# Redundancy

## Overview

OtOpcUa supports OPC UA **non-transparent** warm/hot redundancy. Two (or more) OtOpcUa Server processes run side-by-side, share the same Config DB, the same driver backends (Galaxy ZB, MXAccess runtime, remote PLCs), and advertise the same OPC UA node tree. Each process owns a distinct `ApplicationUri`; OPC UA clients see both endpoints via the standard `ServerUriArray` and pick one based on the `ServiceLevel` that each server publishes.

The redundancy surface lives in `src/ZB.MOM.WW.OtOpcUa.Server/Redundancy/`:

| Class | Role |
|---|---|
| `RedundancyCoordinator` | Process-singleton; owns the current `RedundancyTopology` loaded from the `ClusterNode` table. `RefreshAsync` re-reads after `sp_PublishGeneration` so operator role swaps take effect without a process restart. CAS-style swap (`Interlocked.Exchange`) means readers always see a coherent snapshot. |
| `RedundancyTopology` | Immutable `(ClusterId, Self, Peers, ServerUriArray, ValidityFlags)` snapshot. |
| `ApplyLeaseRegistry` | Tracks in-progress `sp_PublishGeneration` apply leases keyed on `(ConfigGenerationId, PublishRequestId)`. `await using` the disposable scope guarantees every exit path (success / exception / cancellation) decrements the lease; a stale-lease watchdog force-closes any lease older than `ApplyMaxDuration` (default 10 minutes) so a crashed publisher can't pin the node at `PrimaryMidApply`. |
| `PeerReachabilityTracker` | Maintains last-known reachability for each peer node over two independent probes — OPC UA ping and HTTP `/healthz`. Both must succeed for `peerReachable = true`. |
| `RecoveryStateManager` | Gates transitions out of the `Recovering*` bands; requires dwell + publish-witness satisfaction before allowing a return to nominal. |
| `ServiceLevelCalculator` | Pure function `(role, selfHealthy, peerUa, peerHttp, applyInProgress, recoveryDwellMet, topologyValid, operatorMaintenance) → byte`. |
| `RedundancyStatePublisher` | Orchestrates inputs into the calculator, pushes the resulting byte to the OPC UA `ServiceLevel` variable via an edge-triggered `OnStateChanged` event, and fires `OnServerUriArrayChanged` when the topology's `ServerUriArray` shifts. |

## Data model

Per-node redundancy state lives in the Config DB `ClusterNode` table (`src/ZB.MOM.WW.OtOpcUa.Configuration/Entities/ClusterNode.cs`):

| Column | Role |
|---|---|
| `NodeId` | Unique node identity; matches `Node:NodeId` in the server's bootstrap `appsettings.json`. |
| `ClusterId` | Foreign key into `ServerCluster`. |
| `RedundancyRole` | `Primary`, `Secondary`, or `Standalone` (`RedundancyRole` enum in `Configuration/Enums`). |
| `ServiceLevelBase` | Per-node base value used to bias nominal ServiceLevel output. |
| `ApplicationUri` | Unique-per-node OPC UA ApplicationUri advertised in endpoint descriptions. |

`ServerUriArray` is derived from the set of peer `ApplicationUri` values at topology-load time and republished when the topology changes.

## ServiceLevel matrix

`ServiceLevelCalculator` produces one of the following bands (see `ServiceLevelBand` enum in the same file):

| Band | Byte | Meaning |
|---|---|---|
| `Maintenance` | 0 | Operator-declared maintenance. |
| `NoData` | 1 | Self-reported unhealthy (`/healthz` fails). |
| `InvalidTopology` | 2 | More than one Primary detected; both nodes self-demote. |
| `RecoveringBackup` | 30 | Backup post-fault, dwell not met. |
| `BackupMidApply` | 50 | Backup inside a publish-apply window. |
| `IsolatedBackup` | 80 | Primary unreachable; Backup says "take over if asked" — does **not** auto-promote (non-transparent model). |
| `AuthoritativeBackup` | 100 | Backup nominal. |
| `RecoveringPrimary` | 180 | Primary post-fault, dwell not met. |
| `PrimaryMidApply` | 200 | Primary inside a publish-apply window. |
| `IsolatedPrimary` | 230 | Primary with unreachable peer, retains authority. |
| `AuthoritativePrimary` | 255 | Primary nominal. |

The reserved bands (0 Maintenance, 1 NoData, 2 InvalidTopology) take precedence over operational states per OPC UA Part 5 §6.3.34. Operational values occupy 2..255 so spec-compliant clients that treat "<3 = unhealthy" keep working.

