Glossary · System coordination, integration and orchestration
Event ordering
Also known as: message ordering, event sequence
German: Ereignisreihenfolge
In distributed and event-driven systems, event ordering is the guarantee, or the lack of it, that events are delivered to and processed by consumers in a defined order, such as the order in which they were produced by one source or in causal order.
- System integration
In one sentence
Event ordering describes whether and how a system guarantees that consumers receive and process events in a defined sequence.
Example
The event broker guarantees order per machine partition, so “order started” for a machine always arrives before “order finished” for the same machine.
How it applies
- Engineering: Most brokers guarantee order only within a partition, topic or connection, not globally. Design keys so that events whose order matters travel together, and make consumers robust to out-of-order arrival elsewhere.
- Data quality: Retries and reconnections can reorder or duplicate events. Sequence numbers and timestamps (see Timestamp) from the source help consumers detect gaps and reorder.
- Documentation: Interface documentation should state the ordering guarantee explicitly. Readers of event logs and alarm lists should be told whether the displayed order is the order of occurrence or the order of arrival.
Event ordering vs. time synchronization
Event ordering concerns the sequence in which events are handled. Time synchronization concerns the accuracy of clocks. Synchronized clocks help reconstruct order, but ordering guarantees can also be achieved without them.