Why Redis and not in-memory cache? Does the number of instances affect the choice?

Redis is a distributed in-memory cache with rich functionality, often chosen over simple in-memory cache embedded in an application for several reasons:

• Shared access: Redis operates as a separate server that multiple application instances can access, providing a consistent cache for all.
• Scalability: As the number of application instances increases, the local in-memory cache of each instance becomes isolated, leading to data duplication and consistency issues. Redis solves this problem by providing a centralized cache.
• Functionality: Redis supports complex data structures, TTL, atomic operations, which are difficult to implement in a simple in-memory cache.

Thus, the number of application instances directly influences the choice: if there is only one application, a local cache can be used, but with scaling and multiple instances, Redis ensures consistency and caching efficiency.