# OPC UA communication (Laborious)

Laborious exports predictions to OPC UA servers through `OpcRepository` ([`laborious/utils/repository/opc_repository.py`](../laborious/utils/repository/opc_repository.py)) and the synchronous Temporal activity layer in [`laborious/activities/opc.py`](../laborious/activities/opc.py). The repository uses `asyncua.sync.Client` (asyncio on a background thread) so activities remain blocking without `async def`.

Implementation plan for session/channel recovery on Tier-1 `Bad*` errors: [`.cursor/plans/opc_bad_reconnect_ac4c6045.plan.md`](../.cursor/plans/opc_bad_reconnect_ac4c6045.plan.md).

## Architecture

```text
Worker (long-lived)
  └── OpcRepository per OPC server id (from OPC_CONFIG / env)
        ├── connect / disconnect / validate_connection (read-only)
        ├── _connect_locked / _reconnect_locked (under _connection_lock)
        ├── write_data (single attempt per call)
        └── background reconnect on Tier-1 Bad*, closed protocol, or stale session

Temporal activity write_opc_data
  └── OPC.manage_output_tags → write_data per tag (sequential per activity)
```

One worker process holds one `OpcRepository` instance per configured server. Multiple Temporal activities can call `write_data` concurrently on the same repository.

## Connection lifecycle

| Phase | Behavior |
|-------|----------|
| Startup | `init_opc()` creates repositories and calls `connect()` → `_connect_locked()` |
| Steady state | `validate_connection()` is read-only (`protocol.state` only); `_session_ready` is checked in `write_data` |
| Tier-1 Bad* / protocol closed / session not ready | `_start_reconnect(reason)` → `_run_reconnect` (thread) → `_reconnect_locked()` (respects `reconnection_interval`) |
| Write | `write_data()` checks in-flight reconnect thread, `_session_ready`, validates, then one `get_node` + `write_value` |
| Shutdown | `disconnect()` sets `_allow_reconnect = False`, then tears down session |

### Session and channel timeouts

Requested session and secure-channel lifetime: **10 minutes** (`OPC_UA_SESSION_AND_CHANNEL_TIMEOUT_MS` in `opc_repository.py`). The server may revise these values; negotiated values are logged after connect and exposed as `opc_session_revised_timeout_milliseconds`.

### Reconnection interval

`OPC_RECONNECTION_INTERVAL` is in **seconds** (default `120`). It gates **background** reconnect after Tier-1 `Bad*` (`last_reconnection_time` is updated only in `_reconnect_locked()`). It limits load on the OPC server when many workflows fail at once.

## Concurrency: connection lock and session readiness

To allow **multiple concurrent writes** when the session is healthy, but **block all writes** while the connection is being torn down or re-established:

| Primitive | Role |
|-----------|------|
| `_connection_lock` (`threading.Lock`) | Held for the entire `disconnect` → `connect` path. Only one connection-maintenance task at a time. |
| `_session_ready` (`threading.Event`) | Set when a session is ready for writes; cleared before reconnect starts and set again after a successful connect. |
| `_allow_reconnect` | Cleared in `disconnect()` so shutdown does not spawn reconnect threads |

**Connection methods (caller holds `_connection_lock` for `_*_locked` helpers):**

| Method | Role |
|--------|------|
| `_create_client()` | Create asyncua `Client` + optional `set_security`; raises if `client` already exists |
| `_open_session()` | `client.connect()` + metrics; raises if session already open or client missing |
| `_connect_locked()` | `_create_client()` (when needed) + `_open_session()`; raises if already connected |
| `_disconnect_locked()` | Teardown session and clear `client` |
| `_reconnect_locked()` | `_disconnect_locked()` + `_connect_locked()`; sets `last_reconnection_time` |

Public `connect()` / `disconnect()` acquire the lock and call `_connect_locked()` / `_disconnect_locked()`.

**Write path (`write_data`):**

1. If a reconnect **thread** is alive → `reconnect_in_progress`.
2. If `_session_ready` is cleared → schedule `SessionNotReady` reconnect; return `reconnect_in_progress` or `connection_lost`.
3. If `validate_connection()` fails (protocol closed) → schedule `ProtocolClosed` reconnect; return `connection_lost`.
4. Single `get_node` + `write_value` (no retry in the same call).

**Reconnect path (`_run_reconnect`):**

1. `_start_reconnect` clears `_session_ready` and starts a daemon thread when the interval allows and `_allow_reconnect` is true.
2. `with _connection_lock:` → `_reconnect_locked()`.
3. `_session_ready` is set on successful `_open_session()`.

