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ADR-008 draft-2: reframe from fixed model to graduated framework

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Draft-1 proposed one model of multi-tenancy with fixed characteristics.
Rejected: the estate needs a framework that can hold several situations at
once, including repos that do not separate tenants rigorously today and must
be developed toward doing so.

What changed:

- Every plane now carries an ordered ladder (I0-I3, A0-A3, E0-E4, P0-P4),
  not just placement. A service is a posture vector, not a verdict.
- D3 reversed. Draft-1 forbade RLS as a control that "looks like a database
  guarantee without being one". The observation was right, the conclusion
  wrong: RLS is E3, materially stronger than E2, and the actual error was
  calling E3 by E4's name. Platform enforcement is now the direction of
  travel and an obligation on the platform, not only on consumers.
- New governing rule: conformance is accuracy, not altitude. Declaring E0 is
  conformant; concealing it is not. Overclaiming is the only violation.
- Fixed a flaw of draft-1's own making: R0 "shared tables, tenant column" was
  an enforcement state mislabelled as placement. Removed; P ladder renumbered.
- Added methodology (analyze/establish/improve/guard) and per-level evidence
  artifacts. Guarding checks a service against its own declaration, which is
  what makes the verification problem tractable at all.
- D7 softened per operator: tiers carry minimum levels internally, marketing
  language stays free, the constraint is on evidence not vocabulary. One hard
  line kept: "another tenant cannot reach your data" requires E4.

Ratification now also tests the framework — each reviewing repo publishes its
posture vector, and if a repo cannot express itself in these ladders the
ladders are wrong, not the repo.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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canon/architecture/adr-008-multi-tenancy-model.md
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---
id: ADR-008
type: architecture-decision-record
title: "Multi-Tenancy Model: Four Planes, One Isolation Ladder"
title: "Multi-Tenancy Framework: Four Planes, Graduated Levels, Declared Posture"
status: proposed
decided_by: Bernd Worsch
date: "2026-08-17"
tags: ["architecture", "multi-tenancy", "isolation", "placement", "tenant-engine", "flex-auth", "rapp-postgres", "scaling"]
revision: "draft-2"
tags: ["architecture", "multi-tenancy", "isolation", "placement", "maturity", "tenant-engine", "flex-auth", "rapp-postgres", "scaling"]
---
# ADR-008: Multi-Tenancy Model — Four Planes, One Isolation Ladder
# ADR-008: Multi-Tenancy Framework — Four Planes, Graduated Levels, Declared Posture
## Status
**Proposed.** Drafted 2026-08-17 at the operator's request. This document
decides things currently owned by `tenant-engine`, `flex-auth`,
`rapp-postgres`, `adaptive-pricing`, and `railiance-platform`. It must be
reviewed by those owners before ratification; §12 lists what each is being
asked to accept. Until ratified it governs nothing, and the existing
per-repo documents remain authoritative for their own repos.
**Proposed, draft-2.** Draft-1 (2026-08-17) proposed a single model with fixed
characteristics. It was rejected on the grounds that the estate does not need
one model of multi-tenancy — it needs a framework able to describe, hold, and
improve several situations at once, including repos that do not separate
tenants rigorously today and must be developed toward doing so. Draft-2 is
that reframe.
## Context
This document decides things currently owned by `tenant-engine`, `flex-auth`,
`rapp-postgres`, `adaptive-pricing`, and `railiance-platform`. It must be
reviewed by those owners before ratification; §18 lists what each is being
asked to accept.
## 1. Context
The estate has been building multi-tenancy for months and has never written
down what it is building. Five documents each cover a slice:
@ -32,260 +38,432 @@ down what it is building. Five documents each cover a slice:
| `rapp-postgres/docs/adr/ADR-0001` | Consumer + tenant isolation in PostgreSQL | Proposed, governs one repo |
| `rapp-postgres/docs/canon-drafts/shared-platform-relational-storage_v0.1` | The stacked-boundary gap | Routed 2026-08-10, **still unratified** |
The slices do not compose, and three specific failures follow.
Four failures follow.
