Expand description
§Hydra Common — Foundation Contract
Status: v1.9 — 2026-08-08 (v1.1 added opaque per-block options
to the production contract, §3.4; v1.2 added the chart fragment item,
§3.3; v1.3 added engine availability and import formats, §2.1–2.3; v1.4
added the recognition contract and its routing rules, §2.5; v1.5 — with a
second engine implemented and able to validate them — added the element
taxonomy contract (§4), the quantity contract (§5), the result-variable
contract (§6), and the run-dispatch layering rule (§2.6); v1.6 added the
optional compact symbol on variable descriptors, §6.1; v1.7 let fragment
numbers, table columns, and chart axes reference quantity keys, §3.3,
joining the fragment model to the quantity contract so consumers format
tagged values in a chosen display family, §5; v1.8 added the
engine-authored category on block descriptors, §3.2; v1.9 — with a second
engine’s criteria implemented to validate it — added the criteria
contract, §7, moving Evolution to §8).
This file is the module documentation
of the hydra-common crate and follows the same spec-first workflow as the
engine specs: implementation changes flow from changes here, never the
reverse.
§1. Purpose and Scope
The common layer is the foundation every engine and every application may depend on. It depends on nothing else in the workspace. It exists so that applications can host any Hydra engine — present or future — through one uniform surface instead of per-engine hardcoded knowledge.
The layer defines five contracts:
- Engine identity — what an engine is, including how it is recognised from a model’s bytes (§2).
- Reportable output — what an engine can contribute to a report (§3).
- Element taxonomy — how an engine describes its model’s element vocabulary so an application can present any engine’s model (§4).
- Quantities — how an engine declares the physical quantities its values carry, so applications can format and convert them (§5).
- Result variables — how an engine describes the per-element result series a completed simulation carries (§6).
Contracts 3–5 were explicit non-goals for v1.0 (ratified 2026-07-28): abstracting them from a single implementation risked baking water-distribution assumptions into the foundation, so they were deferred until a second engine implementation existed to exercise them. The urban drainage engine is that second implementation, and its model — in particular the subcatchment, which is neither a node nor a link — is the stress test these contracts were shaped against.
Still deferred: a cross-engine simulation contract (a neutral session type every engine implements). Two engines with genuinely different run shapes are not yet evidence of the right trait; §2.6 assigns where the uniform run surface lives without defining it here. Nothing in this layer may presuppose the shape that future contract will take.
§2. Engine Identity
§2.1 Engine descriptor
Every engine publishes one immutable descriptor:
| Field | Meaning | Constraints |
|---|---|---|
key | Stable machine identifier | Lowercase ASCII, short domain-umbrella abbreviation (wds, uds, och). Never changes once released — it is persisted in project metadata and report templates. |
label | Human-facing product name | Practitioner-familiar term (e.g. “Water Distribution”). May be revised between releases. |
pill | Two-character badge | Uppercase, exactly 2 characters (e.g. “WD”). |
accent | Brand color for this engine | #rrggbb hex string. |
summary | One-sentence description of the engine’s domain | Plain text, no markup. |
status | Whether this distribution can actually run the engine | available or planned (§2.3). |
import | Source-model formats the engine imports | Ordered list of import-format descriptors (§2.2); may be empty for an engine with no import path. |
The key and the label/pill pair are two deliberately separate naming
systems: the key carries the accurate domain umbrella; the label carries
the familiar practitioner term. They are allowed to diverge and must not
be derived from one another.
§2.2 Import formats
An engine’s models originate in some external tool’s file format. The descriptor names those formats so applications can offer a correctly filtered file picker for any engine without hardcoding per-engine file knowledge:
| Field | Meaning | Constraints |
|---|---|---|
label | Human-facing format name | Plain text, e.g. “EPANET input file”. |
extensions | Filename extensions the format uses | One or more, lowercase ASCII, no leading dot. |
This is deliberately the only file knowledge in the foundation layer.
It names formats; it says nothing about their contents, and nothing here
may be used to decide whether a given file is valid. Validating that a
file really is a model of the named format is the owning engine’s job —
extensions are a picker filter and a first-pass hint, never a check. Two
engines legitimately share the inp extension (EPANET and SWMM both use
it) with entirely incompatible contents, so an application that trusted
the extension would hand a stormwater model to a water-distribution
solver.
§2.3 Availability
A registered engine is either:
available— implemented in this distribution and usable;planned— registered so applications can present it (and so its key is reserved), but carrying no implementation.
