Order of Events in Parallel Subscriptions

The order of events that get distributed to the parallel subscriptions doesn’t always match the order of events in the event bus.

Events are distributed based on the hash value of the partition key. If the partition key values are unique for consecutive events, such as an order ID or the EventUuid field values, the hash values are likely to be unique, although that’s not guaranteed. In this case, the events are probably sent to different partitions for processing and processed out of order.

If the partition key values are identical for consecutive events, like a region value of EMEA, the hash values are identical. The events are sent to the same partition for processing and are processed in order. The hash logic for the partition key is case-sensitive, so the computed hash values for EMEA and emea are different. If the partition key value is null, the empty string is used as the basis of the hash value and events are processed by only one partition.

See Also

Choosing a Partition Key