**The gap has been diagnosed once and the fix stalled.** The
shared-platform-relational-storage draft was written precisely to fill this
hole. It has sat unratified for a week and appears in neither canon
directory. Producing a second orphaned draft would make things worse, not
better; §13 therefore attaches a ratification path to this one.
shared-platform-relational-storage draft was written to fill this hole. It has
sat unratified for a week and appears in neither canon directory. §19 attaches
a ratification path so this one does not join it.
**Placement is owned by nobody.** Whether a workload gets a dedicated
PostgreSQL cluster or a shared one is currently precedent-by-accident:
`user-engine-pg` and `target-revenue-pg` are dedicated; `apps-pg`,
`net-kingdom-pg`, `platform-pg`, `state-hub-db` and `forgejo-db` are shared.
Both patterns are live, neither is written down, and each new service copies
whichever neighbour it happened to consult. `tenant-engine` raised this with
`railiance-platform` on 2026-08-16 and it is unanswered.
**Placement is owned by nobody.** `user-engine-pg` and `target-revenue-pg` are
dedicated; `apps-pg`, `net-kingdom-pg`, `platform-pg`, `state-hub-db` and
`forgejo-db` are shared. Both patterns are live, neither is written down, and
each new service copies whichever neighbour it consulted. `tenant-engine`
raised this with `railiance-platform` on 2026-08-16; unanswered.
**Two contradictory tenancy defaults are already ratified.**
`business-app-service-contract` §1.2 makes *instance-per-client* the default
for business apps. Platform services such as `audit-core` and `tenant-engine`
pool all tenants into one database. Both are correct for their case, but
nothing states which shape a new service takes, and the estate has no
definition distinguishing "business app" from "platform service". A new
repo can read either document and be conformant with one while violating the
other.
`business-app-service-contract` §1.2 makes instance-per-client the default for
business apps. Platform services pool all tenants into one database. Nothing
states which shape a new service takes, and no definition distinguishes the
two categories.
## Decision
**There is no honest way to describe a repo that is not there yet.** This is
the failure draft-1 missed. The estate absorbs and will keep absorbing repos
with weak or absent tenant separation — app-local accounts, no tenant column,
ad-hoc filtering. Today such a repo is simply non-conformant, which gives it
two bad options: misrepresent its posture, or stay outside the framework
entirely. Both are worse than a framework that can say "this repo is at level
1, its target is level 3, here is the gap and the evidence."
### 1. Multi-tenancy is four orthogonal planes, not one property
## 2. What this document is
The recurring confusion is that "is this multi-tenant?" is treated as a
single question. It is four, and they are independent:
**A framework, not a model.** It does not specify one correct multi-tenancy
implementation with a fixed set of characteristics. It supplies:
| Plane | Question | Owner | State |
|---|---|---|---|
| **Identity** | How is a tenant named and validated? | `tenant-engine` / IAM Profile | Ratified, solid |
| **Authorization** | How is a request bound to the tenants it may act for? | `flex-auth` | Contract ratified |
| **Data isolation** | Where does the boundary between two tenants' rows sit? | The consuming service | Stated in ADR-0001, unverified fleet-wide |
| **Placement** | Which physical substrate holds a tenant's data? | *unowned* | Precedent only |
- **Terminology** — four planes, and graduated levels within each (§3, §4).
- **A declaration** — the posture vector, so any repo can state where it
actually is (§5).
- **A conformance rule** — accuracy, not altitude (§6).
- **Methodology** — analyze, establish, improve, guard (§12).
- **Evidence definitions** — what proves a claimed level (§13).
Treating them as one is not a theoretical tidiness problem; it is producing
concrete errors today.
A service is conformant when its declared posture is accurate and its
trajectory is recorded. A service is non-conformant when it claims a level it
cannot evidence — regardless of how high or low that level is.
`rapp-postgres`'s `PostgresConsumer` declaration carries a field
`tenantIsolation: consumer-service-boundary`. That is a **data-isolation**
fact recorded in a **placement** artifact. It reads as though the storage
platform enforces something. It does not, and ADR-0001 §4 is explicit that it
must not pretend to.
## 3. Four orthogonal planes
The "dedicated versus shared" argument routinely mixes placement (a capacity
and blast-radius decision) with data isolation (a correctness decision).
`tenant-engine` drew the right distinction unprompted on 2026-08-16 —
sharing a *cluster* is capacity and cheap to undo; sharing a *database*
entangles schemas and is not — and that distinction is adopted here as
binding.