Standalone nodes (single-instance deployments) report `AuthoritativePrimary` when healthy and `PrimaryMidApply` during publish.

## Publish fencing and split-brain prevention

Any Admin-triggered `sp_PublishGeneration` acquires an apply lease through `ApplyLeaseRegistry.BeginApplyLease`. While the lease is held:

- The calculator reports `PrimaryMidApply` / `BackupMidApply` — clients see the band shift and cut over to the unaffected peer rather than racing against a half-applied generation.
- `RedundancyCoordinator.RefreshAsync` is called at the end of the apply window so the post-publish topology becomes visible exactly once, atomically.
- The watchdog force-closes any lease older than `ApplyMaxDuration`; a stuck publisher therefore cannot strand a node at `PrimaryMidApply`.

Because role transitions are **operator-driven** (write `RedundancyRole` in the Config DB + publish), the Backup never auto-promotes. An `IsolatedBackup` at 80 is the signal that the operator should intervene; auto-failover is intentionally out of scope for the non-transparent model (decision #154).

## Metrics

`RedundancyMetrics` in `src/ZB.MOM.WW.OtOpcUa.Admin/Services/RedundancyMetrics.cs` registers the `ZB.MOM.WW.OtOpcUa.Redundancy` meter on the Admin process. Instruments:

| Name | Kind | Tags | Description |
|---|---|---|---|
| `otopcua.redundancy.role_transition` | Counter<long> | `cluster.id`, `node.id`, `from_role`, `to_role` | Incremented every time `FleetStatusPoller` observes a `RedundancyRole` change on a `ClusterNode` row. |
| `otopcua.redundancy.primary_count` | ObservableGauge<long> | `cluster.id` | Primary-role nodes per cluster — should be exactly 1 in nominal state. |
| `otopcua.redundancy.secondary_count` | ObservableGauge<long> | `cluster.id` | Secondary-role nodes per cluster. |
| `otopcua.redundancy.stale_count` | ObservableGauge<long> | `cluster.id` | Nodes whose `LastSeenAt` exceeded the stale threshold. |

Admin `Program.cs` wires OpenTelemetry to the Prometheus exporter when `Metrics:Prometheus:Enabled=true` (default), exposing the meter under `/metrics`. The endpoint is intentionally unauthenticated — fleet conventions put it behind a reverse-proxy basic-auth gate if needed.

## Real-time notifications (Admin UI)

`FleetStatusPoller` in `src/ZB.MOM.WW.OtOpcUa.Admin/Hubs/` polls the `ClusterNode` table, records role transitions, updates `RedundancyMetrics.SetClusterCounts`, and pushes a `RoleChanged` SignalR event onto `FleetStatusHub` when a transition is observed. `RedundancyTab.razor` subscribes with `_hub.On<RoleChangedMessage>("RoleChanged", …)` so connected Admin sessions see role swaps the moment they happen.

## Configuring a redundant pair

Redundancy is configured **in the Config DB, not appsettings.json**. The fields that must differ between the two instances:

| Field | Location | Instance 1 | Instance 2 |
|---|---|---|---|
| `NodeId` | `appsettings.json` `Node:NodeId` (bootstrap) | `node-a` | `node-b` |
| `ClusterNode.ApplicationUri` | Config DB | `urn:node-a:OtOpcUa` | `urn:node-b:OtOpcUa` |
| `ClusterNode.RedundancyRole` | Config DB | `Primary` | `Secondary` |
| `ClusterNode.ServiceLevelBase` | Config DB | typically 255 | typically 100 |

Shared between instances: `ClusterId`, Config DB connection string, published generation, cluster-level ACLs, UNS hierarchy, driver instances.

Role swaps, stand-alone promotions, and base-level adjustments all happen through the Admin UI `RedundancyTab` — the operator edits the `ClusterNode` row in a draft generation and publishes. `RedundancyCoordinator.RefreshAsync` picks up the new topology without a process restart.

## Client-side failover

The OtOpcUa Client CLI at `src/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) for the command reference.

## Depth reference

For the full decision trail and implementation plan — topology invariants, peer-probe cadence, recovery-dwell policy, compliance-script guard against enum-value drift — see `docs/v2/plan.md` §Phase 6.3.
