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About the Book

What does a digital twin look like when you stop describing it in diagrams and actually give it a database?

From Schema to System is the complete data-backbone reference for the NEXUS-1 companion system. It takes the architecture introduced in From Table to Twin and writes it down as a full SQL Server / EF Core friendly schema atlas: one database, seventeen bounded contexts, hundreds of tables, typed lookup tables, primary keys, foreign keys, indexes, audit columns, and verification queries.

This is not a book about database theory in the abstract. It is a concrete schema reference for an industrial digital-twin platform built around the NEXUS-1 demonstration environment. The plant fleet, instrumentation layer, digital twin, alarm management, event management, maintenance records, root-cause engine, reinforcement-learning policy store, robotics fleet, radiation monitoring, emergency preparedness, compliance, reporting, and audit trail are all modelled as connected sectors of one database.

The central idea is simple:

A digital twin is only as trustworthy as the data backbone that holds it together.

If a unit exists in the reactor fleet, the same UnitId should be visible in instrumentation, alarms, root-cause analysis, maintenance, reporting, and audit. If a signal feeds a twin variable, that binding should be explicit. If an alarm becomes an incident, and the incident becomes a root-cause investigation, and the investigation produces corrective actions, the chain should be queryable. If the system claims that something happened, the database should be able to show where that claim came from.

This book is the schema map for that chain.

What This Book Contains

The book contains the complete NEXUS-1 schema atlas:

17 SQL Server schemas / bounded contexts
654 total tables
246 typed lookup or reference tables
408 substantive operational, historical, audit, fact, or relationship tables
SQL Server / EF Core friendly DDL
Primary-key and foreign-key definitions
Lookup-table conventions
ER diagrams and foreign-key passport maps
Cross-schema dependency mappings
Index definitions
Seed-data examples
Verification queries

The schemas covered are:

CorePlatform
Security
Organization
ReactorFleet
Instrumentation
DigitalTwin
AlarmManagement
EventManagement
Maintenance
RootCause
ReinforcementLearning
Robotics
RadiationMonitoring
EmergencyPreparedness
Compliance
Reporting
Audit

Together, these sectors form the complete data backbone behind the NEXUS-1 digital-twin concept.

Why This Book Exists

The earlier NEXUS-1 books each explained one major part of the platform.

From Grid to Core explained the plant and its physics.
From Flood to Cause explained alarm-flood collapse and root-cause reasoning.
From Trial to Policy explained the interpretable reinforcement-learning agent.
From Table to Twin explained why the data model matters and how a schema becomes a digital twin.

From Schema to System is the technical reference that follows from all of them.

It answers the question:

If all of those systems really lived in one coherent platform, what would the database look like?

The answer is not a single table, not a toy schema, and not a collection of disconnected examples. It is a complete, sector-by-sector architecture where every table has a place, every lookup has a type, and every cross-sector relationship is made explicit by a foreign key.

Who This Book Is For

This book is written for:

software architects
database designers
.NET and EF Core developers
SQL Server developers
engineers interested in digital-twin platforms
readers of the NEXUS-1 series who want the complete data model
anyone studying how large operational systems can be organized into bounded contexts

You do not need nuclear-industry experience to read it. The nuclear-plant setting is the reference system, not the assumption. The real subject is enterprise data architecture for complex industrial systems.

If you have ever wondered how to model a system where telemetry, alarms, incidents, maintenance, root-cause analysis, compliance, reporting, and audit all need to speak about the same physical world, this book is for you.

What Makes It Different

Most database examples are too small to teach architecture. They show customers, orders, products, invoices, and perhaps a few lookup tables. That is useful, but it does not show what happens when a platform grows large enough to need real boundaries.

This book starts where small examples stop.

It shows:

how to divide a large system into schemas without losing referential integrity
how to use lookup tables without falling into hardcoded constants
how to make foreign keys act as passports between bounded contexts
how to keep operational records, history, evidence, and audit separate but connected
how to design for EF Core without giving up SQL Server clarity
how to make a digital twin queryable instead of merely visual

The result is not only a schema. It is a way of thinking about system truth.

Honest Boundary

NEXUS-1 is a Phase-0 demonstration and educational companion system. The schema in this book is a serious software-architecture reference, but it is not a licensed nuclear-plant information model, not safety-class software, not certified for operational use, and not connected to any real facility.

The purpose of this book is to show how a complex digital-twin platform can be structured, keyed, audited, and reasoned about at database level.

It is a schema atlas, not an operating manual.

In One Sentence

From Schema to System is the complete SQL Server / EF Core schema atlas for the NEXUS-1 digital-twin platform: one database, seventeen sectors, fully keyed.

Author: Grigorios Kyriakos Agathangelidis
Greek name: Γρηγόριος Κυριάκος Αγαθαγγελίδης
Also searchable as: Αγαθαγγελίδης Γρηγόριος, Αγαθαγγελιδης Γρηγοριος, Grigorios Agathangelidis.

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