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0009 — All-Rust runtime, no Python

  • Status: Accepted
  • Date: 2026-08-19

Context

v0 shipped with a Python SDK: flows were Python files run as supervised subprocesses, the bundled connectors were Python, and several runtime endpoints (surface/graph/preview/set) shelled out to python3 -m vejas. Once VejasScript (ADR-0001) could express flows natively and run in-process, Python became a second language, a second runtime, and a subprocess-per-flow tax — for no remaining benefit.

Decision

Make the runtime all Rust, Python-free. Port the remaining Python flows to VejasScript; make the bundled connectors native Rust threads (ADR-0007); remove the Python SDK and every python3 shell-out (surface/graph/preview/set are pure Rust over vjs::parse). Ship one binary on debian-slim (with curl for the Slack webhook); docker compose is two containers.

Consequences

  • One language, one process, no subprocess per flow; smaller image, simpler ops, faster reload.
  • The whole system is analyzable and testable in one place: 15 Rust unit tests on the language + 19 golden end-to-end cases (vjs-test).
  • Porting order_sync required one language addition — array concatenation (positions = positions + [{…}] in a for) — to build lists element by element.
  • Cost: contributors write Rust for the core (an external connector can still be any language over the bus, ADR-0007). Old JetStream durable consumers from the Python era should be purged to avoid replaying stale-format messages.

Alternatives considered

  • Keep Python as an escape hatch inside the core: a permanent second language and subprocess model for a capability VejasScript now covers. Rejected; the escape hatch lives at the bus boundary instead (external connectors), not inside the runtime.