What key storage strategies for idempotency exist besides Redis?

In addition to Redis for storing idempotency keys, you can use:

• Databases (SQL/NoSQL) — for example, storing unique keys in a table with a unique index, which guarantees no duplicates.
• File storage — for simple systems, keys can be stored in files with access locking.
• Distributed Key-Value Stores — such as Etcd, Consul, which provide consistency and fault tolerance.
• Message Queues with idempotency support — for example, Kafka with unique message keys.

The choice depends on requirements for performance, fault tolerance, and scalability.