# ScadaBridge Cluster Topology Guide

## Architecture Overview

ScadaBridge uses a hub-and-spoke architecture:
- **Central Cluster**: Two-node active/standby Akka.NET cluster for management, UI, and coordination.
- **Site Clusters**: Two-node active/standby Akka.NET clusters at each remote site for data collection and local processing.

```mermaid
%%{init: {'theme':'base', 'themeVariables': {'textColor':'#111111','lineColor':'#555555','edgeLabelBackground':'#ffffff','fontSize':'15px'}}}%%
flowchart TD
    USERS["Users<br/>(HTTPS / LB)"]

    subgraph CENTRAL["Central Cluster"]
        NA["Node A<br/>Active"]
        NB["Node B<br/>Standby"]
        NA <--> NB
    end

    USERS --> NA
    CENTRAL --> SITE01
    CENTRAL --> SITE02
    CENTRAL --> SITE03
    CENTRAL --> SITEN

    subgraph SITE01["Site 01"]
        S01A["A<br/>Active"]
        S01B["B<br/>Standby"]
    end
    subgraph SITE02["Site 02"]
        S02A["A<br/>Active"]
        S02B["B<br/>Standby"]
    end
    subgraph SITE03["Site 03"]
        S03A["A<br/>Active"]
        S03B["B<br/>Standby"]
    end
    subgraph SITEN["Site N"]
        SNA["A<br/>Active"]
        SNB["B<br/>Standby"]
    end

    classDef start fill:#d5e8d4,stroke:#82b366,color:#111111;
    classDef proc fill:#dae8fc,stroke:#6c8ebf,color:#111111;
    classDef dec fill:#fff2cc,stroke:#d6b656,color:#111111;
    classDef warn fill:#ffe6cc,stroke:#d79b00,color:#111111;
    classDef muted fill:#f5f5f5,stroke:#999999,color:#666666;
    class USERS dec
    class CENTRAL proc
    class NA,S01A,S02A,S03A,SNA start
    class NB,S01B,S02B,S03B,SNB muted
    class SITE01,SITE02,SITE03,SITEN warn
```

## Central Cluster Setup

### Cluster Configuration

Both central nodes must be configured as seed nodes for each other:

**Node A** (`central-01.example.com`):
```json
{
  "ScadaBridge": {
    "Node": {
      "Role": "Central",
      "NodeHostname": "central-01.example.com",
      "RemotingPort": 8081
    },
    "Cluster": {
      "SeedNodes": [
        "akka.tcp://scadabridge@central-01.example.com:8081",
        "akka.tcp://scadabridge@central-02.example.com:8081"
      ]
    }
  }
}
```

**Node B** (`central-02.example.com`):
```json
{
  "ScadaBridge": {
    "Node": {
      "Role": "Central",
      "NodeHostname": "central-02.example.com",
      "RemotingPort": 8081
    },
    "Cluster": {
      "SeedNodes": [
        "akka.tcp://scadabridge@central-02.example.com:8081",
        "akka.tcp://scadabridge@central-01.example.com:8081"
      ]
    }
  }
}
```

> **Seed order is load-bearing — each node lists ITSELF first** (decision 2026-07-22). Note Node B's list is the reverse of Node A's. Akka only lets `seed-nodes[0]` form a *new* cluster, so a node listing its partner first can never boot while that partner is down. `StartupValidator` rejects the boot if the ordering is wrong, comparing host **and** port; use the same spelling of the hostname in `NodeHostname` and in the seed URI, since Akka does no DNS canonicalisation (`central-02` and `central-02.example.com` are different seed identities). See `docs/requirements/Component-ClusterInfrastructure.md` → Seed Node Ordering.

### Cluster Behavior

- **Split-brain resolver**: `auto-down` (`AutoDowning` provider, `auto-down-unreachable-after` = 15s) since the 2026-07-21 availability-over-partition-safety decision — the leader among the *reachable* members downs the unreachable peer, so a hard crash of **either** node fails over. Accepted trade: a real partition leaves both sides active until an operator restarts one. `keep-oldest` (with `down-if-alone = on`) remains a supported `SplitBrainResolverStrategy` value, but in a two-node cluster it cannot survive a crash of the oldest node. See `docs/plans/2026-07-21-auto-down-availability-decision.md`.
- **Minimum members**: `min-nr-of-members = 1` — a single node can form a cluster.
- **Failure detection**: 2-second heartbeat interval, 10-second threshold.
- **Total failover time**: ~25 seconds from node failure to singleton migration.
- **Singleton handover**: Uses CoordinatedShutdown for graceful migration.

### Shared State

Both central nodes share state through:
- **SQL Server**: All configuration, deployment records, templates, and audit logs.
- **JWT signing key**: Same `JwtSigningKey` in both nodes' configuration.
- **Data Protection keys**: Shared key ring (stored in SQL Server or shared file path).

### Load Balancer

A load balancer sits in front of both central nodes for the Blazor Server UI:
- Health check: `GET /health/ready`
- Protocol: HTTPS (TLS termination at LB or pass-through)
- Sticky sessions: Not required (JWT + shared Data Protection keys)
- If the active node fails, the LB routes to the standby (which becomes active after singleton migration).

## Site Cluster Setup

### Cluster Configuration

Each site has its own two-node cluster:

**Site Node A** (`site-01-a.example.com`):
```json
{
  "ScadaBridge": {
    "Node": {
      "Role": "Site",
      "NodeHostname": "site-01-a.example.com",
      "SiteId": "plant-north",
      "RemotingPort": 8081
    },
    "Cluster": {
      "SeedNodes": [
        "akka.tcp://scadabridge@site-01-a.example.com:8081",
        "akka.tcp://scadabridge@site-01-b.example.com:8081"
      ]
    }
  }
}
```

> **Site Node B reverses this list** — `site-01-b` first, `site-01-a` second — per the self-first seed rule above. It applies to site pairs exactly as it does to the central pair: without it, `site-01-b` cannot boot while `site-01-a` is down.

