lmxopcua/docs/Redundancy.md

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 ApplicationUris — 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:

{
  "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:

{
  "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.

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.