====== 5. Reading the Architecture as an Onion ======
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The architecture uses an onion structure.
At the centre are durable concepts. The next layer defines logical relationships. The next layer specializes those relationships for a financial domain or demonstration. The next layer maps the logical model to selected technologies. The outer layer defines physical deployment, testability, and evidence. Figure 5-1 represents these layers.
  Conceptual Architecture
  └── Logical Architecture / PIM
      └── Domain Logical Profile
          └── Implementation Profile / PSM
              └── Deployment, Testability, and Evidence Plan
Figure 5-1: Layered structure of the Financial Systems Archetype document layers
Each layer depends on the layer inside it.
The outer layers may specialize, realize, deploy, or test inner-layer concepts. They must not redefine those concepts.
Table 5-1: Examples of inner-layer concepts and later-layer realisations
^ Inner-layer concept ^ Later-layer realization ^
| Communication Endpoint | DDS Topic, API endpoint, message queue, event stream |
| Data Structure Definition | IDL structure, JSON Schema, XML Schema, UML/SysML model |
| Node | Process, container, pod, service, virtual machine |
| Runtime Plane | Topic namespace, DDS partition, Kubernetes namespace, network segment, security zone |
| Evidence | Logs, captured samples, test reports, deployment records, and Crucible records |

The later-layer realization does not replace the inner-layer concept.

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