SCADA System — Design Documentation

Overview

This document serves as the master index for the SCADA system design. The system is a centrally-managed, distributed SCADA configuration and deployment platform built on Akka.NET, running across a central cluster and multiple site clusters in a hub-and-spoke topology.

Technology Stack

Layer	Technology
Runtime	.NET, Akka.NET (actors, clustering, remoting, persistence, streams)
Central UI	Blazor Server (ASP.NET Core + SignalR)
Inbound API	ASP.NET Core Web API (REST/JSON)
Central Database	MS SQL Server, Entity Framework Core
Site Storage	SQLite (deployed configs, S&F buffer, event logs)
Authentication	Direct LDAP/AD bind (LDAPS/StartTLS), JWT sessions
Notifications	SMTP with OAuth2 Client Credentials (Microsoft 365)
Hosting	Windows Server, Windows Service
Cluster	Akka.NET Cluster (active/standby, keep-oldest SBR)
Logging	Serilog (structured)

Scale

• ~10 site clusters, each with 50–500 machines, 25–75 live tags per machine.
• Central cluster: 2-node active/standby behind a load balancer.
• Site clusters: 2-node active/standby, headless (no UI).

Document Map

Requirements

• HighLevelReqs.md — Complete high-level requirements covering all functional areas.

Component Design Documents

#	Component	Document	Description
1	Template Engine	Component-TemplateEngine.md	Template modeling, inheritance, composition, path-qualified member addressing, override granularity, locking, alarms, flattening, semantic validation, revision hashing, and diff calculation.
2	Deployment Manager	Component-DeploymentManager.md	Central-side deployment pipeline with deployment ID/idempotency, per-instance operation lock, state transition matrix, all-or-nothing site apply, system-wide artifact deployment with per-site status.
3	Site Runtime	Component-SiteRuntime.md	Site-side actor hierarchy with explicit supervision strategies, staggered startup, script trust model (constrained APIs), Tell/Ask conventions, concurrency serialization, and site-wide Akka stream with per-subscriber backpressure.
4	Data Connection Layer	Component-DataConnectionLayer.md	Common data connection interface (OPC UA, custom), Become/Stash connection actor model, auto-reconnect, immediate bad quality on disconnect, transparent re-subscribe, synchronous write failures, tag path resolution retry.
5	Central–Site Communication	Component-Communication.md	Akka.NET remoting/cluster topology, 8 message patterns with per-pattern timeouts, application-level correlation IDs, transport heartbeat config, message ordering, connection failure behavior.
6	Store-and-Forward Engine	Component-StoreAndForward.md	Buffering (transient failures only), fixed-interval retry, parking, async best-effort replication, SQLite persistence at sites.
7	External System Gateway	Component-ExternalSystemGateway.md	HTTP/REST + JSON, API key/Basic Auth, per-system timeout, dual call modes (Call/CachedCall), transient/permanent error classification, dedicated blocking I/O dispatcher, ADO.NET connection pooling.
8	Notification Service	Component-NotificationService.md	SMTP with OAuth2 (M365) or Basic Auth, BCC delivery, plain text, transient/permanent SMTP error classification, store-and-forward integration.
9	Central UI	Component-CentralUI.md	Blazor Server with SignalR real-time push, load balancer failover with JWT, all management workflows.
10	Security & Auth	Component-Security.md	Direct LDAP bind (LDAPS/StartTLS), JWT sessions (HMAC-SHA256, 15-min refresh, 30-min idle), role-based authorization, site-scoped permissions.
11	Health Monitoring	Component-HealthMonitoring.md	30s report interval, 60s offline threshold, monotonic sequence numbers, raw error counts, tag resolution counts, dead letter monitoring.
12	Site Event Logging	Component-SiteEventLogging.md	SQLite storage, 30-day retention + 1GB cap, daily purge, paginated remote queries with keyword search.
13	Cluster Infrastructure	Component-ClusterInfrastructure.md	Akka.NET cluster, keep-oldest SBR with down-if-alone, min-nr-of-members=1, 2s/10s/15s failure detection, CoordinatedShutdown, automatic dual-node recovery.
14	Inbound API	Component-InboundAPI.md	POST /api/{methodName}, X-API-Key header, flat JSON, extended type system (Object/List), script-based implementations, failures-only logging.
15	Host	Component-Host.md	Single deployable binary, role-based component registration, per-component config binding (Options pattern), readiness gating, dead letter monitoring, Akka.NET bootstrap, ASP.NET Core hosting for central.
16	Commons	Component-Commons.md	Namespace/folder convention (Types/Interfaces/Entities/Messages), shared data types, POCOs, repository interfaces, message contracts with additive-only versioning, UTC timestamp convention.
17	Configuration Database	Component-ConfigurationDatabase.md	EF Core data access, per-component repositories, unit-of-work, optimistic concurrency on deployment status, audit logging (IAuditService), migration management.
18	Management Service	Component-ManagementService.md	Akka.NET ManagementActor on central, ClusterClientReceptionist registration, programmatic access to all admin operations, CLI interface.
19	CLI	Component-CLI.md	Standalone command-line tool, System.CommandLine, Akka.NET ClusterClient transport, LDAP auth, JSON/table output, mirrors all Management Service operations.

