disk.
  • Distributed Architecture: It can run on multiple servers, distributing the cache across them to balance the load.
  • Key-Value Storage: Data is stored as key-value pairs, making retrieval straightforward and efficient.
  • Volatile Storage: Data in Memcached is not persistent; it gets lost if the server restarts or if the cache is full and data is evicted.

Core Concepts of Memcached

Memcached is a distributed memory caching system designed to speed up dynamic web applications by alleviating database load. Here are the core concepts and components of Memcached in system design:

  1. Distributed Caching: Memcached distributes data across multiple nodes (servers) using a hashing algorithm. This enables horizontal scaling, where additional nodes can be added to handle more data and increased load.
  2. Key-Value Store: Memcached stores data in key-value pairs. The key is a unique identifier for the data, and the value is the data itself. This simple data model allows for fast retrieval and storage.
  3. In-Memory Storage: Data is stored in RAM, allowing for very fast read and write operations compared to disk-based storage systems. This makes Memcached ideal for caching frequently accessed data.
  4. Least Recently Used (LRU) Eviction: Memcached uses an LRU eviction policy to manage memory. When the cache reaches its memory limit, the least recently used items are evicted to make space for new data.
  5. Client-Server Architecture: Clients communicate with Memcached servers to store and retrieve data. This separation allows multiple clients to access the cache simultaneously and distributes the load across multiple servers.
  6. No Persistence: Memcached is a volatile cache, meaning that data is not persisted to disk. If a Memcached server goes down, all the data in that server’s memory is lost. This design choice is intentional to maximize speed.
  7. Hashing: A consistent hashing algorithm is used to map keys to specific servers. This ensures that each key is mapped to the same server, providing a predictable and balanced distribution of data across servers.
  8. Cache Misses and Hits: Cache Hit Occurs when the requested data is found in the cache. Cache Miss Occurs when the requested data is not found in the cache, prompting the system to retrieve the data from the primary data store (e.g., a database).
  9. Scalability and Load Balancing: Memcached can scale horizontally by adding more servers. Load balancing across these servers can be managed using consistent hashing and other techniques to ensure even distribution of data and requests.
  10. Security: By default, Memcached does not include authentication or encryption, which can be a security risk. It is typically deployed within a secure network, and additional security measures like SASL authentication or TLS encryption can be implemented if needed.

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.

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.

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.

Benefits of Using Memcached

Memcached offers several benefits that make it a popular choice for caching in distributed systems, particularly for dynamic web applications. Here are the key benefits:

  • Improved Performance and Speed
    • Reduced Latency: Memcached stores data in RAM, providing much faster access times compared to disk-based databases.
    • High Throughput: It can handle a large number of read and write operations per second, improving the overall throughput of the application.
  • Scalability
    • Horizontal Scaling: Memcached can easily scale horizontally by adding more nodes (servers). This allows it to handle increasing amounts of data and traffic without significant changes to the application.
    • Load Distribution: Consistent hashing ensures that data is evenly distributed across all available servers, optimizing resource usage and avoiding bottlenecks.
  • Reduced Database Load
    • Offloading Reads: By caching frequently accessed data, Memcached reduces the load on the primary database, freeing up resources for write operations and more complex queries.
    • Efficient Use of Database Resources: Reducing the frequency of database queries helps in maintaining better performance and responsiveness of the database.
  • Simplicity and Flexibility
    • Simple API: Memcached provides a straightforward key-value interface, making it easy to integrate with various programming languages and applications.
    • Minimal Configuration: Setting up and maintaining Memcached is relatively simple, requiring minimal configuration and management overhead.
  • Cost-Effective
    • Reduced Infrastructure Costs: By reducing the load on databases and application servers, Memcached can help in reducing the overall infrastructure costs.
    • Optimal Resource Utilization: Efficient caching leads to better utilization of existing resources, potentially delaying the need for additional hardware or expensive database upgrades.
  • Supports Various Data Structures
    • Versatile Data Storage: While primarily a key-value store, Memcached supports various data structures, including strings, lists, and more complex objects through client-side serialization.
    • Application-Specific Use Cases: This flexibility allows developers to cache different types of data efficiently, catering to diverse application requirements.

