Engines

Hydra is a platform, not a single solver. Each modelling domain gets its own engine, with its own data model, source-model format, and numerical methods. They all sit behind one shared toolchain: the desktop GUI, the hydra CLI, and the hydra-sdk Rust library.

Both engines run from all three surfaces, and both are edited in the GUI. The one difference is how a project starts: a water distribution project can begin from a blank network, while a drainage project begins by importing a SWMM model (there is no smallest-valid drainage model to start from). Once imported, a drainage model is edited like any other.

The engine registry

Every engine is registered in Hydra’s engine registry with an immutable descriptor: a stable key, a display label, a badge, an accent colour, a one-line summary, an availability status, and the file formats it imports. The registry is the single source of truth; applications resolve a project’s stored engine key against it rather than hardcoding names or file filters. hydra engines prints it.

A key this build has never heard of (for example a project created by a newer Hydra) is an error, and it is always surfaced as an explicit unsupported state, never silently substituted with a default engine.

Why the split matters

Two engines can claim the same file extension with wholly incompatible contents: wds and uds both read .inp, one an EPANET model and one a SWMM model. An extension is therefore a file-picker filter, never a validity test: only the owning engine’s parser can decide whether a file really is one of its models. Hydra rejects a mismatched model explicitly rather than loading a stormwater network as a plausible-looking pressurised one. See Foreign .inp dialects for what that rejection looks like in the CLI, the GUI, and the SDK.

The same separation runs through the codebase. hydra-common holds only what every engine shares (engine identity, the reportable-output contract, and the element, quantity, result-variable and criteria contracts) and carries no engine vocabulary. Engines emit neutral content fragments; hydra-report renders them without knowing which engine produced them. See Crate Layout for how the workspace is arranged.

Working with the registry from code

#![allow(unused)]
fn main() {
use hydra_sdk::common::{engine_by_key, ENGINES};

for engine in ENGINES {
    println!("{} ({}) — {:?}", engine.label, engine.key, engine.status);
}

let wds = engine_by_key("wds")?;
assert!(wds.is_available());
}

See the SDK Overview for the full engine-identity API.