Single Authority per Invariant
One writer for each domain whose invariants must hold
Every invariant that must hold across a set of facts is enforced by designating exactly one authority with exclusive write access to that set.
Distributed coordination problems become local correctness problems when only one process may write to a domain whose invariants must hold. A customer cannot spend more than their available balance; if only one writer maintains the balance ledger, only one writer can violate that invariant. Locking achieves the same guarantee at the cost of contention, coupling, and scale — the single writer principle achieves it by architecture instead.
Article 19 clarifies how this principle applies when cross-cutting invariants (compliance, KYC, audit) span multiple product teams: each cross-cutting obligation gets its own platform domain as single writer; product teams consume committed L0 decisions rather than re-implementing shared rules.
This is not organisational centralisation for its own sake. It is the recognition that diffuse write authority produces software and organisations where nobody can answer “who decided this?” Conway’s Law makes the isomorphism explicit: organisations that allow multiple teams to write the same domain without coordination produce systems that do the same. The reverse manoeuvre — design ownership to match desired architecture — is how coherence is maintained at scale.
The reservation pattern extends the principle without restoring synchrony: hold, proceed in parallel, commit or release. Sagas apply the same logic across bounded contexts. In every case, the invariant enforcer is identifiable, the write stream is ordered, and accountability traces to a single authority — not a committee, not a lock manager, not an emergent consensus nobody authored. See the reservation pattern and One Authority Per Invariant.