Planned engines are registered rather than hidden because a user choosing a modelling domain deserves to see what Hydra covers and what is coming, and because the key must be reserved before anything persists it.
Applications must present planned engines as explicitly unavailable and must refuse to create projects, run simulations, or import models for them. Refusing is a hard requirement, not a UI nicety: a persisted project naming a planned engine would be indistinguishable from one whose engine was removed.
Resolving a planned engine’s key is not an error and must not be conflated with the unknown-key case (§2.4) — the descriptor exists and its identity fields are valid; only its implementation is absent.
§2.4 Registry
The registry is the ordered collection of descriptors for every engine compiled into a distribution. It supports:
- Enumeration in a stable, deliberate order (the order engines are presented to users), available and planned engines alike.
- Lookup by key, which either yields the descriptor or a typed “unknown engine” error.
Applications must treat an unknown key (e.g. a project created by a newer Hydra carrying an engine this build lacks) as an explicit unsupported state, never as a fallback to a default engine.
The registry holds three registered engines — wds (available), uds
(available), and och (planned) — in that order. uds shipped planned
through v1 and became available with its engine implementation.
§2.5 Recognition
§2.2 establishes that an extension cannot decide which engine owns a file. Recognition is how that question is answered: given the bytes of a candidate model, each engine reports whether the model is one of its own.
The foundation layer defines only the neutral verdict. It contains no section names, no format grammar, and no engine vocabulary of any kind — the judgement is authored entirely by the engine, and this layer merely gives every engine the same three words to say it in:
| Verdict | Meaning |
|---|---|
definite | The bytes carry a marker that belongs to this engine’s format and to no other. |
plausible | The bytes are shaped like this engine’s format but carry nothing that distinguishes them from another engine claiming the same shape. |
no | The bytes are not this engine’s, either because the format is unrecognised or because they carry another format’s marker. May carry engine-authored text saying what the engine believes the file is instead. |
The optional text on no is the same device the reportable-output contract
uses for an unavailable block (§3.4): the foundation layer holds no words of
its own, and an engine that can say “this is a SWMM model, it declares a
[SUBCATCHMENTS] section” gives an application something far more useful to
report than a bare refusal. It is advisory — an application must behave
identically whether or not it is present.
Recognition is not validation. It answers “whose is this?”, not “can
this run?”. It must be cheap enough to run against every registered
engine before any model is parsed, so it may inspect only as much of the
input as identification requires. A definite verdict is not a promise
that the model is well-formed or simulable — that remains the owning
engine’s parse and validation step, which may still reject it.
Recognition may be stricter than parsing. An engine may decline to claim a file it would nonetheless parse successfully when told to. This is deliberate: automatic routing must not guess, whereas an explicit instruction from the user carries information routing does not have.
§2.5.1 Routing
An application holding a model of unknown provenance resolves it by asking every available engine (§2.3) and applying, in order:
- Exactly one
definite— that engine owns the model. - More than one
definite— ambiguous. This indicates two engines claiming the same marker and is a defect in one of them; report it as ambiguity rather than choosing. - No
definite, one or moreplausible— ambiguous, however few engines answered that way. - Nothing but
no— unrecognised.
Rule 3 holds even when exactly one engine answered plausible, and
even when only one engine is available at all. A plausible verdict means
precisely “I cannot distinguish this from another engine’s model”, so
acting on it is the guess this contract exists to prevent — the model may
belong to an engine that is registered but planned (§2.3), or to one a
later release adds. An engine that can genuinely identify its own models
returns definite; if it cannot, the shortfall is in its recognition, not
something routing should paper over.
The two failures are therefore distinguishable and should be reported differently: ambiguity means “narrow it down for me” and is answered by naming the engine explicitly, whereas unrecognised means no engine here reads this format at all.
Routing must never fall back to a default engine. Ambiguous and unrecognised are terminal outcomes that the application reports, offering the user the means to name the engine explicitly. Choosing arbitrarily would hand a model to a solver that models different physics and return a confident, wrong answer — the failure §2.2 exists to prevent.
Planned engines (§2.3) are not consulted, having no implementation to
consult. An application that can otherwise identify the model as a
planned engine’s — for example because the owning engine returned no
and named the foreign format — should say so rather than reporting a
generic failure: “this is a SWMM model, and that engine is not yet
implemented” is actionable where “unrecognised” is not.
§2.5.2 Layering
The registry (§2.4) is inert data and cannot invoke engines: this layer depends on nothing, and an engine’s recognition lives in the engine. The dispatch that consults each engine and applies §2.5.1 therefore belongs to a layer that sees both this contract and every engine — never to an individual application, which would duplicate the routing policy in every interface and let them drift apart.
