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# ADR-012 — Segment DSL v1 + Incremental Evaluation

> Declarative JSON DSL, versioned, deliberately restricted so per-event incremental evaluation is possible: typed profile attribute conditions · event aggregates with window (count, sum(property), first_seen.

Status: Proposed · Refs: D6, DEC-G5

## Decision

Declarative JSON DSL, versioned, deliberately restricted so per-event incremental evaluation is possible: typed profile attribute conditions · event aggregates with window (`count`, `sum(property)`, `first_seen`, `last_seen` over event\_name in last-N-days|all-time) · membership in other segments (acyclic) and tiers · AND/OR/NOT. Covers RFM (recency=last\_seen, frequency=count, monetary=sum). Prebuilt RFM templates ship per vertical taxonomy.
Membership materialized in `core.segment_members` with entered\_at/exited\_at history; diffs emit `segment.entered|exited`. Segments whose conditions are not incrementally evaluable are flagged `nightly_only` at creation — the builder tells the user. Full recompute per tenant runs at the data-freshness tier (nightly default) as the safety net.

## Consequences

* Real-time segment triggers with bounded compute. − Expressiveness ceiling; extending the DSL is an explicit versioned change, never ad-hoc.