Layer Brett Fundamentals: Core Concepts and Practical Patterns

Layer Brett is a pragmatic approach to layered architecture that favors clear boundaries, stable contracts, and thin adapters. Its aim is simple: keep product velocity high while keeping complexity low. Where many architectures drift into an indistinct mesh of responsibilities, Layer Brett insists that every layer owns one thing, speaks one contract, and protects the domain from accidental coupling.

Why layers, why now

Modern products move fast. New markets, new devices, new regulations—change is the default. In this context, systems fail not because teams are slow but because change ripples unpredictably. Layers give us the ability to change parts in isolation. With Layer Brett, the promise is specific:

• Boundaries reduce cascading changes and accidental dependencies.
• Contracts give teams autonomy by defining how layers communicate.
• Adapters keep technologies swappable and mitigate vendor lock-in.

The four canonical layers

Although Layer Brett is adaptable, most stacks share four canonical layers. These are not rigid dogma; they are a working agreement that aligns teams and tooling.

1) Interface layer

This is where requests enter and responses leave: HTTP handlers, GraphQL resolvers, CLI commands, or event consumers. It validates inputs, translates transport-specific details, and forwards a clean request model to the application layer. Its rule: no business decisions here.

2) Application layer

The application layer sequences use cases. It orchestrates the domain and adapters but does not compute business rules. Think of it as the film director: it decides which scenes play, in which order, with which parameters. Its rule: no infrastructure details here.

3) Domain layer

Pure business logic. Aggregates, value objects, domain services, invariants. The domain layer is intentionally unaware of databases, queues, or HTTP. Its rule: depend only on domain abstractions and pure data types.

4) Infrastructure layer

Integrations and implementations that talk to the world: databases, payment providers, caches, email gateways. It implements the ports the domain and application layers require. Its rule: never leak infrastructure types upward.

Contracts and ports: the real force multipliers

Layer Brett centers on contracts, sometimes called ports. A port is a minimal, stable interface the domain or application layer expects. For example, a PaymentPort might define authorize, capture, and refund. The infrastructure layer implements it for Stripe or Adyen. When a vendor changes, your code changes in one place: the adapter. This narrows blast radius and makes technical choices reversible.

Adapters that stay thin

Adapters translate between the world and your ports. Done well, they are intentionally boring. They map types, handle retries, and hide idiosyncrasies. Done poorly, they become a second domain. Keep them thin:

• Translate, don’t transform. Put semantics in the domain.
• Prefer composition over inheritance for vendor SDKs.
• Log at boundaries and include correlation IDs for traces.

Data flow and idempotency

Layer Brett encourages explicit message shapes between layers. Use idempotency keys at the interface layer to shield the domain from duplicates. Apply retry policies in adapters, not in the domain. When mutation happens, emit events at the application layer so other use cases can subscribe without tight coupling.

Observability as a first-class contract

Every boundary is an observation point. Standardize logs, metrics, and traces at the same time you define ports. A typical convention is to log request_id, user_id, port name, operation, and result. Track golden signals per port—latency, throughput, and error rate—so you can spot regressions when a contract evolves.

Evolution without drama

Systems must evolve. Layer Brett expects versioned contracts: v1 and v2 can run in parallel while clients upgrade. Keep deprecations boring by providing adapters that translate from old models to new domain concepts. Avoid breaking changes unless a clear business reason exists, and always communicate EOL timelines in code and docs.

Common pitfalls (and fixes)

• Leaky infrastructure types in the domain. Fix by wrapping external types at the adapter boundary.
• Too many layers for trivial flows. Fix by collapsing layers where they add no insight.
• Adapters that reimplement business rules. Fix by moving decisions to the domain and leaving mapping to adapters.
• Implicit contracts. Fix by writing the interface, documenting it, and testing it with contract tests.

Minimal checklist to adopt Layer Brett

• Define use cases as application services with clear inputs and outputs.
• Specify ports for external concerns (payments, identity, storage).
• Create thin adapters that implement ports and include telemetry.
• Keep the domain pure; avoid time, I/O, and framework types.
• Introduce contract tests and tracing at each boundary.
• Track latency budgets per layer and enforce them in CI.

A small example journey

Imagine you need to add gift cards to an e-commerce app. Without layers, changes sprawl across controllers, services, and database repos. With Layer Brett, the domain gains a GiftCard entity and rules (issue, redeem, expire). The application layer adds a RedeemGiftCard use case, and the interface layer exposes POST /giftcards/redeem. For storage, you define a GiftCardPort, then implement it with your relational DB. Later, if finance mandates a separate ledger system, you write a new adapter that implements the same port, migrate data behind the scenes, and the domain stays intact.

What “Layer Brett” is not

It is not microservices by default. You can apply Layer Brett inside a monolith and only extract services when you have clear operational reasons. It is not an excuse to build abstractions for their own sake. Abstractions are a tax; spend them only when they reduce actual cost.

Closing thoughts

Layer Brett is about disciplined pragmatism. Boundaries are there to protect attention. Contracts encode agreements that allow teams to move independently. Adapters buy you optionality. When in doubt, choose the simplest design that preserves these three properties. That is how systems stay understandable as they scale—and how your team keeps shipping with confidence.