ADR 0007 · Accepted · Phase 7
Status: Accepted · Phase: 7 · Supersedes: none · Superseded by: none
Context
Arc serves its own customers and partner fintechs from one deployable. Partner data must not leak across boundaries, and every partner needs a sandbox that is genuinely isolated from live.
Decision
A tenant on every record and every event. The event envelope carries tenantId; ledger accounts, journals, outbox rows and idempotency keys are all tenant-scoped.
Sandbox tenants are prefixed (sbx_…) and detectable with isSandboxTenant, which is what gates sandbox-only behaviour such as magic-amount failure injection and POST /v1/sandbox/reset.
Credentials are environment-prefixed: ak_test_ / sk_test_ versus ak_live_ / sk_live_.
Rate limits and idempotency keys are scoped per tenant, so one partner cannot exhaust another’s budget or collide with another’s keys.
Consequences
Good. One deployable, one migration path, one set of code to reason about. A key pasted into the wrong environment is visibly wrong rather than quietly wrong. Sandbox behaviour is decided by tenant, not by a request flag, so the sandbox exercises the same code path production does.
Costs. Isolation is enforced by discipline in queries rather than by physical separation — a missing tenant predicate is a leak, and nothing in the schema prevents one. Postgres row-level security would make it structural and is not implemented. Noisy-neighbour effects are bounded by rate limiting but not eliminated.
Sandbox reset does not clear the ledger — it clears quotes and transfers only. That is a known gap, not a design position.
Alternatives
A database per tenant — the strongest isolation, and unworkable for a simulation and heavy for a small partner base. Worth revisiting at scale for large partners.
A schema per tenant — migrations multiply by tenant count; the operational cost outweighs the isolation benefit at this size.
A separate deployment for sandbox — doubles the infrastructure and, worse, allows sandbox and live to drift apart in behaviour, which defeats the purpose of a sandbox. 
