---
id: DATA-9
family: RT-DATA
level: extended
profiles: [runtime]
posture: normative-target
added: "1.0"
changed: "1.0"
source: https://flowdrop.io/spec/rules/rt-data/data-9
specification: FlowDrop Workflow Specification 1.0-draft
licence: CC BY 4.0
---

# DATA-9 — The unified output port carries exposed outputs only

*RT-DATA (Part II) · level: extended · profiles: runtime · added in 1.0*

## The rule

> **Normative.** This is the rule.
>
> 1. The unified `output` port composes a node's exposed outputs and nothing else.
>
> 2. Keys prefixed with `_` and the reserved `trigger` key are excluded from it.
>
> 3. An output key the node type does not configure counts as exposed and is kept.


## What it means

The exclusion of `_`-prefixed keys and the reserved `trigger` key does not
depend on how the node type configures them. Marking a key like `_debug`
exposed on the node type has no effect: the leading underscore itself marks
the value as plumbing rather than published data, and `trigger` is excluded
the same way whatever its own exposure setting says, because it carries no
data to begin with.

The other clause runs the opposite way from what a stricter reading would
suggest. An output key the node type's own configuration never mentions still
counts as exposed and is composed into `output`. That is what stops a node
whose behaviour produces more than its declared output schema from silently
losing the extra value: nobody has to configure a key for it to be kept.

## Example

A node's result carries an ordinary output alongside an internal key that the
node type happens to mark exposed anyway.

```json title="What the node produced" verdict="produced"
{"result": "ok", "_debug": "trace"}
```

```json title="What the unified output port composes" verdict="composed"
{"result": "ok"}
```

The same node type declares only `result`, but the node also produces
`extra`, a key its configuration never mentions.

```json title="What the node produced, one undeclared key" verdict="produced"
{"result": "ok", "extra": "bonus"}
```

```json title="What the unified output port composes with nothing declared" verdict="composed"
{"result": "ok", "extra": "bonus"}
```

## Related rules

- Names: DATA-8
- Referenced by: DATA-8

---

Rule identifiers are permanent and are never renumbered. This specification carries
no implementation status: each implementation publishes its own standing against
these rules. Licensed CC BY 4.0.