Describe the idempotency mechanism of debit operations: where was the key stored, what was the TTL, how were collisions handled?

The mechanism of idempotency for debit operations is usually implemented by storing a unique operation key (e.g., UUID or request hash) in a fast storage with a limited lifetime (TTL).

• Where the key is stored: the operation key is stored in a distributed cache or database supporting TTL, such as Redis.
• TTL: the lifetime of the key is set based on business logic, for example, 5-10 minutes, to prevent repeated debits on repeated requests within a short period.
• Collision handling: when a request with an existing key is received, the operation is not executed again, and the previous call's result is returned. If the key is absent, the operation is executed, and the key is stored.

This approach guarantees that repeated requests with the same key will not lead to double debiting, ensuring idempotency.