# C1 — Determinism and disjoint parallelism

**Guarantee.** A backtest is a deterministic, synchronous, non-concurrent event
loop that reaches a unique state after each input tick. Same input (incl. seed)
→ bit-identical run. Two backtests are fully disjoint and run concurrently
without locking. The bit-identity is *per run*: one backtest of given (inputs,
seed) reproduces byte-for-byte. It does **not** extend to a *derived metric*
recomputed for the same params by two *different command paths* (e.g. a swept
member's `sqn` vs the same cell re-run under `aura generalize`), which may differ
by floating-point reassociation (≤1 ULP) because the two paths accumulate the
same logical reduction in a different operation order (IEEE-754 non-associativity).
C1 governs the determinism of a single run, not the cross-command bit-identity of
a re-derived statistic. (fieldtest 0078, ratified.)

**Forbids.** Concurrency *within* a single sim; any nondeterministic input that
is not captured as an explicit input (see C11, C12).

**Why.** Real money rides on backtest results; reproducibility and an audit
trail are non-negotiable. Speed comes from parallelism *across* sims, which
disjointness makes lock-free.

## Current state

The per-run event loop is `Harness::run` (`crates/aura-engine/src/harness.rs`):
each cycle picks the live source head with the smallest `(timestamp, source
index)`, pops one record, increments `cycle_id`, and forwards the value —
allocating nothing per cycle beyond a reused scratch buffer. A single run is
fully sequential, so its output is bit-identical by construction; the
end-of-stream `finalize` pass runs after the loop and adds no within-sim
concurrency.

Cross-sim parallelism spans the sweep/member/window fan-out and campaign cells
(#277). The campaign executor `execute` (`crates/aura-campaign/src/exec.rs`)
flattens the (strategy, instrument, window, regime) cell matrix into
document-order `PlannedCell`s, groups them by instrument ordinal, and walks
sequential chunks of K instrument groups — `--parallel-instruments`
(`DEFAULT_PARALLEL_INSTRUMENTS = 4`, `crates/aura-campaign/src/lib.rs`). K is a
structural bound on distinct resident instruments — the RAM lever for the
external data-server's per-reference file retention (~1.4 GB per instrument on
the reference dataset), not a pool-worker count. Within a chunk, cells run
concurrently via `rayon::par_iter` on the process-global pool shared with the
member/window fan-out, and results are written into document-order `slots`, so
outputs stay byte-identical across worker counts and bounds.

Two scheduling-dependent carve-outs sit outside the per-run bit-identity, which
governs successful runs. On the run-fatal path (non-containable faults, e.g. a
dead registry store) a shared `AtomicBool` abort flag latches, and the propagated
fault is the lowest document-order fault among the cells that completed before
the latch; the set of per-cell family lines already written by then is
scheduling-dependent — inert, because no campaign-run record is written on that
path and store reads are name-keyed, never line-ordered.

Duplicate campaign instruments are refused at both the validate tier
(`crates/aura-campaign/src/lib.rs`, a `seen_instruments` scan) and the executor's
`preflight`: the per-cell registry family name embeds the raw instrument string,
so name uniqueness is what keeps concurrent appends from racing one name's
run-index assignment.

## See also

- [C4](c04-cycle-granularity.md) — tie determinism preserves this contract; the
  source-index tie-break is what keeps the merge order deterministic.
- [C11](c11-sources-record-replay.md), [C12](c12-atomic-sim-unit.md) —
  recorded/pinned inputs are how nondeterministic sources become explicit inputs;
  the harness is the atomic disjoint unit.
- [C23](c23-graph-compilation.md) — the raw-index compilation primitive; names
  survive only as non-load-bearing debug symbols.

> History: [c01-determinism.history.md](c01-determinism.history.md)