# Driver-expansion program — overall design

**Status:** program design, 2026-07-15. Umbrella for the next-driver effort: one universal
browse mechanism + six new/extended drivers (MELSEC deferred). This is the authoritative index
and the single statement of the effort's shared architecture, dependency graph, and build order.
Per-driver detail lives in the linked design docs; cross-cutting rules are stated **once here**
so the per-driver docs don't re-derive them.

## 1. Goal & scope

Extend the OtOpcUa OPC UA server's driver fleet under the existing Equipment-kind address-space
model — no new COM gateway, all in-process .NET 10 — and make more of the fleet **browsable**
from the AdminUI `/uns` address picker.

**Curated set (user, 2026-07-15):** MTConnect Agent · MQTT/Sparkplug B · MELSEC SLMP · Omron ·
BACnet/IP · SQL poll · Modbus RTU. **MELSEC is deferred** (commercial-license-forced hand-rolled
framer + the hardest addressing model on the roadmap; revisit on a Mitsubishi-shop need). The
other six are designed and scheduled below.

Alongside the new drivers, a **universal Discover-backed browser** retrofits browse to the
already-shipped fleet (AbCip, TwinCAT, FOCAS) and is the foundation every browsable new driver
builds on.

Existing fleet for context: Modbus(TCP), S7, AbCip, AbLegacy, TwinCAT, FOCAS, Galaxy,
OpcUaClient, Historian.Gateway (`src/Drivers/`).

## 2. Document map

**Program:** this file.

**Research reports** (`docs/research/drivers/`, index: [`README.md`](../research/drivers/README.md)):
- [`mtconnect-agent.md`](../research/drivers/mtconnect-agent.md) ·
  [`mqtt-sparkplug.md`](../research/drivers/mqtt-sparkplug.md) ·
  [`melsec-slmp.md`](../research/drivers/melsec-slmp.md) *(deferred)* ·
  [`omron.md`](../research/drivers/omron.md) ·
  [`bacnet-ip.md`](../research/drivers/bacnet-ip.md) ·
  [`sql-poll.md`](../research/drivers/sql-poll.md) ·
  [`modbus-rtu.md`](../research/drivers/modbus-rtu.md) ·
  [`00-existing-driver-browse-audit.md`](../research/drivers/00-existing-driver-browse-audit.md)

**Design docs** (`docs/plans/2026-07-15-*`):
- [`universal-discovery-browser-design.md`](2026-07-15-universal-discovery-browser-design.md) — **Wave 0**
- [`mtconnect-driver-design.md`](2026-07-15-mtconnect-driver-design.md)
- [`bacnet-ip-driver-design.md`](2026-07-15-bacnet-ip-driver-design.md)
- [`mqtt-sparkplug-driver-design.md`](2026-07-15-mqtt-sparkplug-driver-design.md)
- [`sql-poll-driver-design.md`](2026-07-15-sql-poll-driver-design.md)
- [`omron-driver-design.md`](2026-07-15-omron-driver-design.md)
- [`modbus-rtu-driver-design.md`](2026-07-15-modbus-rtu-driver-design.md)

Prior art the designs build on: [`2026-05-28-driver-browsers-design.md`](2026-05-28-driver-browsers-design.md)
(bespoke `IBrowseSession` pattern), [`2026-06-12-galaxy-standard-driver-design.md`](2026-06-12-galaxy-standard-driver-design.md)
(standard Equipment-kind driver), and the driver-typed tag editors design.

## 3. Shared architecture (the contract every driver in this program follows)

Every driver here is a **standard Equipment-kind driver** — an `IDriver` (lifecycle: `Initialize`
/ `Reinitialize` / `Shutdown` / `GetHealth` / footprint) composing only the capability interfaces
its backend supports, from `src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/`:

| Capability | Interface | This program's drivers |
|---|---|---|
| Discover | `ITagDiscovery.DiscoverAsync(IAddressSpaceBuilder, ct)` | all (device-enumerated or authored-only) |
| Read | `IReadable` | all |
| Subscribe | `ISubscribable` | all (poll-based, except the push drivers: MQTT, BACnet COV, MTConnect's `/sample` stream) |
| Write | `IWritable` | Omron, Modbus-RTU (v1); MTConnect/MQTT/BACnet/SQL read-only in v1 |
| Probe | `IHostConnectivityProbe` | all |
| Re-discover | `IRediscoverable` | only drivers with a native backend change signal (per its doc contract) — e.g. MQTT rebirth; BACnet/SQL are discovery-capable but have no change signal and correctly omit it |