Use Cases of Memcached

Memcached is a versatile caching solution that can be used in various scenarios to improve performance and efficiency in web applications and distributed systems. Here are some common use cases:

  1. Web Page Caching: Store the results of complex database queries or computationally expensive operations to serve dynamic web pages quickly. Cache frequently accessed static resources, such as HTML, CSS, and JavaScript files, to reduce server load and improve page load times.
  2. Session Management: Storage Store user session data in Memcached to allow for quick access and scalability across multiple application servers. This is especially useful in a load-balanced environment. Maintain session state without relying on a single server, providing high availability and reliability.
  3. Database Query Caching: Cache the results of frequently executed database queries to reduce database load and improve response times. Store the results of expensive computations that are reused frequently, reducing the need to perform the same calculations repeatedly.
  4. Application Data Caching: Cache application configuration data that is read frequently but changes rarely, reducing the need to access the database for each read. Store user profile data to allow for quick retrieval and reduce latency in user-related operations.
  5. API Response Caching: Cache responses from external API calls to reduce latency and avoid hitting rate limits. Use Memcached to cache responses between microservices, improving the efficiency and performance of service-to-service communication.
  6. E-commerce Applications: Cache product catalog information to provide faster search and retrieval of product data. Shopping Cart Data: Store shopping cart data to maintain a responsive user experience during shopping sessions.

Features of Memcached

Memcached is a powerful, distributed memory caching system with a variety of features that make it suitable for high-performance applications. Here are some key features:

  • Distributed Memory Caching: Memcached distributes data across multiple nodes, enabling horizontal scaling and load balancing.
  • High Performance: In-memory storage results in very low latency and high throughput for read and write operations.
  • Simple Key-Value Store: Memcached uses a simple key-value store model, which makes it easy to integrate and use.
  • Scalability: Memcached can scale horizontally by adding more servers, which allows it to handle increased load efficiently.
  • Flexible Memory Allocation: Uses a slab allocator to manage memory efficiently, reducing fragmentation and optimizing resource usage.
  • Least Recently Used (LRU) Eviction: Implements an LRU eviction policy to ensure that the most frequently accessed data remains in the cache.
  • Consistent Hashing: Distributes keys across nodes using consistent hashing, ensuring even distribution and minimal data movement when nodes are added or removed.
  • Non-Persistent Storage: Designed to be a volatile cache, meaning it does not persist data to disk, which enhances performance but requires that the application can handle cache misses gracefully.
  • Monitoring and Management Tools: Provides built-in statistics and monitoring capabilities, and integrates well with external monitoring tools.
  • Security Features: While Memcached itself lacks built-in authentication and encryption, it can be deployed securely within a trusted network and with additional layers of security, such as SASL authentication and TLS encryption.

Real-world Examples of Memcached Usage

Memcached is widely used across various industries to enhance application performance and scalability. Here are some notable real-world examples:

  1. Facebook: Facebook uses Memcached extensively to handle billions of requests per second, caching user sessions, and profile data. Significantly reduces database load, ensuring fast and scalable access to user data.
  2. Wikipedia: Wikipedia uses Memcached to cache rendered pages and frequently accessed data. Improves page load times and reduces the load on the primary database servers.
  3. Twitter: Twitter uses Memcached to store timeline and user feed data, which are accessed frequently by users. Enhances the speed of feed generation and ensures a smooth user experience even under heavy load.
  4. YouTube: YouTube uses Memcached to cache video metadata and user preferences. Reduces latency in video recommendations and improves the responsiveness of the platform.
  5. Reddit: Reddit uses Memcached to cache posts, comments, and user session data. Improves the performance of the website, allowing for quick retrieval of popular content and user information.


In conclusion, Memcached is a powerful tool for improving the performance and scalability of web applications. By caching frequently accessed data in memory, it reduces database load and speeds up response times. Its simple key-value storage, distributed architecture, and support for multiple languages make it easy to integrate into various systems. Real-world examples from companies like Facebook, Twitter, and YouTube demonstrate its effectiveness in handling high traffic and enhancing user experience. Overall, Memcached is an essential component for optimizing system design and ensuring efficient, fast, and scalable applications.