Efficient memory management is crucial in Redis because all data is stored in memory, making optimal usage essential for performance and scalability. Proper memory handling ensures fast data retrieval and prevents unexpected service disruptions due to memory exhaustion. Understanding how Redis manages memory helps you design reliable and high-performing data solutions.

Memory Management Explained

Redis is an in-memory data store, so understanding how it manages memory is essential for building reliable and efficient applications. You need to know how Redis allocates memory, stores keys, handles eviction, and optimizes memory usage.

Memory Allocation

• Redis allocates memory from your system's RAM to store all data, including keys and values;
• It uses its own memory allocator or the system's allocator, depending on how it was compiled;
• When you add new data, Redis requests more memory as needed until it reaches the configured maximum.

Key Storage

• Every item in Redis is stored as a key-value pair;
• Keys are always stored as strings, while values can be strings, lists, sets, hashes, or other data types;
• Each key-value pair uses memory for the key, the value, and some internal metadata.

Eviction Policies

• When Redis reaches its memory limit, it uses an eviction policy to decide which data to remove;
• Common policies include noeviction (returns an error when full), allkeys-lru (removes least recently used keys), and volatile-lru (removes least recently used keys with an expiration time set);
• You can set the eviction policy in the Redis configuration file or at startup.

Memory Optimization Basics

• Use the most appropriate data types for your use case to save memory;
• Set expiration times (TTL) on keys that do not need to persist forever;
• Avoid storing large values or unnecessary metadata;
• Monitor memory usage with the INFO MEMORY command and adjust your settings as needed.

Practical Example

Suppose you are building a session store for a web application. You store each user's session data in Redis with a unique key. To control memory usage:

• You set a maximum memory limit in the Redis configuration;
• You use the allkeys-lru eviction policy so that inactive sessions are removed first when memory is full;
• You set an expiration time of 30 minutes on each session key to automatically remove old sessions;
• You monitor memory usage and optimize your data structures to store only necessary session information.

This approach ensures that your Redis instance remains fast and efficient, even as the number of users grows.