Callback Event

Overview

Callback Event is the structured event message delivered through a callback when a monitored operation changes state or reaches a defined condition. It is used to communicate the event type and relevant data to the receiving application. It differs from the callback transport, which is the delivery mechanism rather than the business event itself.

A typical implementation works as follows: The sender assigns an event identifier and type, includes object and status data with timestamps and version, and delivers it to the configured target. A receiver should treat transport acknowledgement and successful downstream processing as separate states for Callback Event.

Common risks include ambiguous event names and mutable payloads. Important failure modes include ambiguous event names, mutable payloads, missing sequence context, duplicate delivery, and schema changes. These failures can lead to incorrect transitions.

Core controls require teams to use stable event types, immutable IDs, versioned schemas, timestamps, idempotent consumers, and authoritative object references. A Callback Event handler should acknowledge only after durable receipt when the provider’s retry contract depends on the response.

Operational evidence should include event ID, type, object ID, previous and new state, created time, schema version, and delivery attempts. Callback Event should be documented alongside Callback, Callback Response, and Callback Verification. Business actions triggered by Callback Event should be idempotent and should verify the current object state before fulfillment or accounting updates.

Monitoring for Callback Event should track delivery age, signature failures, duplicate rate, retry exhaustion, and unresolved business events. Replay of Callback Event should preserve original identifiers and timestamps so historical processing cannot masquerade as a new event.