Explain what Redis is and what it is used for in modern applications.

Redis is an open-source, high-performance in-memory data store that functions as a key-value data structure.

Main features and uses of Redis:

• Caching: speeds up access to frequently used data, reducing load on the database.
• Session storage: often used to store user sessions in web applications.
• Queues and Pub/Sub: supports message queue mechanisms and publish/subscribe.
• Support for various data structures: strings, lists, sets, hashes, sorted sets, etc.

Example of using Redis in C# for caching a value:

using StackExchange.Redis;

var redis = ConnectionMultiplexer.Connect("localhost");
var db = redis.GetDatabase();

// Set a value
db.StringSet("user:1:name", "Ivan");

// Get a value
string name = db.StringGet("user:1:name");
Console.WriteLine(name); // Outputs: Ivan