# Incremental Sync

Two distinct change-detection paths feed the running server: driver-backend rediscovery (Galaxy's `time_of_last_deploy`, TwinCAT's symbol-version-changed, OPC UA Client's upstream namespace change) and generation-level config publishes from the Admin UI. Both flow into re-runs of `ITagDiscovery.DiscoverAsync`, but they originate differently.

## Driver-backend rediscovery — IRediscoverable

Drivers whose backend has a native change signal implement `IRediscoverable` (`src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IRediscoverable.cs`):

```csharp
public interface IRediscoverable
{
    event EventHandler<RediscoveryEventArgs>? OnRediscoveryNeeded;
}
public sealed record RediscoveryEventArgs(string Reason, string? ScopeHint);
```

The driver fires the event with a reason string (for the diagnostic log) and an optional scope hint — a non-null hint lets Core scope the rebuild surgically to that subtree; null means "the whole address space may have changed".

Drivers that implement the capability today:

- **Galaxy** — polls `galaxy.time_of_last_deploy` in the Galaxy repository DB and fires on change. This is Galaxy-internal change detection, not the platform-wide mechanism.
- **TwinCAT** — observes ADS symbol-version-changed notifications (`0x0702`).
- **OPC UA Client** — subscribes to the upstream server's `Server/NamespaceArray` change notifications.

Static drivers (Modbus, S7, AB CIP, AB Legacy, FOCAS) do not implement `IRediscoverable` — their tags only change when a new generation is published from the Config DB. Core sees absence of the interface and skips change-detection wiring for those drivers (decision #54).

## Config-DB generation publishes

Tag-set changes authored in the Admin UI (UNS edits, CSV imports, driver-config edits) accumulate in a draft generation and commit via `sp_PublishGeneration`. The delta between the currently-published generation and the proposed next one is computed by `sp_ComputeGenerationDiff`, which drives:

- The **DiffViewer** in Admin (`src/ZB.MOM.WW.OtOpcUa.Admin/Components/Pages/Clusters/DiffViewer.razor`) so operators can preview what will change before clicking Publish.
- The 409-on-stale-draft flow (decision #161) — a UNS drag-reorder preview carries a `DraftRevisionToken` so Confirm returns `409 Conflict / refresh-required` if the draft advanced between preview and commit.

After publish, the server's generation applier invokes `IDriver.ReinitializeAsync(driverConfigJson, ct)` on every driver whose `DriverInstance.DriverConfig` row changed in the new generation. Reinitialize is the in-process recovery path for Tier A/B drivers; if it fails the driver is marked `DriverState.Faulted` and its nodes go Bad quality — but the server process stays running. See `docs/v2/driver-stability.md`.

Drivers whose discovery depends on Config DB state (Modbus register maps, S7 DBs, AB CIP tag lists) re-run their discovery inside `ReinitializeAsync`; Core then diffs the new node set against the current address space.

## Rebuild flow

When a rediscovery is triggered (by either source), `GenericDriverNodeManager` re-runs `ITagDiscovery.DiscoverAsync` into the same `CapturingBuilder` it used at first build. The new node set is diffed against the current:

1. **Diff** — full-name comparison of the new `DriverAttributeInfo` set against the existing `_variablesByFullRef` map. Added / removed / modified references are partitioned.
2. **Snapshot subscriptions** — before teardown, Core captures the current monitored-item ref-counts for every affected reference so subscriptions can be replayed after rebuild.
3. **Teardown** — removed / modified variable nodes are deleted via `CustomNodeManager2.DeleteNode`. Driver-side subscriptions for those references are unwound via `ISubscribable.UnsubscribeAsync`.
4. **Rebuild** — added / modified references get fresh `BaseDataVariableState` nodes via the standard `IAddressSpaceBuilder.Variable(...)` path. Alarm-flagged references re-register their `IAlarmConditionSink` through `CapturingBuilder`.
5. **Restore subscriptions** — for every captured reference that still exists after rebuild, Core re-opens the driver subscription and restores the original ref-count.

Exceptions during teardown are swallowed per decision #12 — a driver throw must not leave the node tree half-deleted.

## Scope hint

When `RediscoveryEventArgs.ScopeHint` is non-null (e.g. a folder path), Core restricts the diff to that subtree. This matters for Galaxy Platform-scoped deployments where a `time_of_last_deploy` advance may only affect one platform's subtree, and for OPC UA Client where an upstream change may be localized. Null scope falls back to a full-tree diff.

## Active subscriptions survive rebuild

Subscriptions for unchanged references stay live across rebuilds — their ref-count map is not disturbed. Clients monitoring a stable tag never see a data-change gap during a deploy, only clients monitoring a tag that was genuinely removed see the subscription drop.

## Key source files

- `src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IRediscoverable.cs` — backend-change capability
- `src/ZB.MOM.WW.OtOpcUa.Core/OpcUa/GenericDriverNodeManager.cs` — discovery orchestration
- `src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IDriver.cs` — `ReinitializeAsync` contract
- `src/ZB.MOM.WW.OtOpcUa.Admin/Services/GenerationService.cs` — publish-flow driver
- `docs/v2/config-db-schema.md` — `sp_PublishGeneration` + `sp_ComputeGenerationDiff`
- `docs/v2/admin-ui.md` — DiffViewer + draft-revision-token flow