When the Aggregate Gets Heavy - Example Code

Companion code for the Code That Makes Sense post When the Aggregate Gets Heavy: Splitting Behavior Without Anemia.

A blog/CMS Post aggregate grew too big to hold all its behavior cleanly. This repo refactors it in four phases. Each step extracts one concern with a design pattern, without sliding into the anemic-domain trap, and one shared test suite proves the behavior never changed.

The five phases

Each module is a full snapshot of the codebase at one stage. Read them in order, and diff phase N against phase N+1 to see exactly one pattern appear.

Phase	Module	What it extracts	Pattern
0	00-start	nothing yet: the god-class Post, every concern inline	(the "before")
1	01-specification	canBeFeatured() policy + the PostQueries filter example	Specification
2	02-state-strategy	transitionTo + canEdit lifecycle, excerpt() algorithm	State + Strategy
3	03-decorator	render() transformation chain	Decorator
4	04-cor	validateForPublish() ordered guards	Chain of Responsibility

The public Post API is identical in all five modules, only the internals change. That is what lets a single test suite run against every phase.

How behavior preservation is proven

Every module compiles the same tests from shared-tests/ against its own implementation, and all five share one approved golden-master transcript. So a single command runs the identical suite against all five phases:

./gradlew test

Green means every refactoring preserved behavior. To run one phase on its own:

./gradlew :04-cor:test

Mutation testing

Tests can pass without actually catching bugs. PIT (./gradlew pitest) inserts faults into the production code and checks whether the suite notices. A minimum 80% kill rate per module is enforced; on this codebase every phase scores 98-100%.

./gradlew pitest

Hard code-style rules

Two constructs are banned everywhere in this codebase:

• No ternary operator (? :).
• No switch / switch expression.

State and Strategy use polymorphism. Predicates use early-return if / else. Enum-to-instance lookups use immutable Map registries. Verified with a final grep at build time.

Requirements

JDK 25. Gradle provisions it through the toolchain and the wrapper pins Gradle 9.5.1, so there is nothing else to install: ./gradlew fetches what it needs.

Layout

# the five phase snapshots
00-start/
01-specification/
02-state-strategy/
03-decorator/
04-cor/

# unit tests + golden master, shared by all phases
shared-tests/

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