# Writing a connector

A walkthrough from an empty directory to a node on the canvas. The normative
surface is in [the connector contract](../reference/connector-contract.md);
this is how to use it.

## Start from the skeleton

A working connector, kept as small as a correct one can be, lives at
`skeleton/` in the [connectors repository](https://git.stroblme.de/Fluksio/connectors).
Copy it:

```bash
git clone https://git.stroblme.de/Fluksio/connectors.git
cp -r connectors/skeleton fluksio-connector-mydevice
cd fluksio-connector-mydevice
```

Rename the package (`src/fluksio_connector_skeleton/` and the `[project]` name
in `pyproject.toml`), then point the entry point at your class:

```toml
[project.entry-points."fluksio.node_types"]
my_device = "fluksio_connector_mydevice:MyDevice"
```

## Install it into the engine

Connectors are found through installed package metadata, so the engine has to
have yours installed. During development, install it as an editable path
dependency of the backend:

```bash
cd app/backend
uv add --editable ../../fluksio-connector-mydevice
```

Editing your connector's **code** then takes effect on the next engine restart,
with no reinstall. Changing its **metadata** — the entry point, the version,
the package name — needs `uv sync` to regenerate the installed metadata before
a restart picks it up.

Two things to know about this:

- `uv add` writes the path dependency into `backend/pyproject.toml` and
  `uv.lock`. That path does not exist inside the backend's Docker image, so
  **revert both files before committing**; the dependency belongs on your
  machine, not in the repo. Distribution is a later milestone.
- For the same reason, develop against the **standalone backend** rather than
  the Docker stack:

  ```bash
  cd app && make dev-backend    # FastAPI on :8000
  make dev-frontend             # Vite on :5173
  ```

Restart the backend and your node type appears in the add-node palette (⌘K),
labelled with the package it came from.

## Write the node

```python
from typing import Any

from pydantic import Field
from fluksio.flow.connector import CONTRACT_VERSION, ConnectorNode


class MyDevice(ConnectorNode):
    contract = CONTRACT_VERSION
    title = "My device"
    description = "Reads temperature and humidity over the device's HTTP API."

    class Params(ConnectorNode.Params):
        host: str = "192.168.1.50"
        api_key: str | None = Field(
            default=None, json_schema_extra={"x-secret": True}
        )

    async def poll(self) -> dict[str, Any] | None:
        reading = await self._read()
        return {"temperature": reading.temp, "humidity": reading.humidity}
```

Then, on the canvas: place the node, set `poll_interval` to how often the
device should be read, pick the API key from the secrets store, and add output
ports named `temperature` and `humidity`. The port names are the wiring — any
node consuming those messages is downstream of this one.

## If the device is told rather than asked

A connector that commands something implements `write()` instead of, or
alongside, `poll()`. Input ports carry the values in:

```python
    def write(self, **ports: Any) -> None:
        self._device.set(ports["level"])
```

Place the node with *input* ports named `level`, and whatever publishes that
message drives the device. Set `idempotent = False` on the class: a write is a
command, and a redelivery after a crash should not undo a newer one.

Keep the first version off the wire. A boolean setting the code checks before
it sends — `artnet`'s `transmit` is the example — lets a flow be built and
watched in the logs before anything physically moves.

## If the reading is bytes

A camera or a microphone publishes a reference rather than the bytes:

```python
    async def poll(self):
        jpeg = await asyncio.to_thread(self._grab)
        return {"frame": self.save_artifact(jpeg, "f.jpg", media_type="image/jpeg")}
```

Place the node with an `image`-typed output port and a Media widget draws each
frame as it lands. `fluksio-connector-test-media` publishes test frames and
tones this way, so the whole path can be wired up with no camera in the room —
copy it if yours is a media device.

## Try it without a device

Give `poll` something predictable first and confirm the values reach the canvas
before pointing it at hardware:

```python
async def poll(self):
    return {"temperature": 21.5}
```

Press Run, or wait for the poll interval, and the value appears on the edge.
Anything the connector prints shows up in the logs panel in the dock, and a
raised exception shows there with its traceback and marks the node down.

## Checklist before sharing it

- `contract = CONTRACT_VERSION` declared on the class itself.
- Credentials marked `x-secret`, never read from the environment.
- `stop()` is safe to call twice, and after a failed `start()`.
- Blocking I/O wrapped in `asyncio.to_thread`.
- `poll()` returns `None` rather than raising when there is simply nothing new.
- `write()` returns quickly; the thread it runs on is the scheduler's.
- Health reported when the connection comes and goes.