**Per-driver build pattern** (identical across the fleet — see any existing driver + the
driver-typed-editors design):
1. `ZB.MOM.WW.OtOpcUa.Driver.<Name>.Contracts` — options + tag DTOs + parser + enums (no backend
   NuGet dep).
2. `ZB.MOM.WW.OtOpcUa.Driver.<Name>` — the runtime `IDriver` + capabilities.
3. `.Browser` project **only when bespoke browse is required** (see §4).
4. Factory + `DriverType` string + `Register()`, wired in the Host's `DriverFactoryBootstrap`
   (factory + probe on admin nodes via `TryAddEnumerable`).
5. Typed tag editor + validator: a `<Name>TagConfigModel` (`FromJson`/`ToJson`/`Validate`) +
   `TagConfigEditorMap` + `TagConfigValidator` entries.
6. Docker fixture under `tests/.../Docker/`, deployed to `10.100.0.35` via `lmxopcua-fix sync`
   (the `project=lmxopcua` label is a deployment convention applied host-side — no checked-in
   compose file carries a `labels:` entry today).

### 3.1 Cross-cutting rules (mandatory, stated once)

- **Enum-serialization trap** — every enum on a config/tag surface MUST serialize as **names,
  never numbers**, on both sides (AdminUI page serializer **and** probe carry
  `JsonStringEnumConverter`). Two shipped factory-side patterns both satisfy this — Modbus keeps
  DTO fields `string?` + parses via `ParseEnum<T>` (no converter in the factory); OpcUaClient
  uses enum-typed DTOs with the converter in factory **and** probe. Either is fine; a mismatch
  between the authoring side and the factory side **faults the driver** at deploy. This is a
  proven systemic bug — see the driver enum-serialization memory. Non-negotiable for every
  driver here.
- **Per-op deadline (R2-01 frozen-peer lesson)** — every network call (read/write/discover/probe/
  subscribe/agent-HTTP/SQL-query) MUST have a bounded deadline (linked-CTS / socket ReadTimeout /
  `CommandTimeout`). A frozen peer must never wedge a poll. No unbounded waits anywhere.
- **`WriteIdempotent`** — writable drivers flag only tags whose replay is safe. Note the invoker
  seam (`ExecuteWriteAsync(…, isIdempotent, …)`) exists but the host's write dispatch currently
  hardcodes `isIdempotent: false` (the arch-review "hardcoded isIdempotent" Medium) — so **no
  write auto-retries today**; drivers carry the per-tag flag now and retry activates when that
  fleet plumbing lands.
- **Secrets** — connection strings / API keys / broker creds come from env/secret refs (mirror
  `ServerHistorian__ApiKey`), never committed or logged.
- **Ctor is connection-free** — driver constructors must not open connections or touch the
  network; all connects happen in `InitializeAsync`. Every existing driver already obeys this,
  and the universal browser's `CanBrowse` throwaway-instance pattern **depends** on it — a
  ctor-connecting driver would leak connections on every capability probe.
- **Live-verify discipline** — Razor binding + deploy-inertness bugs pass unit tests and review;
  every driver's picker/editor and deploy path gets a docker-dev `/run` live-verify before it's
  trusted.

## 4. Browse architecture — the two-tier decision

The program's central browse decision: instead of a bespoke browser per driver, a **universal
Discover-backed browser** (`DiscoveryDriverBrowser`, Wave 0) captures any driver's
`ITagDiscovery.DiscoverAsync` output and re-presents it to the picker. Bespoke `IBrowseSession`
browsers are reserved for the cases the universal one genuinely can't serve.

