Redis uses a data structure called Sorted Set to store timeout clients. In this data structure, each element is a member with a score associated with it. The score represents the expiration time of the member. Redis uses this data structure to maintain a sorted list of members based on their expiration time. The client with the lowest score (or earliest expiration time) is considered the next one to expire and will be removed from the set when its time arrives. This allows Redis to efficiently manage the expiration of keys in a scalable way.
