7.1 Redis vs Memcached

Suppose an online store caches each product as a 2 KB serialized object, and a price changes. With Memcached, the application reads the whole 2 KB value, changes one number, and writes 2 KB back. With Redis, the application can store the product as a hash and change only the price field on the server, in one atomic command.

That single difference follows from what each system was built to be. Memcached is a deliberately minimal distributed cache: it stores a blob, returns a blob, and evicts when memory is full