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243 lines
7.9 KiB
Markdown
243 lines
7.9 KiB
Markdown
# The connector contract
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Connectors integrate devices and services through typed messages.
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This reference covers the contract the engine supports; start with
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[Writing a connector](../code/connectors.md) for a working package.
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## Contract version
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```python
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from fluksio.flow.connector import CONTRACT_VERSION, ConnectorNode
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class MyConnector(ConnectorNode):
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contract = CONTRACT_VERSION
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title = "My connector"
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description = "Reads a device."
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```
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Declare `contract` on the class itself: inheritance does not count.
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The current contract version is `1`. A mismatch is logged and the connector
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is skipped. Breaking changes bump the version; compatible additions do not.
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`title` defaults to the entry point name; `description` may be empty.
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## Packaging and discovery
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Register the class in your package's `pyproject.toml`:
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```toml
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[project.entry-points."fluksio.node_types"]
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my_device = "fluksio_connector_mydevice:MyConnector"
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```
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The entry point name is the stored node type. Names already registered,
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including built-in types, are skipped. The editor shows the package name
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and version from its installed metadata.
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Discovery runs at engine startup. Install connectors in the engine's
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environment and restart after installation or code changes.
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## Parameters
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Subclass `ConnectorNode.Params` to keep `poll_interval` and add your settings:
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```python
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from pydantic import Field
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class Params(ConnectorNode.Params):
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host: str = "localhost"
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api_key: str | None = Field(
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default=None, json_schema_extra={"x-secret": True}
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)
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```
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The node panel builds a form from this model's JSON schema: switches for
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booleans, inputs for numbers and strings, and JSON fields for objects and
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arrays. Parsed settings are available as `self.config`.
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Mark credentials with `x-secret`. The editor stores a reference such as
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`{"$secret": "device-key"}`, and the engine resolves it while building the node.
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## Ports and messages
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The flow author declares ports as `MessageSpec` values. Each has a message
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name, a `DType`, and optional delivery settings.
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- `poll()` returns values keyed by **output port**.
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- `write()` receives values keyed by **input port**.
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- The base class maps local port names to message names and validates types.
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- Unknown output keys fail the reading. Return only declared ports.
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- Port `interval` limits are enforced by the engine; the connector controls
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how often it reads the device.
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For devices with variable readings, filter to the configured outputs:
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```python
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declared = {spec.port for spec in self.output_ports if spec.name}
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return {port: value for port, value in reading.items() if port in declared}
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```
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Log missing device fields once so a typo does not leave a port silently empty.
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## Polling
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```python
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async def poll(self):
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return {"reading": await self._read_device()}
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```
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Set `poll_interval` in seconds; `0` disables polling. Return `None` when
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there is nothing new. The base loop publishes only changed values and retries
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a failed reading or publication on the next tick. It reports connection health
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around each poll.
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`poll()` runs on the event loop. Use `asyncio.to_thread` for blocking reads.
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The base loop already calls `inject` on a worker thread.
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For pushed readings, leave `poll_interval=0`, manage the subscription in
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`start()` and `stop()`, and call `self.inject({...})` when values arrive.
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Offload `inject` too when calling it from the event loop.
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## Writing
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```python
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def write(self, **ports):
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self._device.set(ports["level"])
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return None
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```
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Settings stay on `self.config`; arguments contain input ports only.
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Return `None`, or a dictionary keyed by output port if the device responds
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with useful values.
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Writes run on a scheduler thread. Return promptly or queue slow work.
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Polling and writing are independent; a connector may support both.
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Set `idempotent = False` for commands that must not be redelivered after a
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crash. Leave `write()` unchanged on a read-only connector.
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## Devices whose readings are bytes
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```python
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import asyncio
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async def poll(self):
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jpeg = await asyncio.to_thread(self._grab)
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return {
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"frame": self.save_artifact(
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jpeg, "frame.jpg", media_type="image/jpeg", volatile=True
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)
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}
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```
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`save_artifact(data, name="", media_type="application/octet-stream",
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volatile=False)` returns a reference for an artifact or media port.
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Match the media type to an `image`, `audio`, or `video` port.
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The artifact store is available after `start()`. Use `volatile=True`
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for disposable live frames: they go to a bounded memory ring and expire as
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new frames arrive. Use durable storage for recordings.
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Local screens receive pushed live frames; portal screens fetch the bytes.
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Choose a modest poll rate for remote viewing.
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## Keeping state across restarts
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Keep connections and device handles in node attributes. Store short strings
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such as pairing tokens or pinned certificates in `self.store`:
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```python
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value = await asyncio.to_thread(self.store.get, "token:device")
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await asyncio.to_thread(self.store.set, "token:device", "value")
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await asyncio.to_thread(self.store.delete, "token:device")
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```
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- Keys and values are strings. `get` returns `None` for a missing key;
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`delete` returns whether it existed.
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- All nodes from the connector's top-level package share the namespace.
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Include a device identifier in each key.
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- Storage is in the engine's database and is available before `start()`.
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- Keys allow 1–200 characters, without `/` or whitespace. Values allow
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64 KiB and each package allows 256 keys; exceeding a limit raises `ValueError`.
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- Calls block on the database: use `asyncio.to_thread` on the event loop.
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Superusers can list keys and clear them under **Stored state** in the node
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panel. Values are never returned. The equivalent API paths are:
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| Method | Path |
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| `GET` | `/api/v1/connectors/{node_type}/state` |
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| `DELETE` | `/api/v1/connectors/{node_type}/state/{key}` |
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| `DELETE` | `/api/v1/connectors/{node_type}/state` |
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Handle missing keys: clearing one asks the connector to pair or learn again.
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### Pinning a device's certificate
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`fluksio.flow.connector_tls` supports trusting a local device's certificate
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on first contact:
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```python
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import httpx
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from fluksio.flow.connector_tls import (
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context_for,
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pinned_certificate,
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raise_if_changed,
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)
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address = f"{self.config.host}:{self.config.port}"
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pem = pinned_certificate(self.store, address, self.config.timeout)
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try:
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with httpx.Client(verify=context_for(pem)) as client:
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response = client.get(f"https://{address}/status")
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response.raise_for_status()
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except httpx.ConnectError as exc:
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raise_if_changed(exc, pem, address, self.config.timeout, "my_device")
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raise
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```
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Run this blocking work off the event loop. First contact pins the certificate
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under `cert:<address>`; a changed certificate is refused before sending a
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credential or command. `raise_if_changed` reports `CertificateChanged`
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with the fingerprint and key to clear.
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Pins trust the exact certificate, including after its expiry. Clearing the
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key accepts the certificate presented on the next connection.
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## Lifecycle and health
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```python
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async def start(self, app=None):
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await super().start(app)
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async def stop(self, app=None):
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await super().stop(app)
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```
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`start` runs when a flow starts or rebuilds. `stop` runs before rebuilding
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or stopping, even after a failed start; make it idempotent. Call `super()`
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when overriding these hooks if you want the base polling loop.
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The optional `app` is the FastAPI application, for connectors that mount
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routes. Most connectors can ignore it.
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Report connection changes if you manage your own subscription:
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```python
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self.report_health("ok")
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self.report_health("degraded", "Some readings timed out")
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self.report_health("down", "Connection lost")
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```
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Repeated statuses do not emit another event. `down` appears among flow issues
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and on Home; `degraded` means the connection still works. Loading failures
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are reported separately.
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Keep shared flow values in messages, credentials in secret parameters, and
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blocking I/O off the event loop.
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