To guarantee message ordering and exactly-once delivery:

• Kafka ensures ordering within individual partitions. To maintain logical sequence, send related events (e.g., from the same user or session) to the same partition.
• Exactly-once delivery is achieved by using Kafka’s idempotent producers and transactional writes, combined with consumers that track message offsets.
• Design idempotent consumers: Ensure that reprocessing a message doesn’t result in duplicated side effects.
• Use unique event IDs: Snowplow provides event-level deduplication support using unique identifiers for every event.

These strategies ensure data integrity even in the face of retries, crashes, or restarts.