CQRS Boundary Criteria: When Separate Models Finally Earn Complexity Tax

# cqrs/decision.py
from dataclasses import dataclass

@dataclass
class CqrsSignals:
 write_p95_ms: float
 read_p95_ms: float
 read_qps: float
 join_depth: int
 projection_lag_slo_ms: float
 team_can_own_two_schemas: bool

def should_split(s: CqrsSignals) -> tuple[bool, str]:
 if not s.team_can_own_two_schemas:
 return False, "no_owner_for_projection"
 if s.read_p95_ms < 80 and s.join_depth <= 2:
 return False, "single_model_still_fast"
 if s.write_p95_ms > 100 and s.read_qps > 500 and s.join_depth >= 3:
 return True, "write_path_contends_with_heavy_reads"
 if s.projection_lag_slo_ms < 200 and s.read_qps > 2000:
 return True, "hot_read_path_needs_denorm"
 return False, "tax_not_justified"

// cqrs/projection.ts
export async function projectOrderPlaced(evt: OrderPlaced) {
 await readDb.upsert({
 pk: `order#${evt.orderId}`,
 customerId: evt.customerId,
 total: evt.total,
 status: "placed",
 version: evt.version, // ✅ monotonic — reject older
 projectedAt: Date.now(),
 });
}

export async function assertLag(orderId: string, writeAt: number, sloMs = 500) {
 const row = await readDb.get(`order#${orderId}`);
 if (!row || row.projectedAt - writeAt > sloMs) {
 metrics.hit("ProjectionLagBreach");
 // ✅ Fall back to write-side get for read-your-writes sessions
 return "fallback_write_model";
 }
 return "ok";
}