§2.6 Run dispatch
Running a model is engine-owned, and each engine’s run has its own shape: one engine solves in phases and streams results as they become final; another steps a single cascade and writes results when it completes. Knowing those shapes — “how do I drive engine X from bytes to a results file?” — is per-engine knowledge of exactly the kind §2.5.2 forbids applications from holding, and for the same reason: an application that encodes it duplicates it in every interface, and the copies drift.
The uniform run surface — open a model for its engine, advance it, observe progress, persist its results, collect its warnings — therefore belongs to the same both-seeing dispatch layer as routing (§2.5.2), and every application drives every engine through that one implementation.
This section deliberately assigns where that surface lives and no more. Its concrete shape is the dispatch layer’s own, documented with its implementation, because a neutral session contract in this layer remains an explicit non-goal (§1): it would have to be abstracted from two run shapes that genuinely differ, and this layer must not freeze a guess. When a later engine proves the common shape, the surface graduates here as a new contract, additively (§8).
§3. Reportable-Output Contract
The contract by which an engine describes — and produces — the content blocks a report can include. Presentation (layout, styling, output formats, templates) is not part of this contract; it belongs to the report layer, which consumes this contract and knows nothing engine-specific.
§3.1 Concepts
| Term | Meaning |
|---|---|
| Block | One self-contained unit of reportable content an engine can produce (e.g. a pressure summary, a pump energy table). |
| Catalog | The engine’s complete list of block descriptors. Queryable statically — without any simulation having run. |
| Fragment | The materialized content of one block for one completed simulation. |
§3.2 Block descriptor
| Field | Meaning | Constraints |
|---|---|---|
id | Stable block identifier | Namespaced by engine key: <engine>.<name> (e.g. wds.pressure-summary). Never changes once released — report templates reference it. |
title | Default human-facing heading | Plain text. |
summary | What this block contains, for the template-builder UI | One or two sentences, plain text. |
category | Engine-authored grouping heading | Plain text, short (one or two words). Blocks sharing the exact string belong together. |
category lets a consumer with many blocks on screen group them — as
tabs, section headings, or not at all; the choice is the consumer’s.
Group order is catalog order: a category first appears where its first
block does. The string is display text with no foundation-defined
vocabulary — two engines using the same word (“Summary”) are not
thereby related, exactly as with quantity keys (§5).
The descriptor otherwise deliberately carries no result-class or prerequisite vocabulary — what a block needs from a simulation is the producing engine’s internal concern, expressed only through the production error contract (§3.4). Encoding result taxonomies (hydraulic vs. quality vs. anything else) here would bake one engine family’s domain into the foundation layer; a category is not that — it is an opaque engine-authored label carrying no semantics the foundation or the report layer can act on beyond equality.
Removing a block id, or changing the meaning of an existing id, is a breaking change to every saved template that references it and must be treated with the same gravity as a file-format break.
§3.2.1 Option descriptors
A block may accept options (§3.4). So that a template-builder UI can offer them without knowing any engine, an engine can describe the options one of its blocks accepts. A description is a list of option descriptors:
| Field | Meaning | Constraints |
|---|---|---|
key | Field name in the options object | Stable per block; renaming one is a break, like a block id. |
label | Human-facing control label | Plain text, engine-authored. |
help | One or two sentences explaining the option | Plain text, engine-authored. |
kind | What shape the value takes, and its bounds | Below. |
unit | Display unit text, or absent | Display text only — never a unit system (§3.3). |
kind is one of: number (optional default, optional inclusive
minimum and maximum), integer (same), boolean (optional default),
text (optional default), number list (optional default, optional
minimum length, and a flag requiring strict ascent — threshold edges),
choice (one of a supplied list of items), or multi-choice (any
subset of one). A choice item is an opaque value plus a label for
display.
Descriptions are resolved against a model, not fixed by the catalog. The catalog (§3.2) is static and model-free, because listing blocks must not require a loaded model. Options are the opposite: their permissible values and their correct defaults are frequently properties of the model in hand — which constituents exist, which land uses, and what unit system the file declares. An engine therefore describes a block’s options given that block’s id and the model, exactly as it produces a fragment given the model (§3.4). Only the description vocabulary lives in this layer; the model type is the engine’s own and is never named here.