Reference Documentation

• AkkaDotNet/ — Akka.NET reference notes covering actors, remoting, clustering, persistence, streams, serialization, hosting, testing, and best practices.
• docs/plans/ — Design decision documents from refinement sessions.

Architecture Diagram (Logical)

                    Users (Blazor Server)
                         │
                    Load Balancer
                         │
┌────────────────────────┼────────────────────────────┐
│                   CENTRAL CLUSTER                    │
│              (2-node active/standby)                 │
│                                                      │
│  ┌──────────┐ ┌──────────┐ ┌──────────┐            │
│  │ Template  │ │Deployment│ │ Central  │            │
│  │ Engine    │ │ Manager  │ │ UI       │ Blazor Svr │
│  └──────────┘ └──────────┘ └──────────┘            │
│  ┌──────────┐ ┌──────────┐ ┌──────────┐            │
│  │ Security  │ │  Config  │ │  Health  │            │
│  │ & Auth    │ │   DB     │ │ Monitor  │            │
│  │ (JWT/LDAP)│ │ (EF+IAud)│ │          │            │
│  └──────────┘ └──────────┘ └──────────┘            │
│  ┌──────────┐                                       │
│  │ Inbound  │  ◄── External Systems (X-API-Key)     │
│  │ API      │      POST /api/{method}, JSON         │
│  └──────────┘                                       │
│  ┌──────────┐                                       │
│  │ Mgmt     │  ◄── CLI (ClusterClient)              │
│  │ Service  │      ManagementActor + Receptionist   │
│  └──────────┘                                       │
│  ┌───────────────────────────────────┐              │
│  │    Akka.NET Communication Layer   │              │
│  │  (correlation IDs, per-pattern    │              │
│  │   timeouts, message ordering)     │              │
│  └──────────────┬────────────────────┘              │
│  ┌──────────────┴────────────────────┐              │
│  │    Configuration Database (EF)    │──► MS SQL    │
│  └───────────────────────────────────┘   (Config DB)│
│                  │                    Machine Data DB│
└─────────────────┼───────────────────────────────────┘
                  │ Akka.NET Remoting
     ┌────────────┼────────────┐
     ▼            ▼            ▼
┌─────────┐ ┌─────────┐ ┌─────────┐
│ SITE A  │ │ SITE B  │ │ SITE N  │
│ (2-node)│ │ (2-node)│ │ (2-node)│
│ ┌─────┐ │ │ ┌─────┐ │ │ ┌─────┐ │
│ │Data │ │ │ │Data │ │ │ │Data │ │
│ │Conn │ │ │ │Conn │ │ │ │Conn │ │
│ │Layer │ │ │ │Layer │ │ │ │Layer │ │
│ ├─────┤ │ │ ├─────┤ │ │ ├─────┤ │
│ │Site │ │ │ │Site │ │ │ │Site │ │
│ │Runtm│ │ │ │Runtm│ │ │ │Runtm│ │
│ ├─────┤ │ │ ├─────┤ │ │ ├─────┤ │
│ │S&F  │ │ │ │S&F  │ │ │ │S&F  │ │
│ │Engine│ │ │ │Engine│ │ │ │Engine│ │
│ ├─────┤ │ │ ├─────┤ │ │ ├─────┤ │
│ │ExtSys│ │ │ │ExtSys│ │ │ │ExtSys│ │
│ │Gatwy │ │ │ │Gatwy │ │ │ │Gatwy │ │
│ └─────┘ │ │ └─────┘ │ │ └─────┘ │
│ SQLite  │ │ SQLite  │ │ SQLite  │
└─────────┘ └─────────┘ └─────────┘
     │            │            │
  OPC UA /     OPC UA /     OPC UA /
  Custom       Custom       Custom
  Protocol     Protocol     Protocol

Site Runtime Actor Hierarchy

Deployment Manager Singleton (Cluster Singleton)
├── Instance Actor (one per deployed, enabled instance)
│   ├── Script Actor (coordinator, one per instance script)
│   │   └── Script Execution Actor (short-lived, per invocation)
│   ├── Alarm Actor (coordinator, one per alarm definition)
│   │   └── Alarm Execution Actor (short-lived, per on-trigger invocation)
│   └── ... (more Script/Alarm Actors)
├── Instance Actor
│   └── ...
└── ... (more Instance Actors)

Site-Wide Akka Stream (attribute + alarm state changes)
├── All Instance Actors publish to the stream
└── Debug view subscribes with instance-level filtering