ADR-004: Define Default Guarantee as At-Least-Once

Status

Accepted

Context

In a pseudo trigger environment, exactly-once delivery introduces excessive complexity. Conversely, ensuring no events are missed is more important than preventing duplicates.

Decision

The default contract is defined as delivery that approximates at-least-once.

Rationale

• Fits naturally with the batch-success-then-commit model
• On crash or commit failure, allowing duplicates is safer than risking loss
• The requirement for an idempotent handler can be communicated clearly to users

Consequences

• Handlers and downstream consumers must be designed to tolerate duplicates
• Documentation must prevent misunderstanding of exactly-once semantics
• A dedup helper is provided as an optional feature