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Architecture & System Design › Domain-Driven Design

Aggregate

A cluster of objects treated as one unit for changes.

Backend Engineer · Senior

Also known as: aggregate, DDD aggregate, consistency boundary

An Aggregate is a cluster of entities and values treated as one consistency unit: everything inside changes together (transactionally), everything outside references only the root. It draws the boundary where invariants hold strongly — inside, immediate consistency; across aggregates, eventual.

Order [root] ─┬─ OrderLine (entity, no outside refs)
              └─ Money total (value)
outside world → Order only (never OrderLine directly)

Sizing aggregates is the craft: too large (whole catalogues in one) creates contention and doomed transactions; too small (every entity its own) pushes invariants into eventually-consistent nowhere. Bound by true invariants — what must change atomically, no more.

The classic mistakes:

  • Giant aggregates. “The Customer with all Orders, Invoices and Tickets” serialises everything through one lock and fails constantly under contention. Split by real invariant needs.
  • Navigable internals. Outside code reaching inside (order.lines.add) bypasses root enforcement silently. Encapsulate: all mutation through root methods.
  • Cross-aggregate transactions. Needing atomicity across roots signals misdrawn boundaries (merge) or eventual consistency acceptance (sagas/events) — not distributed transactions by default.
  • Lazy-loading webs. ORM graphs spanning aggregates load the world per operation and blur boundaries. Fetch per aggregate; reference others by id.
  • Anemic aggregates. Roots as data bags with logic in services hollow the pattern. Behaviour (especially invariant enforcement) lives inside.
  • Eventual-everything confusion. Inside = transactional; between = eventual. Mixing the two (eventual inside, transactional across) inverts the model’s guarantees.
  • Technical boundaries as aggregates. Table-shaped or service-shaped clusters instead of invariant-shaped ones. Boundaries follow consistency needs, discovered with experts.

How to draw them: cluster by atomic invariants, expose only roots, reference across by id, accept eventuality between. Aggregates are consistency made structural.