**The gate:** a new default-interface member `ITagDiscovery.SupportsOnlineDiscovery => false`
(mirrors the existing `RediscoverPolicy` default member). A driver opts in when its `DiscoverAsync`
**enumerates from the device**; flat-address drivers leave it false and stay manual-entry. This
member is **added in the universal browser's P1** — **every browsable driver in this program
depends on it**, which is why the universal browser is the Wave-0 gate.

| Driver | Browse mechanism | Verdict |
|---|---|---|
| **MTConnect** | `/probe` device model via `DiscoverAsync` | **Universal** (`SupportsOnlineDiscovery=true`) |
| **BACnet/IP** | Who-Is + object-list via `DiscoverAsync` | **Universal** for small sites; bespoke-lazy `IBrowseSession` a documented P-later for large multi-device sites |
| **AbCip** *(existing)* | CIP Symbol 0x6B/0x6C in `DiscoverAsync` | **Universal** (+ `PatchForBrowse` `EnableControllerBrowse:true`) |
| **TwinCAT** *(existing)* | ADS symbol upload in `DiscoverAsync` | **Universal** (+ patch); bespoke lazy only if symbol set too big |
| **FOCAS** *(existing)* | FixedTree curated discovery | **Universal** (curated; + patch `FixedTree.Enabled:true` + an `UntilStable` settle — its tree fills post-connect, so a one-shot capture would be empty) |
| **MQTT/Sparkplug** | passive topic/birth observation window | **Bespoke** `MqttBrowseSession` — runtime discovery is authored-only, so universal would replay only authored tags |
| **SQL poll** | `INFORMATION_SCHEMA` schema walk | **Bespoke** `SqlBrowseSession` — schema walk ≠ `DiscoverAsync` |
| **Omron** | libplctag `@tags` unsupported on NJ/NX | **None live** — offline Sysmac tag-export importer (P-later); FINS never browsable |
| **Modbus RTU** | flat registers, no discovery | **None** — not browsable |
| S7, AbLegacy, Modbus(TCP) *(existing)* | flat / no on-wire symbols | **None** — documented in the audit |

Bespoke browsers **override** the universal fallback simply by being registered for their
`DriverType` (`BrowserSessionService` resolves bespoke-first). See the universal-browser design §3
for the required `BrowserSessionService` fallback change (the `ToDictionary` dup-key constraint
means the universal browser is a **separate `IUniversalDriverBrowser`**, not one of the injected
`IDriverBrowser` set).

## 5. Library & license decisions

| Driver | Library | License | Note |
|---|---|---|---|
| MTConnect | TrakHound MTConnect.NET (`-Common` + `-HTTP`) | MIT | netstd2.0 → loads on net10; pin + license-review; hand-roll fallback |
| BACnet/IP | System.IO.BACnet (ela-compil fork, `BACnet`) | MIT | multi-targets net10; YABE engine; callback API → TCS adapter |
| MQTT/Sparkplug | MQTTnet v5 + **hand-rolled Tahu protobuf** | MIT | **Not SparkplugNet** — its MQTTnet-4.x transitive pin collides with the repo's deliberately-OFF transitive pinning (Roslyn-split constraint) |
| Omron (CIP) | libplctag.NET | MPL-2.0 | same dep AbCip already ships (`plc=omron-njnx`); FINS hand-rolled |
| SQL poll | Microsoft.Data.SqlClient | — | already in-repo at 6.1.1 — zero new deps for P1 |
| Modbus RTU | extend existing Modbus | — | **RTU-over-TCP only** behind the **existing** `IModbusTransport` seam; zero new deps. Direct serial (`System.IO.Ports`) **descoped** (user, 2026-07-15) — serial buses are reached via serial→Ethernet gateways |
| ~~MELSEC~~ | ~~hand-rolled 3E~~ | — | deferred; HslCommunication is a commercial-license blocker |