This is why descriptors carry values rather than pre-rendered text: an
engine resolving minPressure for a US-customary model returns a default
of 20 with unit psi, and for an SI model 14 with unit m. A consumer
displays what it is given and computes nothing.
A description is advisory. It tells a UI what to offer; it is not the validation authority. Production (§3.4) validates independently and remains the sole judge of a malformed options value, so an engine is free to accept values no description advertised, and a consumer that skips the description entirely — as a template authored by hand does — is unaffected. Describing no options for a block means a UI offers none, not that none are accepted.
§3.3 Fragment model
Fragments are neutral data — no colors, fonts, page geometry, or format hints. A fragment is a titled sequence of items; each item is one of:
| Item | Shape | Notes |
|---|---|---|
| Key-value list | Ordered pairs of (label, value) | For scalar summaries (“Total demand”, “Simulation duration”). |
| Table | Column descriptors + row-major values | Column descriptor: name, optional unit text, value kind. |
| Note | Plain text paragraph | For caveats and methodological remarks (e.g. “Convergence relaxed at 3 timesteps”). |
| Chart | Axis labels/units + chart data (below) | Declarative data only — engines describe what is charted, never colors, geometry, or layout. |
Chart data is one of:
- Bar — parallel category labels and values (distributions, rankings). Single-series in this revision.
- Line — one or more named series of (x, y) points over a continuous x axis (time series).
Every chart must be table-derivable: renderers without graphics support present the chart as a data table derived mechanically from its data (bar → category/value rows; line → x column plus one column per series, absent where a series lacks that x). A chart therefore never gates information behind a graphics-capable format.
Values are typed: number (with optional unit text), integer, boolean,
text, timestamp, or absent. Unit strings are display text; a structured
unit system in common remains an explicit non-goal (§1).
Nested sections and images are deferred to a later revision.
Quantity-tagged numbers (v1.7). A number value, a table column, and a chart axis may each additionally reference a quantity key from the producing engine’s quantity catalog (§5). The tag changes what the number is:
- A tagged value is expressed in that quantity’s SI display unit — the same convention §5 already fixes for every quantity-bearing value crossing an engine boundary. Its unit text, when present, is the quantity’s SI label. The producer performs no display formatting beyond this; choosing a display family is the consumer’s decision, which is the reason the tag exists.
- A consumer holding the producing engine’s quantity catalog may re-express the value in either display family using only the descriptor: convert by the affine map, label from the family’s unit text, round by the family’s advisory decimals. The catalog reaches such a consumer from the application, which is the composition root (§3.5) — this layer still never resolves a key to a descriptor itself.
- A consumer holding no catalog renders the value and its unit text as written. A fragment therefore remains self-describing: the tag refines presentation, it never gates content.
- An untagged number is what every number was before v1.7: engine-authored display text, rendered as given. Tags are per-value facts, not a fragment-wide mode — one table may carry tagged and untagged columns side by side (a flow column beside an engine-spelling text column, a count beside a pressure).
A column’s tag applies to every number in that column; a chart axis’s tag applies to that axis’s coordinates in the chart data. Keys are opaque and engine-scoped exactly as in §5; a tag naming a key the engine’s catalog does not declare is a producer defect, and consumers treat the value as untagged rather than failing the fragment.
§3.4 Production
An engine produces a fragment given:
- a block
idfrom its catalog, - the artifacts of one completed simulation (results and derived analytics — the engine defines internally what it needs), and
- an optional options value: JSON-shaped structured data whose
meaning is defined entirely by the producing engine (thresholds,
top-N counts, tolerances). The foundation layer and the report layer
treat it as fully opaque — carrying it, never interpreting it. An
absent options value means the engine’s documented defaults; a
malformed options value fails production with the
failederror naming the problem. No option vocabulary may be defined in this layer.
Production is read-only and deterministic: the same simulation artifacts, block id, and options always yield the same fragment. Production deliberately takes no display-family input: a producer emits quantity-tagged values in SI display units (§3.3) and untagged values as engine-authored text, and which family a reader sees is decided where the fragment is presented, not where it is produced. Production fails with one of three neutral, typed errors:
- unknown block — the id is not in this engine’s catalog;
- unavailable — the block does not apply to this run, with a human-readable reason supplied by the engine, written as a complete sentence because a consumer may show it standing alone rather than after a label (e.g. “The run has no water-quality results.”); an expected condition, not a fault;
- failed — reading or deriving from the simulation artifacts failed.
The report layer decides how an unavailable or failed block renders (placeholder, omission) — the engine never does, and the contract carries no engine vocabulary for why beyond the engine-authored reason text.
