shopdb-flask/docs/ASSET-COMPOSITION.md

Composition: several devices under one parent

When more than one physical device answers to a single identifier, model each device as its own asset and file it under a parent. This page is the pattern: when to reach for it, what you get for free, and how to make the collector do it without writing code.

The mechanism is not new - partof and its propagation rails are the ADR-001 platform contract. What is new is the recipe.

The problem it solves

A machine number is supposed to identify one thing. Sometimes it does not.

At West Jefferson, several Telesis part markers serve one operation number: 0613, 0615 and WJPRT each have more than one. Their configurations differ, most often by COM port. Treating the operation as the device collapsed them into a single record, and the damage was quiet:

• Their backups overwrote each other, so the history read as one device flip-flopping between configurations that were really two devices.
• No question about an individual device could be asked at all - how many there are, which port one is on, which one failed.
• Every PC driving one contested the single controls link to the operation, so whichever reported last appeared to own it.

None of that announced itself. It surfaced as duplicate backup rows, weeks later.

When to use it

Reach for composition when ALL of these hold:

• Several physical devices share one identifier, and that is CORRECT - not a numbering mistake. Two PCs carrying the same machine number by accident is a fault to fix on the PC, not a model to build. flask relationships check-shared-machines tells the two apart.
• The devices are individually interesting: they fail, get replaced, carry their own configuration or calibration.
• Something already identifies each device. One device per PC is the easy case, because the PC names it.

Do NOT reach for it when the parent is simply a location. A room holding six printers wants a Location, not a parent asset.

What you get

Filing a device partof a parent buys behaviour already built:

• Control propagates. controls propagates through partof (ADR-001, and seeded by flask seed reference-data). A PC controlling a device therefore controls its parent by inheritance, so the PC does NOT need - and must not have - a direct link to the parent. That is what stops several devices contesting a link only one can hold.
• Position inherits. Map-position resolution walks partof first, so a device with no coordinates of its own shows at its parent's position.
• History separates. Anything keyed on the asset - backup revisions, relationships, notes, audit - is per device instead of merged.

Making the collector do it

A PC that drives a subordinate device declares it in plugins/computers/pctypemap.py:

SUBORDINATE_DEVICE_MAP = {
    'gea-shopfloor-partmarker': {
        'assettype': 'machine',        # core AssetType for the device
        'typename': 'Part Marker',     # type within that vocabulary
        'description': 'Telesis part marker',
        'suffix': 'PARTMARKER',        # device asset number = <PC>-<suffix>
        'partof': True,                # file under the reported machine number
        'label': 'collector:partmarker',
    },
}

partof: True is the switch that matters. It files the device under the operation the PC reports AND stops that PC claiming the operation directly. Leave it False for a device that does not share an identifier - a CMM is a subordinate device too, but no two CMMs answer to one number, so it links to its PC and nothing more.

label is the relationship origin marker. Only rows carrying it are archived by a collector push, so links made by hand are never touched. Do not change an existing label: those exact strings are in production databases.

A site adds or retargets an entry without a code change, per ADR-015:

Setting: subordinatedevice_<pctype>   category: pctypemapping
Value:   {"assettype": "machine", "typename": "Marking Laser",
          "suffix": "LASER", "partof": true, "label": "collector:partmarker"}

A malformed override falls back to the built-in default rather than failing the collector push - a bad setting must not stop a bay reporting its inventory.

Making a backup kind follow the device

A backup posted by a PC that drives a device belongs to the DEVICE, not the parent. plugins/backups/services/registry.py resolves this through markerforsource: it finds the PC by the reported sourcehostname, follows the PC's active device link, and files against what it finds. It falls back to the machine number whenever the device cannot be resolved - no hostname on the payload, a lean build without the computers plugin, or a PC that has not reported yet - because filing under the parent beats rejecting a backup.

Two things that matter if you add a kind:

• sourcehostname is load-bearing, not informational. Without it a backup files against the parent and merges two devices' histories.
• A revision chain is (asset, kind, source hostname), so two devices on one parent keep separate chains even before they become separate assets.

Finding the next one

flask relationships check-shared-machines

Read-only. Lists every machine number claimed by more than one PC and separates the two cases by whether child assets exist:

  0615       11 PCs, 11 child asset(s) - modelled
  2026       2 PCs, NO child assets - F31N20R3, F4Z7S7J4

The first is composition working. The second is two PCs carrying one number - fix that on the PC.

The parent is not disposable

Once devices are filed under it, the parent is doing a real job even though it may look empty: it is the thing that says those devices belong together. Deactivating it (isactive = 0, which is what "delete" does in the UI) breaks filing, because the resolver requires an ACTIVE parent - every subsequent report warns and files nothing.

A hard SQL DELETE is worse: backuprevisions.assetid is ON DELETE CASCADE, so it destroys history still attached to the parent.

If the parent looks wrong in a list - an operation number appearing among machines - that is a CLASSIFICATION question, not a deletion one.

See also

• docs/adr/ADR-001-asset-as-platform-contract.md - relationship types, propagation rails, position inheritance
• docs/adr/ADR-015-site-specific-configuration.md - why the device map is settings-overridable
• docs/COLLECTOR-INTEGRATION.md - the collector contract and the part-marker case end to end