"Is this multi-tenant?" is treated as one question. It is four, and they are
independent:
**Decision:** every document, declaration, and plan tier that says
| Plane | Question | Vocabulary owner |
|---|---|---|
| **Identity (I)** | How is a tenant named and validated? | `tenant-engine` / IAM Profile |
| **Authorization (A)** | How is a request bound to the tenants it may act for? | `flex-auth` |
| **Enforcement (E)** | Where, mechanically, is the tenant boundary enforced? | This framework |
| **Placement (P)** | Which physical substrate holds a tenant's data? | `railiance-platform` |
Treating them as one produces concrete errors today. `rapp-postgres`'s
`PostgresConsumer` carries a field `tenantIsolation: consumer-service-boundary`
— an **E**-plane fact recorded in a **P**-plane artifact, reading as though the
storage platform enforces something it does not. The "dedicated versus shared"
argument routinely mixes P (capacity, blast radius) with E (correctness).
The planes are separated **precisely so that each may sit at a different
level**. A repo can be strong on identity and weak on enforcement, or the
reverse, and the framework must state that without flattening it to a single
verdict. That is the whole point, and draft-1 lost it by collapsing E into a
single fixed rule.
**Decision 3.1:** every document, declaration, and plan tier that says
"isolation" MUST name which plane it means.
### 2. The identity and authorization planes are settled; this ADR does not reopen them
**Decision 3.2:** the planes couple at their top levels — `E4` is only
reachable at `P3` or above — and that coupling MUST be stated where it applies
rather than used to argue the planes are one.
Recorded here only so the model is complete and so no future document
re-derives them:
## 4. Graduated levels
- Tenant identifiers are `tenant:<grouping>:<name>` per ADR-0013, with
`tenant:platform` and `tenant:coulomb` reserved and ungrouped. `tenant-engine`
owns which values exist; the IAM Profile owns the wire format.
- Capability roles `PLTF`/`IAM`/`VEN`/`CUS` are non-exclusive and are a
separate fact from grouping. `tenant-engine` is the source of truth; the
`tenant_roles` token claim is a point-in-time cache that MUST NOT be trusted
for privileged, destructive, credential-vending, or `aal2`-class decisions.
- `flex-auth` makes authorization decisions; `tenant-engine` is a data source
and makes none.
- Storage platforms MUST NOT mint, parse, or authorize on tenant identifiers.
To them these are opaque strings.
Each plane carries an ordered ladder. Higher is stronger, not better: the
right level is the one a service can evidence and its risk warrants.
### 3. Data isolation is enforced by the consuming service, and the platform must never imply otherwise
### 4.1 Identity (I) — how the tenant is named
This is ADR-0001 §4 promoted to fleet scope.
| Level | State |
|---|---|
| **I0** | No tenant concept. Data is not attributable to a tenant. |
| **I1** | A local tenant notion exists but does not conform to `tenant:<grouping>:<name>`. |
| **I2** | Canonical identifiers used, validated against the ratified vocabulary; `tenant-engine` is the source of existence. |
| **I3** | I2 plus capability roles (`PLTF`/`IAM`/`VEN`/`CUS`) honoured, with live `tenant-engine` re-query for privileged, destructive, credential-vending or `aal2`-class decisions rather than trusting the cached `tenant_roles` claim. |
A consumer connects through a pooled role shared across all of its tenants.
The database therefore sees one principal, not the requesting tenant, and has
no trustworthy basis on which to filter. Any database-level tenant filter
would depend on the consumer correctly setting a session variable — which is
the application-level check the consumer already owes, relocated somewhere it
*looks* like a database guarantee without being one.
Note `business-app-service-contract` §2.1 sets app-local accounts as the v1
baseline for business apps — an explicitly sanctioned low level with recorded
triggers for moving up. That is the pattern this framework generalises.
- **Mandatory:** consumer-owned data is tenant-keyed from the first
migration, even where the consumer serves exactly one tenant today.
- **Mandatory:** the consumer binds the authenticated identity to the tenants
it may act for, at its own service boundary.
- **Available, opt-in:** row-level security keyed on a session GUC set at pool
checkout, as defence in depth behind a correct service-side check — never
as a substitute for one.