Block options arrived in v1.1 as production inputs only. Since v1.3 an engine can additionally describe the options a block accepts (§3.2.1), so a template-builder UI can offer them generically. Production is unchanged by this: it validates the options value it is given regardless of what was described, and a hand-authored template that never consults a description behaves exactly as before.
§3.5 Consumers and dependency rules
| Layer | May depend on | Must not depend on |
|---|---|---|
common | nothing in the workspace | — |
engine-* | common | the report layer, applications |
| report layer | common | any engine-*, applications |
| applications (CLI, GUI) | everything above, via the umbrella | — |
Applications are the composition root: they obtain catalogs and fragments from engines and hand fragments to the report layer for rendering. The report layer never invokes an engine; engines never render.
§4. Element Taxonomy Contract
The contract by which an engine describes its model’s element vocabulary, so that an application can enumerate, render, and inspect any engine’s model without knowing what a junction or a subcatchment is. It follows the same discipline as recognition (§2.5) and reportable output (§3): engine-specific meaning travels only through opaque ids and engine-authored text; this layer contributes structure, never domain vocabulary.
§4.1 Element classes
The single piece of structural vocabulary this layer owns is the element class — the geometric and referential nature of an element, which an application must know to render and organise it, and which is genuinely engine-independent:
| Class | Nature | Application obligations |
|---|---|---|
point | A located element: one coordinate. | Render as a marker; selectable; may anchor polyline ends and region outlets. |
polyline | A connecting element: references a from-point and a to-point, with optional intermediate vertices. | Render as a line/path between its endpoints; selectable. |
region | An areal element: a polygon boundary, with an optional reference to a point element it discharges to. | Render as a filled polygon; selectable; the discharge reference may be visualised as a connector. |
collection | A non-spatial named object (a curve, a pattern, a time series, a control). | Enumerable and countable; presentation is application-defined and may be engine-specific. |
The class list is closed in this revision; extending it is an additive
spec change here, not an engine decision. A subcatchment is the proof case
for region: it is neither a node nor a link, and any taxonomy that
offered only those two classes would have baked one engine family’s shape
into the foundation.
§4.2 Element kinds
Within those classes, an engine describes its kinds — junction, tank, conduit, subcatchment, rain gage — as an ordered catalog of descriptors:
| Field | Meaning | Constraints |
|---|---|---|
id | Stable kind identifier | Opaque to this layer; stable per engine — persisted data and application preferences may reference it. |
label | Human-facing singular name | Plain text, engine-authored. |
label_plural | Human-facing plural name | Plain text, engine-authored. |
class | The kind’s element class (§4.1) | One of the four classes. |
role | What the kind does in the network (§4.3) | One of the three roles, or absent. |
badge | Short glyph for dense UI (markers, chips) | One or two characters, engine-authored. |
The catalog is static and model-free, like the block catalog (§3.2): an application must be able to build its chrome — tables, filters, layer toggles, legends — before any model is loaded. Kind ids follow the block-id stability rule: removing one, or changing the meaning of one, is a break on the order of a file-format break.
Identity: every element carries an engine-scoped string identifier. This layer requires only that the pair (kind id, element id) is unique within a model; whether identifiers are additionally unique across kinds (as they are within one engine’s node family) is the engine’s own rule, expressed through its validation, not through this contract.
§4.3 Element roles
A class says what an element is geometrically. A role says what it does in the network:
| Role | Meaning |
|---|---|
conveyance | Carries flow without imposing a boundary or a control on it — a junction, a pipe, a conduit. The bulk of any model. |
boundary | Where the model meets what it does not simulate: a fixed head or stage, a storage volume, an outfall. Flow enters or leaves the modelled system here. |
control | Acts on the flow rather than merely passing it — a pump, a valve, a weir, an orifice, a flow divider. |
Role exists because it is the distinction an application must draw to
present an unsimulated model at all. Before any results exist there is
nothing to colour by, and a network drawn in one uniform tone tells a
reader nothing; what they need to see is where the system is fed and
drained, and where something acts on the flow. Class cannot answer that —
a pump and a pipe are both polyline, a reservoir and a junction both
point — and kind cannot either without the application naming kinds it
should not know.
A kind may have no role at all. A rain gage is located but conveys nothing; a curve, a pattern and a control rule are not in the flow network to begin with. Those declare no role, and an application draws them by whatever means suits — the absence is information, not an omission to be defaulted away.
