# Subscriptions

Driver-side data-change subscriptions live behind `ISubscribable` (`src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/ISubscribable.cs`). The interface is deliberately mechanism-agnostic: it covers native subscriptions (Galaxy MXAccess advisory, OPC UA monitored items on an upstream server, TwinCAT ADS notifications) and driver-internal polled subscriptions (Modbus, AB CIP, S7, FOCAS). Core sees the same event shape regardless — drivers fire `OnDataChange` and Core dispatches to the matching OPC UA monitored items.

## Driver vs virtual dispatch

Per [ADR-002](v2/implementation/adr-002-driver-vs-virtual-dispatch.md), `DriverNodeManager` routes subscriptions across both driver tags and virtual (scripted) tags through the same `ISubscribable` contract. The per-variable `NodeSourceKind` (registered from `DriverAttributeInfo` at discovery) selects the backend:

- `NodeSourceKind.Driver` — subscribes via the driver's `ISubscribable`, wrapped by `CapabilityInvoker` (the rest of this doc).
- `NodeSourceKind.Virtual` — subscribes via `VirtualTagSource` (`src/ZB.MOM.WW.OtOpcUa.Core.VirtualTags/VirtualTagSource.cs`), which forwards change events emitted by `VirtualTagEngine` as `OnDataChange`. The ref-counting, initial-value, and transfer-restoration behaviour below applies identically.

Because both kinds expose `ISubscribable`, Core's dispatch, ref-count map, and monitored-item fan-out are unchanged across the source branch.

## ISubscribable surface

```csharp
Task<ISubscriptionHandle> SubscribeAsync(
    IReadOnlyList<string> fullReferences,
    TimeSpan publishingInterval,
    CancellationToken cancellationToken);

Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken);

event EventHandler<DataChangeEventArgs>? OnDataChange;
```

A single `SubscribeAsync` call may batch many attributes and returns an opaque handle the caller passes back to `UnsubscribeAsync`. The driver may emit an immediate `OnDataChange` for each subscribed reference (the OPC UA initial-data convention) and then a push per change.

Every subscribe / unsubscribe call goes through `CapabilityInvoker.ExecuteAsync(DriverCapability.Subscribe, host, …)` so the per-host pipeline applies.

## Reference counting at Core

Multiple OPC UA clients can monitor the same variable simultaneously. Rather than open duplicate driver subscriptions, Core maintains a ref-count per `(driver, fullReference)` pair: the first OPC UA monitored-item for a reference triggers `ISubscribable.SubscribeAsync` with that single reference; each additional monitored-item just increments the count; decrement-to-zero triggers `UnsubscribeAsync`. Transferred subscriptions (client reconnect → resume session) replay against the same ref-count map so active driver subscriptions are preserved across session migration.

## Threading

The STA thread story is now driver-specific, not a server-wide concern:

- **Galaxy** runs its MXAccess COM objects on a dedicated STA thread with a Win32 message pump (`src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Sta/StaPump.cs`) inside the standalone `Driver.Galaxy.Host` Windows service. The Proxy driver (`Driver.Galaxy.Proxy`) connects to the Host via named pipe and re-exposes the data on a free-threaded surface to Core. Core never touches COM.
- **Modbus / S7 / AB CIP / AB Legacy / TwinCAT / FOCAS** are free-threaded — they run their polling loops on ordinary `Task`s. Their `OnDataChange` fires on thread-pool threads.
- **OPC UA Client** delegates to the OPC Foundation stack's subscription loop.

The common contract: drivers are responsible for marshalling from whatever native thread the backend uses onto thread-pool threads before raising `OnDataChange`. Core's dispatch path acquires the OPC UA framework `Lock` and calls `ClearChangeMasks` on the corresponding `BaseDataVariableState` to notify subscribed clients.

## Dispatch

Core's subscription dispatch path:

1. `ISubscribable.OnDataChange` fires on a thread-pool thread with a `DataChangeEventArgs(subscriptionHandle, fullReference, DataValueSnapshot)`.
2. Core looks up the variable by `fullReference` in the driver's `DriverNodeManager` variable map.
3. Under the OPC UA framework `Lock`, the variable's `Value` / `StatusCode` / `Timestamp` are updated and `ClearChangeMasks(SystemContext, false)` is called.
4. The OPC Foundation stack then enqueues data-change notifications for every monitored-item attached to that variable, honoring each subscription's sampling + filter configuration.

Batch coalescing — coalescing multiple pushes for the same reference between publish cycles — is done driver-side when the backend natively supports it (Galaxy keeps the v1 coalescing dictionary); otherwise the SDK's own data-change filter suppresses no-change notifications.

## Initial values

A freshly-built variable carries `StatusCode = BadWaitingForInitialData` until the driver delivers the first value. Drivers whose backends supply an initial read (Galaxy `AdviseSupervisory`, TwinCAT `AddDeviceNotification`) fire `OnDataChange` immediately after `SubscribeAsync` returns. Polled drivers fire the first push when their first poll cycle completes.

## Transferred subscription restoration

When an OPC UA session is resumed (client reconnect with `TransferSubscriptions`), Core walks the transferred monitored-items and ensures every referenced `(driver, fullReference)` has a live driver subscription. References already active (in-process migration) skip re-subscribing; references that lost their driver-side handle during the session gap are re-subscribed via `SubscribeAsync`.

## Key source files

- `src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/ISubscribable.cs` — capability contract
- `src/ZB.MOM.WW.OtOpcUa.Core/Resilience/CapabilityInvoker.cs` — pipeline wrapping
- `src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Sta/StaPump.cs` — Galaxy STA thread + message pump
- Per-driver subscribe implementations in each `Driver.*` project
