0003 — No default scripting engine
A bare chart is candles-only; scripting engines are opt-in.
Status: Accepted — strengthened: no engine ships in the package at all (see Decision).
Context
Vela™ can run indicator scripts, but "run a script" is not one thing — different languages need different engines, and engines are heavy, opinionated pieces (parsers, runtimes, worker transports). Bundling one engine as an always-on default would push that weight onto every chart, including charts that only ever show candles, and would quietly privilege one scripting language as the language.
We wanted the engine layer to be exactly as present as the application asks for, and we wanted failure modes to be loud rather than surprising.
Decision
Vela™ ships no default scripting engine. A bare chart renders candles only. Engines are registered explicitly — one at a time via registerEngine(language, engine), or in bulk through the dependency-injection entry — and are selected per indicator by language id.
If an indicator is added and no engine is registered for its language, the chart throws an actionable error rather than failing silently or guessing. Re-registering a language is last-wins, and the change applies to future indicators only, so already-running indicators are not yanked out from under a live chart.
No engine is shipped either. Vela™ originally bundled a Pine engine as an opt-in swappable default; it now lives in its own package, @luxalgo/vela-pinets. The decisive argument was licensing: the Pine runtime it executes is AGPL-3.0, and keeping it in the tree meant an Apache-2.0 library whose most-used feature dragged copyleft obligations behind it. Extracting it made the boundary structural rather than a matter of which import you happened to write. The engine layer is therefore the one layer with no bundled default at all — unlike the native renderer and the cache-wrapped provider feed, which remain swappable defaults that are present when chosen.
Consequences / Trade-offs
- Pay only for what you use. Charts that just plot price carry no scripting runtime — and the published bundle carries none either.
- No privileged language. The architecture treats every engine as a peer; Pine is one addon among possible others, on exactly the same port as an engine you write yourself.
- License containment. A copyleft runtime cannot reach Vela™'s license by accident: the ACL bans the import outright, so the obligation is taken on only by an application that installs the addon.
- Loud, actionable failures. Running an indicator with no matching engine fails immediately with a clear message — easier to diagnose than silent no-ops.
- Explicit setup required. Applications that want scripting must register an engine; there is a deliberate one-line cost to enabling it.
- Predictable re-registration. Last-wins-for-future-indicators avoids surprising mutation of in-flight execution at the price of two indicators briefly running on two engine versions.
Invariant
No engine ships in the package. A bare chart is candles-only; engines are registered explicitly and chosen per indicator by language; an indicator with no matching engine throws. Re-registration is last-wins and affects future indicators only.
See also: 0002 — Core owns market data, 0004 — Enforce layering with an import ACL.