# Redundancy (v2)

## Overview

OtOpcUa supports OPC UA **non-transparent** warm/hot redundancy. Two or more `OtOpcUa.Host` processes run side-by-side, share the same Config DB, and join the same Akka.NET cluster. Each process owns a distinct `ApplicationUri`; OPC UA clients discover both endpoints by reading `Server.ServerArray` (NodeId `i=2254`) on either node and pick one based on the `ServiceLevel` byte that each server publishes.

> **Discovery surface.** The `ServerArray` path on the `Server` object is what each node populates with self + peer `ApplicationUri`s — see `OpcUaApplicationHost.PopulateServerArray` and the per-node `PeerApplicationUris` option below. The redundancy-object-type `ServerUriArray` proper (a child of `Server.ServerRedundancy`) remains deferred pending an SDK object-type upgrade; clients should read `Server.ServerArray` for peer discovery today.

> **v2 change.** v1's operator-managed `ClusterNode.RedundancyRole` column + `RedundancyCoordinator` / `ApplyLeaseRegistry` / `PeerHttpProbeLoop` are gone. Primary/secondary is now derived from **Akka cluster role-leader** for the `driver` role. The operator no longer writes a role into the DB; cluster topology + health drive ServiceLevel automatically.

The runtime pieces live in:

| Component | Project | Role |
|---|---|---|
| `ServiceLevelCalculator` | `OtOpcUa.ControlPlane.Redundancy` | Pure function `(NodeHealthInputs) → byte`. No side effects. |
| `RedundancyStateActor` | `OtOpcUa.ControlPlane.Redundancy` | Admin-role cluster singleton; subscribes to cluster topology events, debounces 250ms, broadcasts `RedundancyStateChanged` on the `redundancy-state` DPS topic. |
| `DbHealthProbeActor` | `OtOpcUa.Runtime.Health` | Per-node; runs `SELECT 1` against ConfigDb every 5s. Read by health endpoint + redundancy calc. |
| `PeerOpcUaProbeActor` | `OtOpcUa.Runtime.Health` | Per-node; pings peer `opc.tcp://peer:4840` (real probe call is staged for follow-up F12). |
| `ClusterRoleInfo` | `OtOpcUa.Cluster` | Live view of cluster membership + role-leader; exposes `IClusterRoleInfo` to the rest of the host. |

## ServiceLevel tiers (Part 5 §6.5)

`ServiceLevelCalculator.Compute(NodeHealthInputs)` returns a byte in 0..255 by tier:

| Tier | Byte | Condition |
|---|---|---|
| Down | 0 | Member status is not `Up` or `Joining` (leaving, removed, exiting). |
| Critically degraded | 100 | ConfigDb unreachable AND data is stale. |
| Stale | 200 | Data stale but ConfigDb reachable. |
| Healthy follower | 240 | DB ok + OPC UA probe ok + not stale. |
| Healthy leader | 250 | Healthy + this node is the `driver` role-leader. |

Drivers write their computed byte into the OPC UA `ServiceLevel` Variable on each refresh. Clients with the standard redundancy heuristic ("pick the highest ServiceLevel") therefore prefer the role-leader and fall back to followers on its degradation.

## Data flow

```
Cluster topology event ──┐
DB health probe ─────────┤
OPC UA peer probe ───────┤
                          ▼
              RedundancyStateActor (admin singleton)
                          │  debounce 250ms
                          ▼
              DPS topic "redundancy-state"
                          │
                          ▼
                Driver nodes' OpcUaPublishActor
                          │
                          ▼
              ServiceLevelCalculator → byte
                          │
                          ▼
              OPC UA ServiceLevel Variable
```

The admin singleton is the cluster's only `RedundancyStateActor`. If the admin leader fails over, the new admin node spins up its replacement, re-subscribes to cluster events, and publishes a fresh snapshot from the current `Cluster.State`. There is no DB-persisted state to recover.

## Configuration

Per-node identity comes from `appsettings.json` + the `OTOPCUA_ROLES` env var:

```json
{
  "Cluster": {
    "Hostname": "0.0.0.0",
    "Port": 4053,
    "PublicHostname": "node-a.lan",
    "SeedNodes": ["akka.tcp://otopcua@node-a.lan:4053"],
    "Roles": ["admin", "driver"]
  }
}
```

```
OTOPCUA_ROLES=admin,driver
```

Both nodes share the same `ConfigDb` connection string; `Cluster.PublicHostname` + `Roles` are what makes them distinct in cluster gossip. The first node bootstraps the cluster (its address goes in `SeedNodes`); the second node joins via the same `SeedNodes` list.

There is no longer a `Node:NodeId` setting, no `ClusterNode.RedundancyRole`, no `ServiceLevelBase`. NodeId is derived as `host:port` of the cluster `PublicHostname` (see `ClusterRoleInfo.LocalNode` for the formula).

### Peer URI advertising

Each node advertises its partner via `OpcUaApplicationHostOptions.PeerApplicationUris` (an `IList<string>`, default empty). `OpcUaApplicationHost.PopulateServerArray` appends each configured peer URI to the SDK's `IServerInternal.ServerUris` string table after server startup, so that `Server.ServerArray` reads served by `OnReadServerArray` return both self + peers. Set this per-node in `appsettings.json`:

```json
{
  "OpcUaServer": {
    "PeerApplicationUris": ["urn:node-b:OtOpcUa"]
  }
}
```

Node A lists Node B's `ApplicationUri` and vice-versa. Validated by `DualEndpointTests` in `tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/` — boots two `OpcUaApplicationHost` instances on loopback, asserts a real OPCFoundation client `Session` reading `Server.ServerArray` from Node A sees both URIs.

## Split-brain

`akka.conf` configures Akka's split-brain resolver with `active-strategy = keep-oldest`, `stable-after = 15s`, and `failure-detector.threshold = 10.0`. Under a clean partition: the oldest member stays up + the smaller (or younger) side downs itself within ~15 seconds. The `RedundancyStateActor` on the surviving partition re-computes from the post-partition `Cluster.State`.

There is no operator-driven role swap during a partition. Failover is what the cluster does automatically.

## Client-side failover

The OtOpcUa Client CLI at `src/Client/ZB.MOM.WW.OtOpcUa.Client.CLI` supports `-F` / `--failover-urls` for automatic client-side failover; for long-running subscriptions the CLI monitors session KeepAlive and reconnects to the next available server, recreating the subscription on the new endpoint. See [`Client.CLI.md`](Client.CLI.md).

## Depth reference

For the full design — message contracts, tiered calculator truth table, recovery semantics — see `docs/plans/2026-05-26-akka-hosting-alignment-design.md` §6.