# C3 — One merge, at ingestion only

**Guarantee.** Heterogeneous timestamped sources are k-way-merged by timestamp
into one chronological cycle stream at the ingestion boundary; source-native
time units (e.g. data-server's Unix-`time_ms`) are normalized there to the
canonical epoch-ns `timestamp` of C7.

**Forbids.** Any merge / as-of join *inside* the graph.

**Why.** A single ordered timeline is the mechanism that makes heterogeneous-rate
sources (news daily-bias + M5 + ticks) causally combinable without leaking the
future. Keeping the merge at one boundary keeps the graph semantics simple.

## Current state

The k-way merge is `Harness::run`'s per-cycle pick of the live source head with
the smallest `(timestamp, source index)` (`crates/aura-engine/src/harness.rs`).
Each `Source` yields records in ascending timestamp order — the ingestion
precondition the merge relies on — and there is no merge or as-of join anywhere
inside the DAG; the engine only routes in-graph edges.

The ms→epoch-ns normalization is a single seam, `aura_ingest::unix_ms_to_epoch_ns`
(`crates/aura-ingest/src/lib.rs`): the data-source ingestion edge transposes the
data-server's Array-of-Structs `M1Parsed` records into aura's Structure-of-Arrays
base columns (C7) and normalizes their Unix-millisecond time to the canonical
epoch-ns `Timestamp` at this one boundary. Both the eager `load_m1_window`
transpose and the lazy streaming `M1FieldSource` route through that same seam.

## See also

- [C4](c04-cycle-granularity.md) — the merged stream is the data-driven cycle
  clock; ties break by source declaration order.
- [C2](c02-causality.md) — one ordered timeline is what makes heterogeneous-rate
  sources combinable without look-ahead.
- [C7](c07-scalar-soa.md) — the canonical epoch-ns `Timestamp` and the four SoA
  base columns the merge normalizes into.