Definition
An idempotent operation is one where repeated executions yield exactly the same result as a single execution. In distributed systems and APIs, idempotency is critical because messages may be delivered multiple times (retry logicPattern for handling temporary failures in distributed systems - automatic retry with exponential backoff., network recovery, timeouts). HTTP methods: GET, PUT and DELETE are idempotent by definition. POST is not - sending the same POST twice may create two resources. Idempotency is typically implemented via idempotency keys: a unique ID per operation that the system stores, and on duplicates returns the previous result rather than re-executing the operation. In the context of ElhubNational hub for metering data and market processes in the Norwegian electricity market., idempotency is relevant because messages may be resent following communication failures - the system must recognise and discard duplicates without triggering two supplier switches.