DRY, SRP, IoC, DI, and Service Contracts

Enterprise problem: A platform that can generate Research Brief, Service Desk, and Field Inspection applications becomes fragile when shared code means shared ownership, one service controls unrelated jobs, or business logic constructs databases and cloud clients directly. Whole-course context: The customer lifecycle now reaches verified export and retirement; today uses that evidence to draw internal boundaries that can evolve without breaking the lifecycle. Today’s slice: We apply Don’t Repeat Yourself (DRY), Single Responsibility Principle (SRP), Inversion of Control (IoC), Dependency Injection (DI), and contract tests to the control plane, runtime, and generated applications. End-of-day evidence: A service map, interface contract, positive contract test, rejected incompatible implementation, ownership decision, environment, timestamp, source revision, run, and trace IDs show replaceability. Still unsolved: Kubernetes packaging, infrastructure ownership, Terraform, and AWS isolation remain deferred.

The customer outcome is a reviewable dry, srp, ioc, di, and service contracts change, not a collection of requirements. This day starts with the implementation boundary, then uses the command or manifest below to produce positive, denied, and recovery evidence.

HTTP controllers, domain services, ports, injected adapters, and evidence adapter; service containers; each service owns its database/object evidence; cache: not involved until a measured read path needs it.

This map names the implementation boundary for this day. The service or controller changes only the state it owns; runtime and audit evidence let the operator distinguish a declared change from an effective one.

Inject an adapter at one composition root

Domain code that constructs AWS clients cannot run locally without cloud coupling and cannot expose its required authority. This labelled excerpt reflects the public monorepo’s actual Python ports and composition root at services/control_plane/app.py and services/control_plane/adapters.py.

def build_controller() -> ForgeController:
    database = Database(os.getenv("CONTROL_DATABASE_URL", "sqlite:///.lab/control.db"))
    repository = SqlProjectRepository(JsonProjectRepository(database))
    artifact_store = S3ArtifactStore(os.environ["ARTIFACT_BUCKET"]) if os.getenv("ARTIFACT_BUCKET") else SqlArtifactStore(Database(os.getenv("ARTIFACT_DATABASE_URL", "sqlite:///.lab/artifacts.db")))
    outbox = OutboxPublisher(database, publisher_from_url(os.getenv("BROKER_TOPIC", "http://broker:8080"), SERVICE_TOKEN)); outbox.start()
    forge = Forge(repository, GeneratorClient(os.getenv("GENERATOR_URL", "http://generator:8080")), RuntimeClient(os.getenv("RUNTIME_URL", "http://runtime:8080")), artifact_store, outbox)
    return ForgeController(forge)

Good boundaries preserve customer behavior while allowing one owned implementation to change.

• DRY centralises decisions, not every similar line.
• SRP follows reasons to change and ownership.
• IoC and DI make dependencies explicit but never bypass authorization.

A service boundary is credible only when ownership and compatibility are testable.

• [ ] Each record and policy has one mutating owner
• [ ] Ports state tenant, identity, idempotency, and failure semantics
• [ ] Composition occurs at a small number of visible roots
• [ ] Breaking contract and unaffected positive control are proven