- **Prohibited:** describing the opt-in layer as a platform guarantee.
### 4.2 Authorization (A) — how a request is bound to a tenant
**This is the model's largest residual risk and §9 does not soften it.**
| Level | State |
|---|---|
| **A0** | No authorization, or tenant context not carried on the request. |
| **A1** | Ad-hoc checks scattered through handlers. |
| **A2** | A single local authorization boundary; tenant context bound once, centrally. |
| **A3** | Decisions delegated to `flex-auth` as PDP, with live re-query where the IAM Profile requires it. |
### 4. The isolation ladder
### 4.3 Enforcement (E) — where the tenant boundary actually holds
Placement decisions are expressed as rungs. Each rung states what it
actually buys, and — critically — what it does *not* change.
This is the reframed core. Draft-1 asserted one rule for everyone; this is a
ladder whose top is platform-enforced.
| Rung | Shape | Data-isolation guarantee | Blast radius of a leaked runtime credential | Live occupants |
|---|---|---|---|---|
| **R0** | Shared tables, tenant column | App-enforced | All tenants of that consumer | Inside every consumer today |
| **R1** | Database per consumer, shared cluster | App-enforced *(unchanged)* | All tenants of that consumer | `audit-core`, `tenant-engine` on `platform-pg` |
| **R2** | Dedicated cluster per consumer | App-enforced *(unchanged)* | All tenants of that consumer | `user-engine-pg`, `target-revenue-pg` |
| **R3** | Dedicated cluster per tenant | Structural | One tenant | Business apps per `business-app-service-contract` §1.2 |
| **R4** | R3 + separate region/jurisdiction | Structural + residency | One tenant | None |
| Level | Mechanism | Bypassed by |
|---|---|---|
| **E0** | None. Data not tenant-keyed; separation is incidental or absent. | Anything. |
| **E1** | Data tenant-keyed, filtering applied per query at call sites. | One missing predicate. |
| **E2** | Filtering centralised at a single service-side choke point that binds authenticated identity to permitted tenants. | Code that bypasses the choke point. |
| **E3** | Platform-assisted: row-level security keyed on a session GUC set at pool checkout, or an equivalent data-access layer the application cannot trivially route around. | A `SECURITY DEFINER` function, a missing policy, or a wrong GUC. |
| **E4** | Structural: the connection a workload holds cannot address another tenant's data at all. Requires per-tenant credentials and per-tenant substrate. | Only a platform-level compromise. |
The column that matters is the third one. **R0 → R1 → R2 does not improve
tenant isolation at all.** Those rungs buy consumer isolation, capacity
predictability, independent backup retention, and a smaller operational blast
radius. The tenant boundary stays exactly where §3 puts it: in application
code. Only R3 makes it structural.
Three properties of this ladder matter.
**Decision:** R1 is the default for platform services. R3 is the default for
client-facing business apps, as already ratified. A service that is unsure
which it is MUST resolve that before choosing a rung (§12.4).
**E3 is real and is not forbidden.** Draft-1 rejected RLS on the grounds that
a control depending on the consumer setting a session variable "looks like a
database guarantee without being one". The observation is correct; the
conclusion was wrong. E3 is materially stronger than E2 — it converts "every
call site must remember" into "one checkout path must be right" — and it is
the standard mitigation in pool-model systems. What was actually wrong was
*calling E3 by E4's name*. The honesty requirement survives; the prohibition
does not.
### 5. Movement between rungs must be an operational change, not a rebuild
**Higher enforcement is the direction of travel, not an optional extra.**
Services holding data whose cross-tenant exposure would be a reportable breach
SHOULD target E3 or above. The framework's default expectation for a new
platform service is **E2 at first serve, E3 recorded as target**.
Adopted from `tenant-engine`'s TEN-WP-0009 design, which got this right:
**Platform enforcement is a platform obligation, not only a consumer one.**
Draft-1 placed the whole burden on consumers permanently. It is now stated
that reaching E3 requires the storage platform to *offer* the mechanism —
provisioned RLS policies, a documented GUC contract, and a probe.
`rapp-postgres` ADR-0001 §4 already commits to provisioning it on request;
that offer becomes an obligation with a defined contract (§18.6). Where a
consumer wants E3 and the platform has not supplied the mechanism, the gap is
the platform's, not the consumer's.
