Eventhash

Eventhash is a compact identifier derived from the data of an individual event in an event-driven or event-sourced system. It serves as a content-based address or fingerprint for the event, enabling efficient deduplication, integrity checking, and fast lookups in event stores and log streams.

Typically, eventhash is computed by applying a cryptographic hash function such as SHA-256 to a canonical representation of the event. A canonical form might include the event type, timestamp, and a stable encoding of the payload and metadata. Some implementations use JSON with field order normalization; others use a byte-oriented serialization. The resulting hash is stored as the event's hash and may be exposed as a separate field or used as the event's identifier.

Common uses include deduplicating events across retries or multiple producers, implementing idempotent event processing, enabling content-addressable storage, and facilitating cross-system reconciliation where the same logical event should map to the same eventhash.

Considerations include the risk of hash collisions, though with modern hash sizes it's extremely small; privacy implications if sensitive data is included in the input; performance costs for hashing large payloads; and governance about mutability—ideally, once created, an event's hash should not change.

Related concepts include cryptographic hash functions, message digests, content-addressable storage, and event-sourcing patterns such as event IDs and stream indexing.