Glossary · Data, timing and communication
Idempotency
Also known as: idempotence
German: Idempotenz
In systems engineering, idempotency is the property that repeating the same operation has the same effect as performing it once. It makes retries and duplicate messages harmless.
- Data and interfaces
- Integrated systems
In one sentence
Idempotency makes repeated commands and duplicate messages harmless, which is a precondition for safe retries between integrated systems.
Example
The command “set conveyor speed to 0.5 m/s” is idempotent; “increase speed by 0.1 m/s” is not, and a duplicated message would speed the conveyor up twice.
How it applies
- Integrated systems: Design commands as absolute target states rather than increments, or give each request a unique ID so the receiver can discard duplicates. State in the interface specification which operations are idempotent.
- Functional safety: A non-idempotent command combined with retry logic can cause unintended motion or double dosing. Such hazards belong in the risk assessment of the integration.
- Evidence: Test idempotency explicitly by sending duplicate messages and by replaying operations after a partial failure.
Idempotent vs. non-idempotent operations
Reading a value, setting an absolute position or confirming a known job ID are idempotent. Counting pieces, incrementing a setpoint or starting a new job without an ID are not, and need duplicate protection before they may be retried.