The role list is closed in this revision, and extending it is an additive spec change here rather than an engine decision, exactly as the class list is. Roles carry no presentation: an application decides what a boundary looks like, and this layer decides only which kinds are boundaries.
Assignment is the engine’s judgement, not a lookup. A storage unit is a boundary in drainage because it is where volume leaves the routed network, while a tank is a boundary in distribution for the same reason expressed differently. Where a kind is arguably two roles, the engine picks the one an application should draw it as.
§4.4 Attribute schemas
For each kind, an engine describes the attributes an application may display for elements of that kind — an ordered list of attribute descriptors reusing the option-descriptor vocabulary of §3.2.1:
| Field | Meaning | Constraints |
|---|---|---|
key | Field name in the element’s attribute data | Stable per kind; renaming one is a break, like a block id. |
label | Human-facing name | Plain text, engine-authored. |
kind | Value shape and bounds | The §3.2.1 kinds, unchanged. |
quantity | Key of the physical quantity the value carries (§5), or absent | Absent means dimensionless or textual. |
An attribute schema is advisory in exactly the §3.2.1 sense: it tells a generic UI what to show; it is not the validation authority, and an engine remains free to hold data no schema advertises. This revision defines attribute schemas for display. Editability, defaults, and creation flows are a later additive revision — describing them before a second engine’s editor exists would repeat the mistake §1 warns against.
§5. Quantity Contract
Fragments carry unit strings as display text (§3.3) because a rendered report needs no arithmetic. Live applications do: they let the user choose a display unit system, format values in it, and accept input in it. The quantity contract is how an engine declares the physical quantities its values carry so that applications can do that generically.
An engine publishes a static catalog of quantity descriptors:
| Field | Meaning | Constraints |
|---|---|---|
key | Stable quantity identifier | Opaque to this layer; referenced by attribute schemas (§4.3) and result variables (§6). |
si_label | Unit text in the SI display system | Plain text, e.g. “m”, “L/s”, “mm/hr”. |
us_label | Unit text in the US-customary display system | Plain text, e.g. “ft”, “gpm”, “in/hr”. |
si_to_us | Affine conversion from SI display value to US display value | A scale factor and an offset (offset 0 for all but temperature-like quantities). |
si_decimals / us_decimals | Suggested display precision per system | Advisory formatting hints. |
Values crossing an engine boundary for a quantity-bearing field are in that quantity’s SI display unit; the application converts for display and converts back on input, using only the descriptor. Engines never format, and applications never hardcode a conversion — the descriptor is the single authority, so a quantity this layer has never heard of (a rainfall intensity, an infiltration rate) costs an application nothing to support.
Quantity keys are engine-scoped: two engines may both declare a flow
quantity, and nothing requires their descriptors to agree, because no
value ever crosses between engines. The catalog is static; which
attributes and variables reference which quantities is declared where
those are declared (§4.3, §6).
This contract deliberately does not model unit systems beyond the two display families applications offer. Since v1.7 it reaches into the fragment model: a fragment number, table column, or chart axis may reference a quantity key (§3.3), in which case the value is in the quantity’s SI display unit and formatting for a display family belongs to whichever consumer presents it — the report layer at render time, a live application at display time. “Engines never format” thereby holds for fragments too: a producer that tags a value stops choosing its display family, and the engine-side conversion code that used to make that choice is deleted, not parameterised.
§6. Result-Variable Contract
The contract by which an engine describes the per-element time-series variables a completed simulation carries — pressure, flow, depth, runoff — so an application can offer result exploration (map colouring, legends, per-element series, period scrubbing) for any engine.
§6.1 Variable descriptors
For each element class it produces results for (§4.1), an engine publishes an ordered catalog of variable descriptors:
| Field | Meaning | Constraints |
|---|---|---|
id | Stable variable identifier | Opaque to this layer; application preferences and saved views may reference it. |
label | Human-facing name | Plain text, engine-authored. |
symbol | Compact notation for space-starved surfaces (column headers, chips), or absent | Engine-authored, at most three characters, ideally the domain’s standard notation (Q for discharge, y for depth, Ø for diameter). Absent means the application derives its own fallback, e.g. the label’s initial. |
quantity | Key of the quantity the values carry (§5), or absent | Absent means dimensionless. |
ramp | How values are meaningfully mapped to a colour scale | One of the ramp hints below. |
Ramp hints are the only presentation vocabulary this layer contributes, and they are shape statements, never colours:
| Hint | Meaning |
|---|---|
sequential | Magnitude on a continuous low→high scale. |
diverging | Signed values around a meaningful zero (e.g. flow direction). |
banded | Values classed into user-configurable threshold bands. |
categorical | A closed set of discrete states; the descriptor carries the engine-authored items described below, as a §3.2.1 choice does. |
An application chooses palettes, band edges, and legend styling; the engine says only which shape is truthful for the data.
