# Component: Cluster Infrastructure

## Purpose

The Cluster Infrastructure component manages the Akka.NET cluster setup, active/standby node roles, failover detection, and the foundational runtime environment on which all other components run. It provides the base layer for both central and site clusters.

## Location

Both central and site clusters.

## Responsibilities

- Bootstrap the Akka.NET actor system on each node.
- Form a two-node cluster (active/standby) using Akka.NET Cluster.
- Manage leader election and role assignment (active vs. standby).
- Detect node failures and trigger failover.
- Provide the Akka.NET remoting infrastructure for inter-cluster communication.
- Support cluster singleton hosting (used by the Site Runtime Deployment Manager singleton on site clusters).
- Manage Windows service lifecycle (start, stop, restart) on each node.

## Implementation Note — Code Placement

This component is a **design responsibility**, not a single buildable project that
contains all of the code. The cluster-infrastructure responsibilities above are
realised across two projects:

- **`src/ScadaLink.ClusterInfrastructure`** owns the cluster **configuration model**:
  the `ClusterOptions` POCO (seed nodes, roles, remoting/gRPC ports, failure-detection
  timings, split-brain settings) bound from `appsettings.json` via the Options pattern.
- **`src/ScadaLink.Host`** owns the cluster **bootstrap and runtime wiring**: it
  builds the Akka.NET HOCON from `ClusterOptions`, starts the `ActorSystem`,
  configures the keep-oldest split-brain resolver (`down-if-alone = on`), wires
  `CoordinatedShutdown` into the service lifecycle, and provides active-node /
  cluster-membership health checks. See `Component-Host.md` (REQ-HOST-*) for detail.

This split is deliberate — the Host is the single deployable binary and the only
project that performs Akka.NET bootstrap, so the cluster bring-up lives there
alongside role-based component registration. The `ClusterInfrastructure` project
remains the home of the configuration contract that the Host consumes.

## Cluster Topology

### Central Cluster
- Two nodes forming an Akka.NET cluster.
- One active node runs all central components (Template Engine, Deployment Manager, Central UI, etc.).
- One standby node is ready to take over on failover.
- Connected to MS SQL databases (Config DB, Machine Data DB).

### Site Cluster (per site)
- Two nodes forming an Akka.NET cluster.
- One active node runs all site components (Site Runtime, Data Connection Layer, Store-and-Forward Engine, etc.).
- The Site Runtime Deployment Manager runs as an **Akka.NET cluster singleton** on the active node, owning the full Instance Actor hierarchy.
- One standby node receives replicated store-and-forward data and is ready to take over.
- Connected to local SQLite databases (store-and-forward buffer, event logs, deployed configurations).
- Connected to machines via data connections (OPC UA).

## Failover Behavior

### Detection
- Akka.NET Cluster monitors node health via heartbeat.
- If the active node becomes unreachable, the standby node detects the failure and promotes itself to active.

### Central Failover
- The new active node takes over all central responsibilities.
- In-progress deployments are treated as **failed** — engineers must retry.
- The UI session may be interrupted — users reconnect to the new active node.
- No message buffering at central — no state to recover beyond what's in MS SQL.

### Site Failover
- The new active node takes over:
  - The Deployment Manager singleton restarts and re-creates the full Instance Actor hierarchy by reading deployed configurations from local SQLite. Each Instance Actor spawns its child Script and Alarm Actors.
  - Data collection (Data Connection Layer re-establishes subscriptions as Instance Actors register their data source references).
  - Store-and-forward delivery (buffer is already replicated locally).
- Active debug view streams from central are interrupted — the engineer must re-open them.
- Health reporting resumes from the new active node.
- Alarm states are re-evaluated from incoming values (alarm state is in-memory only).

## Split-Brain Resolution

The system uses the Akka.NET **keep-oldest** split-brain resolver strategy:

- On a network partition, the node that has been in the cluster longest remains active. The younger node downs itself.
- **Stable-after duration**: 15 seconds. The cluster membership must remain stable (no changes) for 15 seconds before the resolver acts to down unreachable nodes. This prevents premature downing during startup or rolling restarts.
- **`down-if-alone = on`**: The keep-oldest resolver is configured with `down-if-alone` enabled. If the oldest node finds itself alone (no other reachable members), it downs itself rather than continuing as a single-node cluster. This prevents the oldest node from running in isolation during a network partition while the younger node also forms its own cluster.
- **Why keep-oldest**: With only two nodes, quorum-based strategies (static-quorum, keep-majority) cannot distinguish "one node crashed" from "network partition" — both sides see fewer than quorum and both would down themselves, resulting in total cluster shutdown. Keep-oldest with `down-if-alone` provides safe singleton ownership — at most one node runs the cluster singleton at any time.