### Site Cluster Behavior

- Same split-brain resolver as central (keep-oldest).
- Singleton actors: Site Deployment Manager migrates on failover.
- Staggered instance startup: 50ms delay between Instance Actor creation to prevent reconnection storms.
- SQLite persistence: each node owns its own consolidated LocalDb database, kept in step by
  asynchronous CDC replication over a gRPC sync stream (LocalDb Phase 1 + 2). The nodes do NOT
  share a SQLite file.

### Site Pair Upgrades — stop and start BOTH nodes together

**A rolling upgrade of a site pair, one node at a time, is no longer supported.** It worked while
the bespoke replicator kept a legacy `SfBufferSnapshot` compatibility handler so a new standby
could still apply an old active node's monolithic snapshot. LocalDb Phase 2 deleted that handler
along with the replicator, so a mixed-version pair has no common replication path: the two nodes
will run, but they will not converge, and the divergence is silent.

Stop both nodes of a site pair, upgrade both, then start both.

**Related bound — do not leave one node of a pair offline for long.** A node absent for longer than
`LocalDb:Replication:TombstoneRetention` (default **7 days**) can **resurrect deleted rows** when
it rejoins: deletes replicate as HLC-ordered tombstones, and once a tombstone is pruned there is
nothing left to suppress the stale row the returning node still holds. Within the retention window
a rejoin is safe and self-correcting (verified live: a node stopped and restarted mid-load rejoined
with both nodes byte-identical and zero duplicates). Beyond it, rebuild the returning node's
database from its peer rather than letting it rejoin.

### Central-Site Communication

Three transports cross the boundary, not one:

- **Akka ClusterClient** — command/control. Sites list every central node in
  `ScadaBridge:Communication:CentralContactPoints`; contact rotation reaches whichever node
  answers, so no "active central" needs to be identified. (There is no `Communication:CentralSeedNode`
  setting — earlier revisions of this guide named one that never existed in the code.)
- **gRPC** — real-time data and audit pull. Note the direction is inverted from the data flow:
  each **site node hosts the gRPC server** on `GrpcPort` (default 8083, h2c) and central dials in.
- **Plain HTTP** — the deploy config itself, fetched by the site with a per-deployment token.

#### gRPC control-plane preshared key (required)

Every site node must set `ScadaBridge:Communication:GrpcPsk`, and central must hold the same
value for that site. **`StartupValidator` refuses to boot a site node without it**, deliberately:
the gate is fail-closed, so an unset key would leave the node joined, healthy-looking and
answering heartbeats while refusing every gRPC call — no live subscriptions, no audit pull, no
cached-telemetry ingest.

| Side | Where the key lives |
|---|---|
| Site node (both nodes of the pair, identical) | `ScadaBridge:Communication:GrpcPsk`, in production `${secret:SB-GRPC-PSK-<siteId>}` |
| Central | secret `SB-GRPC-PSK-<siteId>` in its store — **or** `ScadaBridge:Communication:SitePsks:<siteId>` |

The store is the source that matters in production, because sites are added at runtime and their
keys cannot be enumerated in configuration at boot; `SitePsks` covers a host running without a
master key (the docker rig) and one-off pins.

One key **per site**, never one for the fleet: a compromised site must not yield another site's
key. And never share it with `LocalDb:Replication:ApiKey` — that authenticates the *pair partner*
for database replication, a different trust relationship on the same listener.

**Rotation:** set the new value on both sides, then restart the pair (pairs restart together
anyway — see above). **Upgrading to a build that has this gate requires seeding the key first**,
including in the on-host `deploy/` overlays.

The key is a bearer token over plaintext h2c, so it is readable and replayable by anyone on the
path. That is the accepted posture today — the same trusted-network assumption the boundary
already made, now with authentication rather than none. TLS on these listeners is follow-on
hardening and needs no change to the key design.

## Scaling Guidelines

### Target Scale

- 10 sites maximum per central cluster
- 500 machines (instances) total across all sites
- 75 tags per machine (37,500 total tag subscriptions)

### Resource Requirements

| Component | CPU | RAM | Disk | Notes |
|-----------|-----|-----|------|-------|
| Central node | 4 cores | 8 GB | 50 GB | SQL Server is separate |
| Site node | 2 cores | 4 GB | 20 GB | SQLite databases grow with S&F |
| SQL Server | 4 cores | 16 GB | 100 GB | Shared across central cluster |

### Network Bandwidth

- Health reports: ~1 KB per site per 30 seconds = negligible
- Tag value updates: Depends on data change rate; OPC UA subscription-based
- Deployment artifacts: One-time burst per deployment (varies by config size)
- Debug view streaming: ~500 bytes per attribute change per subscriber

## Dual-Node Failure Recovery

### Scenario: Both Nodes Down

1. **First node starts**: Forms a single-node cluster (`min-nr-of-members = 1`).
2. **Central**: Reconnects to SQL Server, reads deployment state, becomes operational.
3. **Site**: Opens SQLite databases, rebuilds Instance Actors from persisted configs, resumes S&F retries.
4. **Second node starts**: Joins the existing cluster as standby.

### Automatic Recovery

No manual intervention required for dual-node failure. The first node to start will:
- Form the cluster
- Take over all singletons
- Begin processing immediately
- Accept the second node when it joins