app/docs/reference/payload-types.md

Payload types

Every port declares a dtype, and every value that passes through it is checked against that declaration.

This is not decoration. It is what lets the dashboard editor offer you only the messages a gauge can actually draw, what lets the canvas refuse a binding before anything runs, and what lets a downstream node know the shape of what it is getting before the flow starts.

Everything on the wire is JSON. That is what lets the same value pass through the state backend, the work queue and the worker protocol unchanged.

The scalars

dtype	Accepts
float	any number — int or float, but not bool
int	a whole number, not bool
str	a string
bool	exactly true or false

bool is an int subclass in Python and deliberately not a number here: a flag is not a measurement, and a switch bound to a temperature is a mistake worth catching.

NaN and infinity are refused, wherever they sit — including inside a json, record, series or list. JSON cannot spell either, so one that travelled would come back as a response nobody can parse and a row the database rejects, a long way from the node that made it. An empty subset or a division with no denominator is what usually produces one; publish None instead.

The structured ones

These are declared shapes rather than "some JSON", which is what makes a widget binding checkable.

record

Flat named scalars.

{"title": "Boiler", "body": "Pressure low", "severity": "warning"}

Nesting is deliberately out: a record that can contain a record is a schema language, and the shape stops being readable from the declaration alone.

Read by the Notification widget. It is also what an alert channel of kind dashboard writes.

list

Ordered items of one declared shape. The port also declares item:

item	Meaning
unset	record — what the agenda and forecast widgets read
float, int, str, bool	a list of scalars
json	anything

A list of lists, or a list of series, is refused. One declared level is the point.

series

Labelled lines of (timestamp, value) pairs — what a chart draws.

{
  "lines": [
    {"label": "living", "points": [[1717000000, 21.4], [1717000060, 21.5]]}
  ],
  "range": "-24h"
}

Keys beside lines are carried through untouched, which is how a querying chart puts the window and resolution it asked for on the request and reads them back off the answer. That is what stops an answer to a different question from overwriting the picture.

GET /runs/series/compare answers in this shape, which is why comparing three training curves is a widget binding rather than a screen of its own.

artifact

A reference to stored bytes.

{"digest": "sha256:…", "size": 4194304, "media_type": "application/octet-stream", "name": "weights.pt"}

Binary payloads — tensors, checkpoints, images — never travel as a message. The bytes go to a content-addressed store and the message carries this. A thirty-megabyte checkpoint never sits in the state backend, and the reference stays valid wherever the store is reachable from, including on another machine.

Node code produces one with fluksio.save_artifact and opens one with fluksio.load_artifact. See Writing node code.

json

Anything JSON-serializable. The escape hatch, and the right answer when a payload genuinely has no fixed shape.

Reach for it last. A json port tells the canvas, the widget picker and the next author nothing.

Naming a run's output

Any run parameter is also accepted as text, since nobody wants to paste an object into a shell. @run:<id>.<output> names what an earlier run produced — whatever its type, an artifact reference or a json config alike — and a bare sha256:… digest names content in the artifact store. Both resolve before the run starts, so the CLI, the run dialog and a python caller all mean the same thing by the same string.

Both spellings are reserved on every input, str included: an input that has to carry one of them literally is asking for a value this engine reads as a name.

What a widget will bind to

Widget	Accepts
Gauge, Chart, Slider, Bar	float, int
Switch	bool
Agenda, Forecast	list
Notification	record
Value	anything
Icon	weather strings, booleans and numbers alike
Clock, Text	nothing — they bind to no message

Enforced on the server as well as in the editor.

Type failures

A value that does not match its port's declaration raises on the node that published it, naming the port and what arrived. It does not get published, and it does not reach anything downstream — a wrong value stopping at its source is much easier to diagnose than one propagating.

See also

• Flows, nodes and messages
• Node types
• Dashboards and panels