The layers are integrated in a top-down communication model: each layer can hold a dependency only on the layer directly beneath it, as shown in Figure 8-5. This enfor‐ ces decoupling of implementation concerns and reduces the knowledge shared between the layers. In Figure 8-5, the presentation layer references only the business logic layer. It has no knowledge of the design decisions made in the data access layer.

Figure 8-5. Layered architecture

2 Such as AWS S3 or Google Cloud Storage.

3 In this context, the message bus is used for the system’s internal needs. If it were exposed publicly, it would belong to the presentation layer.