Idempotent Consumers

As Retries and Concurrency Limits defined it for a retried charge, an operation is idempotent when performing it twice leaves the same result as performing it once. Setting a booking's status to CANCELED is idempotent: the second time, the status is already CANCELED and nothing changes. Sending an email is not: two calls, two emails.

A consumer is idempotent when it can receive the same message twice and the outside world sees the effect once