Decomposition & DDD

A Bounded Context is an explicit boundary where one domain model and one ubiquitous language are consistent; each maps to a service with its own model and database, and contexts talk through explicit context maps.

Event Storming maps commands, aggregates, and events with experts to find boundaries; at high TPS keep aggregates small, make the aggregate the transaction boundary, and reference other aggregates by ID only.

Entity has identity, Value Object is immutable value, Aggregate is one consistency unit via its root, Repository persists aggregates, Domain Service holds cross-entity logic, Application Service orchestrates use cases, and strategically Bounded Context, Ubiquitous Language, Context Mapping, and Domain Events organize everything.

Merge two services back when the boundary causes coordination without autonomy, such as shared team, lockstep deploys, synchronous chains, shared consistency, and lockstep schema changes, and treat deleting or merging services as a normal, healthy operation.

Decompose by business capability or DDD subdomain for clear ownership, keep tightly coupled transactions in one service, and split by scalability when one part has very different load, always maximizing cohesion and minimizing coupling.

Decompose along DDD bounded contexts, give each service a private database, extract one module at a time with the Strangler Fig, and expect the hard parts to be the shared database and the shift from local transactions to sagas.

Every architecture style exists to solve one dominant problem -- pick the style whose problem matches yours, because choosing the wrong one buys complexity you do not need.

Put the business rules in the center with technology-agnostic ports, plug concrete adapters onto those ports, and make everything depend inward so you can swap any technology without touching the domain.

Line count is not a reason to split; split only where you have evidence of independent change, ownership, scaling, or failure needs that a stronger internal boundary cannot satisfy.