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docs(v2): update for gap-closeout — peer-URI discovery, role overlays, release status

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Joseph Doherty
2026-05-26 11:58:06 -04:00
parent bdae749b2b
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docs/Redundancy.md
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## 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 see both endpoints via the standard `ServerUriArray` and pick one based on the `ServiceLevel` byte that each server publishes.
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.
@@ -78,6 +80,20 @@ Both nodes share the same `ConfigDb` connection string; `Cluster.PublicHostname`
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`.