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 (specifically the driver role-leader) drives ServiceLevel automatically.

The runtime pieces live in:

Component	Project	Role
RedundancyStateActor	OtOpcUa.ControlPlane.Redundancy	Admin-role cluster singleton; subscribes to cluster topology events, debounces 250ms, broadcasts RedundancyStateChanged on the redundancy-state DPS topic.
OpcUaPublishActor	OtOpcUa.Runtime.OpcUa	Per-driver-node; subscribes to the redundancy-state topic, maps the local node's role to a ServiceLevel byte (see below), and forwards it to IServiceLevelPublisher.
IServiceLevelPublisher / SdkServiceLevelPublisher	OtOpcUa.Commons.OpcUa / OtOpcUa.OpcUaServer	Writes the byte into the SDK's Server.ServiceLevel Variable. Production binds DeferredServiceLevelPublisher, which swaps in the real SdkServiceLevelPublisher once the SDK is up (it needs IServerInternal, available only after StandardServer.Start); until then writes route through NullServiceLevelPublisher.
ServiceLevelCalculator	OtOpcUa.ControlPlane.Redundancy	Pure function (NodeHealthInputs) → byte — the fuller DB/probe-aware tiering (see truth table below). Covered by ServiceLevelCalculatorTests; not yet wired into the live driver publish path, which uses the coarse role mapping in OpcUaPublishActor.
DbHealthProbeActor	OtOpcUa.Runtime.Health	Per-node; runs SELECT 1 against ConfigDb every 5s. Read by health endpoint.
PeerOpcUaProbeActor	OtOpcUa.Runtime.Health	Per-node; pings peer opc.tcp://peer:4840 with a TCP connect (2s timeout) and publishes the result on the redundancy-state topic. A full secure-channel Hello handshake is a possible future upgrade; the TCP connect is the current real probe.
ClusterRoleInfo	OtOpcUa.Cluster	Live view of cluster membership + role-leader; exposes IClusterRoleInfo to the rest of the host.

ServiceLevel tiers

Live driver-side mapping (current)

OpcUaPublishActor.HandleRedundancyStateChanged maps the local node's role (from the RedundancyStateChanged snapshot) to a ServiceLevel byte and forwards it through IServiceLevelPublisher to the SDK's Server.ServiceLevel Variable:

Local role	Byte
Primary and driver role-leader	240
Primary (not role-leader)	200
Secondary	100
Detached (no driver role)	0

Roles come from RedundancyStateActor.BuildSnapshot: a node with the driver role is Primary when it holds the driver role-leader lease, otherwise Secondary; a node without the driver role is Detached.

Full health-aware tiering (ServiceLevelCalculator)

ServiceLevelCalculator.Compute(NodeHealthInputs) is the fuller, DB/probe-aware calculation. It is unit-tested but not yet on the live publish path — the driver-side mapping above is what actually drives the SDK today.

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 follower (240) + a +10 bonus when this node is the driver role-leader.

Either way, clients with the standard redundancy heuristic ("pick the highest ServiceLevel") prefer the driver role-leader and fall back to followers on its degradation.

Data flow

Cluster topology event ──┐
                          ▼
              RedundancyStateActor (admin singleton)
                          │  debounce 250ms
                          ▼
              DPS topic "redundancy-state"
                          │
                          ▼
                Driver nodes' OpcUaPublishActor
                          │  role → byte (240/200/100/0)
                          ▼
              IServiceLevelPublisher (SdkServiceLevelPublisher)
                          │
                          ▼
              OPC UA Server.ServiceLevel Variable

Today only cluster topology drives the published ServiceLevel. PeerOpcUaProbeActor and DbHealthProbeActor also run per-node — the peer probe publishes OpcUaProbeResult onto the redundancy-state topic and the DB probe backs the health endpoint — but their outputs are not yet consumed by RedundancyStateActor or folded into the published byte. They are the inputs the fuller ServiceLevelCalculator truth table is designed to use once that path goes live.

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 and no ClusterNode.RedundancyRole column (the V2 migration dropped it — primary/secondary is now derived from cluster role-leadership). NodeId is derived as host:port of the cluster PublicHostname (see ClusterRoleInfo.LocalNode for the formula).

The ClusterNode.ServiceLevelBase column still exists and is editable in the Admin UI (NodeEdit / Cluster Redundancy pages), but it no longer drives the runtime ServiceLevel — that value is computed from cluster role/health and published per the mapping above, independent of this stored preference.

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. The options bind from the OpcUa config section (see Program.cs — AddValidatedOptions<OpcUaApplicationHostOptions>(…, "OpcUa")). Set this per-node in appsettings.json:

{
  "OpcUa": {
    "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.

Observability

OpcUaPublishActor emits one metric on every ServiceLevel transition (it suppresses no-op repeats of the same byte):

Metric	Type	Notes
otopcua.redundancy.service_level_change	Counter ({change})	OPC UA Server.ServiceLevel transitions emitted by the redundancy state. Tagged with level = the new byte.

The meter is defined on OtOpcUaTelemetry (src/Core/ZB.MOM.WW.OtOpcUa.Commons/Observability/OtOpcUaTelemetry.cs); it surfaces through whatever OpenTelemetry exporter the host configures.

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.