A second `_connect_locked()` while a session is already open raises `OpcSessionAlreadyConnectedError` (disconnect first).

**asyncua note:** Concurrent `write_value` on the same session is only safe if the stack tolerates it. If production shows issues, serialize writes while keeping the connection lock semantics above.

## Tier-1 `Bad*` errors and reconnect

When the server invalidates the session (e.g. `BadSessionIdInvalid`) but the client still sees transport as open, `write_data` fails once, records the OPC status in metrics, and **schedules** reconnect if:

- The exception is a `UaStatusCodeError` whose name is in `RECONNECTABLE_OPC_BAD_NAMES` (see plan), and
- `reconnection_interval` has elapsed since `last_reconnection_time`, and
- No reconnect task is already running.

There is **no write retry**: the failed export is not sent again in the same activity.

## Prediction confidence and PostgreSQL comments

| `prediction_confidence` | Meaning |
|-------------------------|---------|
| (unchanged) | Successful OPC export |
| **12** | Generic OPC write failure (`OPC_WRITTING_ERROR_CONFIDENCE`) |
| **14** | Tier-1 session/channel `Bad*` on export (`OPC_SESSION_BAD_CONFIDENCE`) |
| **14** | Write while reconnect in progress (`OPC_SESSION_BAD_CONFIDENCE`, comment `OPC UA reconnect in progress`) |
| **13** | PI Web API write failure (separate path) |

Session/channel errors use a stable comment for counting:

```text
OPC UA session/channel error: BadSessionIdInvalid
```

Reconnect-in-progress exports use:

```text
OPC UA reconnect in progress
```

Example SQL:

```sql
SELECT count(*) FROM predictions WHERE prediction_confidence = 14;
SELECT count(*) FROM predictions WHERE comments LIKE 'OPC UA session/channel error:%';
```

## Prometheus metrics (`opc_*`)

Defined in [`laborious/metrics.py`](../laborious/metrics.py). Do not rename in production without a dashboard migration.

| Metric | Purpose |
|--------|---------|
| `opc_connections_initiated_total` | Connection attempts |
| `opc_connections_failed_total` | Failed connects |
| `opc_connection_status` | Gauge 1=connected, 0=disconnected |
| `opc_session_created_total` | Session established after connect |
| `opc_session_closed_total` | Disconnect initiated |
| `opc_session_revised_timeout_milliseconds` | Negotiated session timeout (ms) |
| `opc_write_attempts_total` | Per write; label `result` = `OK` or exception name |
| `opc_write_inter_arrival_over_session_timeout_total` | Successful writes spaced longer than revised session timeout |

Legacy activity metrics: `laborious_prediction_opc_writing_count`, `laborious_prediction_opc_writing_response_time_monitor`.

## Environment variables

| Variable | Default | Description |
|----------|---------|-------------|
| `OPC_CONFIG` | — | JSON map of server configs (overrides single-server env) |
| `OPC_ID` | `1` | Server id |
| `OPC_URL` | `opc.tcp://localhost:4840` | Endpoint |
| `OPC_SERVER_NAME` | `default_server` | Label for metrics/logs |
| `OPC_SERVER_URI` | same as URL | Application URI / cert SAN |
| `OPC_CERT_PATH` | — | Client certificate (secure mode) |
| `OPC_PRIVATE_KEY_PATH` | — | Client private key |
| `OPC_SERVER_CERT_PATH` | — | Server certificate |
| `OPC_RECONNECTION_INTERVAL` | `120` | Minimum seconds between reconnects |

## Operations checklist

- Correlate `BadSessionIdInvalid` in `opc_write_attempts_total` with `opc_session_closed_total` / `opc_session_created_total` (reconnect may finish after the row is stored with confidence 14).
- Use confidence **14** and comment prefix for session invalidation rates; use **12** for other OPC failures.
- Respect `OPC_RECONNECTION_INTERVAL` under parallel load; bursts of confidence 14 are expected until the next successful cycle.

## Related tests

- Unit: [`tests/laborious/utils/repository/test_opc_repository.py`](../tests/laborious/utils/repository/test_opc_repository.py)
- Unit: [`tests/laborious/activities/test_opc.py`](../tests/laborious/activities/test_opc.py)
- E2E (mock OPC): [`e2e/test_predictions_batch_format_export.py`](../e2e/test_predictions_batch_format_export.py)
- E2E (in-process asyncua server + real `OpcRepository`): [`e2e/test_opc_real_server.py`](../e2e/test_opc_real_server.py) — scenarios 3.1.2, 3.2.2, 3.2.4, 3.2.5
- Scenarios: [`e2e/scenarios.md`](../e2e/scenarios.md)