## Single-Node Operation

`akka.cluster.min-nr-of-members` must be set to **1**. After failover, only one node is running. If set to 2, the surviving node waits for a second member before allowing the Cluster Singleton (Site Runtime Deployment Manager) to start — blocking all data collection and script execution indefinitely.

## Failure Detection Timing

Configurable defaults for heartbeat and failure detection:

| Setting | Default | Description |
|---------|---------|-------------|
| Heartbeat interval | 2 seconds | Frequency of health check messages between nodes |
| Failure detection threshold | 10 seconds | Time without heartbeat before a node is considered unreachable |
| Stable-after (split-brain) | 15 seconds | Time cluster must be stable before resolver acts |
| **Total failover time** | **~25 seconds** | Detection (10s) + stable-after (15s) + singleton restart |

These values balance failover speed with stability — fast enough that data collection gaps are small, tolerant enough that brief network hiccups don't trigger unnecessary failovers.

## Dual-Node Recovery

If both nodes in a cluster fail simultaneously (e.g., site power outage):

1. **No manual intervention required.** Since both nodes are configured as seed nodes, whichever node starts first forms a new cluster. The second node joins when it starts.
2. **State recovery** (each node has its own local copy of all required data):
   - **Site clusters**: The Deployment Manager singleton reads deployed configurations from local SQLite and re-creates the full Instance Actor hierarchy. Store-and-forward buffers are already persisted locally. Alarm states re-evaluate from incoming data values.
   - **Central cluster**: All state is in MS SQL (configuration database). The active node resumes normal operation.
3. The keep-oldest resolver handles the "both starting fresh" case naturally — there is no pre-existing cluster to conflict with.

## Graceful Shutdown & Singleton Handover

When a node is stopped for planned maintenance (Windows Service stop), `CoordinatedShutdown` triggers a **graceful leave** from the cluster. This enables the Cluster Singleton (Site Runtime Deployment Manager) to hand over to the other node in seconds (limited by the hand-over retry interval) rather than waiting for the full failure detection timeout (~25 seconds).

Configuration required:
- `akka.coordinated-shutdown.run-by-clr-shutdown-hook = on`
- `akka.cluster.run-coordinated-shutdown-when-down = on`

The Host component wires CoordinatedShutdown into the Windows Service lifecycle (see REQ-HOST-9).

## Node Configuration

Each node is configured with:
- **Cluster seed nodes**: **Both nodes** are seed nodes — each node lists both itself and its partner. Either node can start first and form the cluster; the other joins when it starts. No startup ordering dependency.
- **Cluster role**: Central or Site (plus site identifier for site clusters).
- **Akka.NET remoting**: Hostname/port for inter-node and inter-cluster communication (default 8081 central, 8082 site).
- **gRPC port** (site nodes only): Dedicated HTTP/2 port for the SiteStreamGrpcServer (default 8083). Separate from the Akka remoting port — gRPC uses Kestrel, Akka uses its own TCP transport.
- **Local storage paths**: SQLite database locations (site nodes only).

## Windows Service

- Each node runs as a **Windows service** for automatic startup and recovery.
- Service configuration includes Akka.NET cluster settings and component-specific configuration.

## Platform

- **OS**: Windows Server.
- **Runtime**: .NET (Akka.NET).
- **Cluster**: Akka.NET Cluster (application-level, not Windows Server Failover Clustering).

## Dependencies

- **Akka.NET**: Core actor system, cluster, remoting, and cluster singleton libraries.
- **Windows**: Service hosting, networking.
- **MS SQL** (central only): Database connectivity.
- **SQLite** (sites only): Local storage.

## Interactions

- **All components**: Every component runs within the Akka.NET actor system managed by this infrastructure.
- **Site Runtime**: The Deployment Manager singleton relies on Akka.NET cluster singleton support provided by this infrastructure.
- **Communication Layer**: Built on top of the Akka.NET remoting provided here.
- **Health Monitoring**: Reports node status (active/standby) as a health metric.
