Aggregate (DDD)

A cluster of objects changed as one unit, with a single root that enforces the business rules and marks the boundary of one transaction.

An order with its lines is an aggregate: you change lines only through the order, and the order checks that the total stays within the credit limit. One transaction changes one aggregate; other aggregates are referenced by id and updated through events. Aggregates that are too large cause lock contention; too small, and the rules leak into services.

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Related terms

DDD (domain-driven design): Designing software around the business’s own language and rules, with the code split where that language changes meaning.
Transactional outbox: Saving an event to an outbox table in the same database transaction as the change itself, then publishing it to the broker separately.
Message ordering: Whether a queue delivers messages in the order they were sent; usually guaranteed only per key or partition, if at all.

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