**Avoid HslCommunication** (commercial license; the "MIT" claim is a stale abandoned fork) for both
MELSEC and Omron.

## 6. Dependency graph & build order

```
Wave 0 ─ Universal browser  ── adds ITagDiscovery.SupportsOnlineDiscovery ──┐
   │        (DiscoveryDriverBrowser + CapturingAddressSpaceBuilder +         │ every browsable
   │         BrowserSessionService fallback)                                 │ driver depends
   │                                                                         │ on this member
   ├── immediately lights up AbCip / TwinCAT / FOCAS pickers (existing)      │
   ▼                                                                         │
Wave 1 ─ Modbus RTU (extend, lowest effort)   ·   SQL poll (bespoke schema browser) ◄┘
   ▼
Wave 2 ─ MTConnect (universal browse)   ·   MQTT/Sparkplug (bespoke observation browser)
   ▼
Wave 3 ─ BACnet/IP (universal browse + COV)   ·   Omron (CIP-first; last — no NJ/NX CIP sim)
```

| Wave | Item | Browse | Write v1 | Effort |
|---|---|---|---|---|
| 0 | Universal `DiscoveryDriverBrowser` | — | — | S–M (~3–5 d) |
| 1 | Modbus RTU | none | Yes | **S (lowest)** |
| 1 | SQL poll | bespoke schema | No | S–M |
| 2 | MTConnect Agent | universal | No | S–M |
| 2 | MQTT/Sparkplug B | bespoke observation | No | M–L |
| 3 | BACnet/IP | universal (+ bespoke-lazy P-later) | No | M–L |
| 3 | Omron | offline import | Yes | M–L |

**Sequencing rationale:** Wave 0 first because the `SupportsOnlineDiscovery` seam gates every
browsable driver and it retrofits browse to three shipped drivers for near-zero marginal cost.
Wave 1 is the low-effort/high-leverage pair (RTU extends an existing driver; SQL adds zero deps).
Wave 2 is the strategic telemetry/UNS pair. Wave 3 is the heavier/test-gated pair — Omron last
because there is **no free NJ/NX CIP simulator**, so its wire correctness is a live-hardware gate.
MELSEC is out of the sequence (deferred).

## 7. Testing & fixtures

- Fixtures live under `tests/.../Docker/`, deployed to the shared host `10.100.0.35` via
  `lmxopcua-fix sync` (which owns the `project=lmxopcua` labelling host-side). Per-driver sims: `mtconnect/cppagent`, Mosquitto/EMQX +
  Sparkplug simulator, `bacnet-stack`/ela-compil `BasicServer`, central SQL Server
  (`10.100.0.35,14330`) + seeded table, `rtu_over_tcp` pymodbus profile (no serial rig —
  direct-serial descoped).
- **Env-gated live suites** for what a sim can't cover: BACnet Who-Is/BBMD UDP broadcast, Omron CIP
  wire (live hardware), MQTT broker soak.
- Per driver: unit (framer/parser/type-map with golden vectors + fakes) → integration (docker
  fixture) → **live `/run`** on docker-dev (picker + editor + deploy path).

## 8. Deferred / out of scope

- **MELSEC SLMP** — deferred (§1). Research complete (`melsec-slmp.md`); revisit on demand.
- **Modbus direct-serial transport** (`ModbusRtuTransport` / `System.IO.Ports`) — descoped
  (user, 2026-07-15); design record kept in the RTU doc §2b. RTU-over-TCP via a
  serial→Ethernet gateway is the only shipped RTU mode.
- **Write-back** for MTConnect (Interfaces), MQTT (NCMD/DCMD), BACnet (WriteProperty priority
  array), SQL (parameterized UPSERT) — all documented as later phases in the respective designs.
- **Bespoke lazy browsers** for large BACnet sites and large ControlLogix/TwinCAT symbol sets —
  graduate from universal only when eager one-shot discovery proves too heavy.
- Shared `Cip.Core` extraction (AbCip ↔ Omron CIP) — a follow-up, not a gate; keep Omron
  self-contained in v1.

## 9. Status

Research: **done** (8 reports). Designs: **done** (7 design docs). Code: **Wave 0 landed** —
the universal Discover-backed browser (P1) is code-complete and unit-tested on
`feat/universal-discovery-browser` (`CapturingAddressSpaceBuilder` + `CapturedTreeBrowseSession`
+ `DiscoveryDriverBrowser`/`IUniversalDriverBrowser` + `BrowserSessionService` fallback +
`ITagDiscovery.SupportsOnlineDiscovery` gate + AbCip/TwinCAT/FOCAS opt-ins & picker browse UI),
with the live `/run` gate outstanding. Next action: complete the Wave-0 live gate, then Wave 1.
