In-memory caching and distributed caching are both techniques used to enhance the speed of data retrieval. However, they have key distinctions that make each appropriate for different scenarios.
An in-memory cache stores data in the system's main memory (RAM). This allows for much faster access than disk-based storage because read/write operations to/from RAM are quicker. In-memory caches often serve as temporary storage subsystems for frequently accessed data to reduce latencies associated with primary databases or storage systems. The key drawback is that this kind of cache is usually limited to a single server and its available memory resources. Consequently, it does not scale horizontally.
Example: Redis is a popular in-memory cache system.
A distributed cache spreads its data across multiple servers or nodes, thereby allowing more data to be stored and processed. This type of cache scales well since adding more nodes increases the total available storage space. It also provides redundancy; if a node fails, the cache system can retrieve the data from another node. However, the complexity of managing data consistency and the potential for latency between nodes are challenges of a distributed cache.
Example: Memcached is commonly used for distributed caching.
To summarize, an in-memory cache is best when you need extremely fast access to data and don't have scalability requirements. On the other hand, a distributed cache is preferable when you need to store larger amounts of data and need horizontal scalability and high availability.