§Categorical items
Each item of a categorical variable carries:
| Field | Meaning | Constraints |
|---|---|---|
value | The number the result series stores for this state | Engine-authored; unique within the variable. |
label | Human-facing name for the state | Plain text, engine-authored. |
severity | Whether the state is unremarkable, worth attention, or wrong, or absent when the states carry no such judgement | One of nominal, caution, alarm. |
Severity is a statement about the domain, not about presentation: a closed pipe is an abnormal condition in a pressurised network whoever is looking at it, and only the engine knows that. Without it an application can order states but cannot rank them, so it must colour a closed pipe and an open one as merely different — losing a distinction the engine already held. It stays optional because it is a real claim: a state set that is genuinely just a partition (a land-use class, a material) must not be forced to invent a judgement, and absent means exactly that.
As with every hint here, this fixes no colours. An application decides what caution and alarm look like, and remains free to ignore severity entirely.
§6.2 Presence
Not every catalog variable exists in every run — a quality variable is absent from a run with quality disabled. An engine therefore reports, for a given completed simulation’s results, which of its catalog variables are present, resolved the way block options are resolved against a model (§3.2.1): the catalog stays static, presence is per-run. An application offers only present variables and treats an absent one the way the report layer treats an unavailable block — an expected state, not an error.
§6.3 Addressing
Consumers address results by (element class, variable id, reporting period), and per-variable minimum/maximum envelopes are addressed by (element class, variable id). Wire encodings, caching, and file formats are the consumer’s own concern and are not part of this contract — but they must be derived from the catalog rather than fixing a variable list, or they re-create the closed-set coupling this contract exists to remove.
§7. Criteria Contract
The contract by which an engine describes — and consumes — the assessment criteria a user asserts over a model’s simulated behaviour: minimum service pressure, a self-cleansing velocity, a freeboard allowance. Criteria are engineering judgements about the network, not display settings and not part of the model; they belong to the person assessing, travel with a project, and outlive any single run.
§7.1 Concepts
| Term | Meaning |
|---|---|
| Criterion | One field of the assessment standard, described by the engine. |
| Criteria catalog | The engine’s complete, static, model-free list of criterion descriptors. |
| Valuation | A caller-held assignment of values to criterion keys. |
The foundation stays engine-blind, exactly as in §4–§6: criterion keys are opaque, meaning travels only through engine-authored text, and no criterion vocabulary (pressure, freeboard, anything else) enters this layer.
§7.2 Criterion descriptor
| Field | Meaning | Constraints |
|---|---|---|
key | Stable criterion identifier | Unique within the engine; persisted by applications, so renaming one is a break. |
label | Human-facing name | Plain text, engine-authored. |
help | One or two sentences on what the criterion judges | Plain text, engine-authored. |
quantity | §5 quantity key, or absent | Values of this criterion are expressed in the quantity’s SI display unit; absent means dimensionless. |
kind | Shape of the value | Below. |
kind is one of:
- value — a single number, with a required
default; - band — an ordered list of named cut points, each
{key, label, default}, defaults strictly ascending. A band’s value is a same-length list of numbers, ascending in the same order.
Defaults are the engine’s judgement of a conventional standard; they are advisory for editors and binding for consumption (§7.4).
§7.3 Valuation
A valuation is a JSON object: criterion key → number (value kind) or array of numbers (band kind), every number in the criterion’s SI display unit. A key absent from the valuation means the criterion’s defaults; a key the catalog does not declare is ignored, so a persisted valuation survives catalog growth. A value of the wrong shape or holding a non-finite number is malformed, and consumption refuses it with a message naming the criterion. A band value out of ascending order is well-formed but degenerate — an editor mid-edit produces one transiently, so it must not poison the whole valuation; consumption handles it per §7.4.
§7.4 Consumption
An engine derives per-block options from a valuation: given a valuation and a model, it answers with an options object (§3.2.1 shapes) for each of its criteria-shaped blocks. This mapping is the engine’s own — which blocks a criterion drives, and in what units their options are expressed, is engine knowledge that never leaks to the caller. A criterion no block consumes may still be cataloged: applications judge with criteria in more places than block production (a map colour scale), and the catalog is the single description of the standard.
