# Flows, nodes and messages

Three ideas hold the whole system up. They are worth twenty minutes, because
almost everything else follows from them.

## A flow is a graph you did not draw

A **flow** is a set of **nodes**. Each node declares the messages it needs
(`requires`) and the messages it produces (`provides`). The graph is whatever
those declarations imply:

```python
# node "read"
def process():
    return {"temperature": read_sensor()}

# node "decide"
def process(temperature, setpoint=21.0):
    return {"heat": temperature < setpoint}
```

`decide` is downstream of `read` because it needs `temperature` and `read`
produces it. Nobody drew a wire.

This is the one structural decision everything else rests on, so it is worth
being explicit about the consequences:

- **Fan-in is free.** Two nodes providing `temperature` are two producers of
  one message. The consumer does not change.
- **A node runs when something it reads was published.** Not merely when
  something upstream of it ran: a node that produced nothing this time — held
  back by a rate limit, say — leaves what reads it on the value it already
  has, and so does everything behind that.
- **A wire cannot be wrong.** There is no wire. There is a name that either
  matches or does not, and the canvas tells you at edit time which it is.
- **Layout is not a document.** The canvas computes the arrangement, so a flow
  has no stored positions to maintain, merge or fight over.
- **Flows stay small.** A graph nobody can hand-arrange is one worth keeping
  small — which is the intent. Several atomic flows that name each other beat
  one flow with sixty nodes in it.

### Message names are namespaced

Inside flow `house`, a message named `temperature` is really
`house.temperature`. A bare name is qualified with its own flow; a dotted name
is used as written. That is how two flows share a value:

```python
# in flow `dashboard`, reading a message that flow `house` produces
def process(house_temperature):     # port bound to "house.temperature"
    ...
```

The canvas draws messages arriving from another flow as labelled endpoints, so
you can see where they come from without opening the other flow.

## A node is a function with declared ports

Most nodes are **Function** nodes: a Python file defining `process(...)`. Its
arguments are its input ports by name; its return value is a dict keyed by
output ports.

```python
def process(reading, unit="C"):
    return {"shown": reading if unit == "C" else reading * 1.8 + 32}
```

`reading` is a port. `unit` is a **setting** — a constant of this node's code,
typed into its panel and stored with the flow. Both arrive as arguments, which
is why a setting may not share a name with a port. See
[Where a node's values come from](values.md).

The rest of the node types are the ones that would be tedious or unsafe to
write yourself: MQTT, HTTP, InfluxDB, schedules, switches, notifications. Each
one is configured by filling in a form the editor generates from its parameter
schema, so they all behave the same way. The full list is in
[Node types](../reference/node-types.md).

### Ports are typed

A port declares a `dtype`: `float`, `int`, `str`, `bool`, `json`, `record`,
`list`, `series` or `artifact`. Every value that passes through is checked
against it.

Types are not decoration. They are what lets the dashboard editor offer you
only the messages a gauge can actually draw, and what lets the canvas refuse a
binding before anything runs. See [Payload types](../reference/payload-types.md).

Everything on the wire is JSON. Bytes — a checkpoint, an image, a model —
travel as an `artifact`: the bytes go to a content-addressed store and the
message carries a small reference to them.

### Nodes are pure

A node is called with the values of the messages it declares and returns the
values of the messages it provides. There is no context object, no global
store, no handle to reach for.

That is deliberate: a node with hidden state cannot run twice in parallel,
cannot be replayed, and cannot be moved to another machine. Plenty of real
automations do need to remember something, and there is a specific way to say
so — see [Keeping state in a flow](state.md).

## Two shapes of flow

Set `mode` on the flow:

| | `live` (default) | `batch` |
|---|---|---|
| Runs | continuously | once per run, on request |
| Started by | subscriptions, schedules, webhooks | `POST /runs/flows/{name}` |
| Ends | never | when the graph drains |
| Keeps | the last value of each message | a run record: params, result, metrics, artifacts |
| Is | a thermostat, an ETL job on a cron | an experiment, a CI-style job |

A batch flow is built and validated like any other, appears on the same canvas
and is type-checked the same way. It is simply never *activated*: no
subscriptions, no schedules, no webhooks. See
[Runs: pipelines that finish](runs.md).

## Editing is separate from running

Every flow has a published version and, while you are working, a draft.

- **Saving** writes the draft. The engine keeps running the published version.
- **Publishing** promotes the draft. The engine reloads and picks it up.
- **Discarding** throws the draft away.

The store is a git repository — `flow.json` for the structure, `nodes/*.py` for
the code — and each save is a commit. So a flow's history is readable with
ordinary git tooling, and copying a flow between installations is copying a
directory.

Saving carries the version you last saw. If someone else saved in between, you
get a 409 instead of quietly overwriting their work.

## What can be wrong, and when you find out

The canvas validates continuously and names problems on the nodes they belong
to:

| Issue | What it means |
|---|---|
| `unconnected_input` | a port needs a message nothing in reach provides |
| `missing_initial_value` | the message exists but has never held a value, and nothing will give it one. Not reported on a batch flow: its inputs arrive with the run |
| `cycle` | A waits for B and B waits for A — nothing could ever start |
| `self_loop_needs_initial` | a node reads a message it also writes, with no starting value |
| `node_error` | the node's code did not load: a syntax error, a missing import |
| `unauthenticated_hook` | advisory — a webhook with no shared secret is open to anyone |

A flow with any of these except the advisory one does not run. The health
summary on Home counts them, so "why is nothing happening?" has an answer that
does not involve reading logs.

## What happens at runtime

- **A flow can be started and stopped.** Stopped means its subscriptions and
  schedules are torn down.
- **A flow can be paused and stepped.** Paused holds messages instead of
  running them; step releases exactly one. This is how you test something
  before it moves a relay.
- **A failing node does not take the flow down.** It reports an error, keeps
  its last error visible after it recovers, and can fire an alert.
- **A flow whose background tasks keep crashing is quarantined.** The engine
  stops restarting them and says so, rather than spinning. Publishing a change
  gives it another chance.

## Values that arrive from outside

Some messages are not computed by any node: a dashboard control writes them,
the API publishes them, a batch run passes them in. Declare those as the flow's
**inputs**, with the value they start from:

```json
{"inputs": [{"spec": {"name": "setpoint", "dtype": "float"}, "initial": 21.0}]}
```

Without that, the node reading `setpoint` waits for something nothing provides,
and the canvas says so. With it, the flow starts at 21.0 and whatever writes
the message afterwards takes over.

## Where to next

- [Where a node's values come from](values.md) — ports, settings, flow inputs
- [Keeping state in a flow](state.md) — the sanctioned way to remember something
- [Runs: pipelines that finish](runs.md) — the batch half of the engine
- [Writing node code](../code/nodes.md) — the practical side of `process()`