An engine omits a block from its answer when the valuation cannot shape it (a degenerate band, §7.3); the block then runs on its documented option defaults. Consumption of a well-formed valuation never fails.
§7.5 Persistence and dependency rules
Persistence is the application’s concern: where a valuation lives, and per what scope (a project, a scenario), is not this contract’s business. The layering of §3.5 applies unchanged: engines depend on this crate, applications compose catalogs, valuations, and production, and this crate depends on nothing.
§8. Evolution
- All contracts evolve additively; fields are added, never repurposed.
- The element, quantity, and result-variable contracts (§4–§6) arrived in v1.5 exactly this way: as new sections, gated on a second engine implementation existing to validate them, requiring no change to the identity or report contracts. The one remaining deferred contract — a neutral simulation session — follows the same path when a further engine proves its shape (§2.6); until then only its dispatch home is assigned.
- Known additive follow-ups already anticipated: editability, defaults, and creation flows on attribute schemas (§4.3), and additional element classes (§4.1) should an engine need one.
- If a future revision must break a contract, the break follows the library release track’s semver discipline.
Structs§
- Attribute
Descriptor - Description of one attribute an application may display for elements of a kind (spec §4.4).
- BandCut
- One named cut point of a band criterion (spec §7.2).
- Block
Descriptor - Descriptor of one block in an engine’s catalog (spec §3.2).
- Category
Item - One discrete state of a
RampHint::Categoricalvariable. - Chart
- A declarative chart (spec §3.3): data plus axis labels only — engines describe what is charted, never colors, geometry, or layout. Every chart is table-derivable so it never gates information behind a graphics-capable format.
- Choice
Item - One selectable item of a
OptionKind::ChoiceorOptionKind::MultiChoice(spec §3.2.1). - Column
- Column descriptor of a table (spec §3.3).
- Criterion
Descriptor - Descriptor of one criterion in an engine’s criteria catalog (spec §7.2).
- Element
Kind - Descriptor of one element kind in an engine’s catalog (spec §4.2).
- Engine
Descriptor - Immutable identity of one Hydra engine (spec §2.1).
- Fragment
- The materialized content of one block for one completed simulation (spec §3.1): a titled sequence of items.
- Import
Format - One source-model file format an engine imports (spec §2.2).
- KeyValue
- One (label, value) pair in a key-value list (spec §3.3).
- Line
Series - One named series of (x, y) points in x order (spec §3.3).
- Option
Descriptor - Description of one option a block accepts (spec §3.2.1).
- Quantity
Descriptor - Descriptor of one physical quantity in an engine’s catalog (spec §5).
- Table
- Column descriptors plus row-major values (spec §3.3).
- Unknown
Engine Error - Lookup failure for
engine_by_key. - Variable
Descriptor - Descriptor of one result variable in an engine’s per-class catalog (spec §6.1).
Enums§
- Block
Error - Failure producing a block (spec §3.4). The report layer decides how an unavailable or failed block renders (placeholder, omission) — the engine never does, and the contract carries no engine vocabulary for why beyond the engine-authored reason text.
- Category
Severity - How remarkable one categorical state is (spec §6.1).
- Chart
Data - Chart data (spec §3.3).
- Criterion
Kind - Shape of one criterion’s value (spec §7.2).
- Display
Family - One of the two display families applications offer (spec §5).
- Element
Class - The geometric and referential nature of an element kind (spec §4.1).
- Element
Role - What an element kind does in the network, as distinct from what it is geometrically (spec §4.3).
- Engine
Status - Whether a registered engine is implemented in this distribution (spec §2.3).
- Fragment
Item - One item of a fragment (spec §3.3).
- Option
Kind - Shape and bounds of one describable block option (spec §3.2.1).
- Ramp
Hint - How a variable’s values are meaningfully mapped to a colour scale (spec §6.1).
- Recognition
- How strongly an engine claims a candidate model as its own (spec §2.5).
- Value
- One typed value inside a fragment (spec §3.3). Unit strings are display text — a structured unit system in this layer is an explicit non-goal (spec §1).
- Value
Kind - Kind of a
Value, used in column descriptors (spec §3.3).
Constants§
- ENGINES
- Every engine compiled into this distribution, in presentation order (spec §2.4) — planned engines included, so applications can present the full modelling scope rather than only what ships today.
- HYDRA_
COMMON_ VERSION - The crate version, taken from
Cargo.tomlat compile time.
Functions§
- engine_
by_ key - Resolve an engine key to its descriptor (spec §2.2).