- Connect by injected credential only. No cluster, host, namespace, or
database name anywhere in source.
### 4.4 Placement (P) — which substrate holds the data
Draft-1's `R0` ("shared tables, tenant column") is removed: that was an
E-plane state mislabelled as placement. Corrected ladder:
| Level | Shape | Live occupants |
|---|---|---|
| **P0** | Shares a database with another consumer (schema or tables co-resident). | None sanctioned; the state absorbed repos often arrive in. |
| **P1** | Database per consumer, shared cluster. | `audit-core`, `tenant-engine` on `platform-pg` |
| **P2** | Dedicated cluster per consumer. | `user-engine-pg`, `target-revenue-pg` |
| **P3** | Dedicated cluster per tenant. | Business apps per `business-app-service-contract` §1.2 |
| **P4** | P3 plus separate region or jurisdiction. | None |
**P0 → P1 → P2 does not raise the E level.** Those steps buy consumer
isolation, capacity predictability, independent backup retention, and a
smaller operational blast radius. They leave the tenant boundary exactly where
the E level puts it. Only P3 makes E4 reachable. This is the single most
misusable fact in the framework and §11 governs how it may be described
commercially.
**Decision 4.5:** P1 is the default for platform services; P3 is the default
for client-facing business apps, as already ratified. A service unsure which
it is must resolve that before choosing (§18.4).
## 5. The posture vector
A service states its tenancy posture as one level per plane, plus a target and
a date:
```yaml
tenancy:
current: { I: 2, A: 3, E: 2, P: 1 }
target: { I: 2, A: 3, E: 3, P: 1 }
reviewed: "2026-08-17"
gap:
E: "Choke point exists and is tested; RLS not yet provisioned. Blocked on
rapp-postgres offering the GUC contract. Target Q4."
```
Worked examples, as best assessed today and subject to owner correction:
| Service | Current | Notes |
|---|---|---|
| `tenant-engine` | `I2 A3 E2 P1` | Moving to P1 under TEN-WP-0009; E2 via its own service boundary. |
| `audit-core` | `I2 A3 E2 P1` | Same shape; holds audit data, so E3 is the more urgent target. |
| A newly absorbed repo | `I1 A1 E1 P0` | Conformant **if declared**, with a recorded path. |
The vector is what makes the framework hold both situations honestly. A repo
at `I1 A1 E1 P0` is not a failure — it is a described state with a direction.
A repo claiming `E3` with no RLS policy is a failure regardless of how good the
rest of it is.
**Decision 5.1:** the posture vector is declared in the repo, not in the hub,
consistent with local-files-are-source-of-truth.
## 6. Honest absorption — conformance is accuracy, not altitude
The governing rule of this framework:
> **A service is conformant when its declared posture is accurate, its target
> is recorded, and it does not claim a level it cannot evidence. It is
> non-conformant when it overclaims — at any altitude.**
Consequences, stated deliberately:
- Declaring `E0` is conformant. Concealing `E0` is not.
- A repo may be absorbed at any posture. It may not be absorbed silently.
- No service is blocked from the estate for being low on a ladder. Services
MAY be blocked from *specific work* — serving a tenant grouping, holding a
data class, carrying a plan tier — by requirements expressed as minimum
levels (§11).
- Downgrading is permitted and must be declared. A regression discovered by
guarding (§12) is a defect; a regression declared in advance is a decision.
This is what the plane separation is *for*. Without it, "not rigorous about
tenant separation" is a single verdict that a repo either passes or fails.
With it, the same repo is `I1 A1 E1 P0` with a path — which is a plan, not an
indictment.
## 7. Portability across placement levels
Movement between P levels must be an operational change, not a rebuild.
Adopted from `tenant-engine`'s TEN-WP-0009 design:
- Connect by injected credential only — no cluster, host, namespace, or
database name in source.
- Own a whole database, never tables inside someone else's. Sharing a cluster
is reversible; sharing a database makes relocation a schema merge.
- Idempotent schema creation, so a fresh target comes up correct.
- Idempotent schema creation.
- No cross-database joins or co-location assumptions.
A service meeting these moves R1 → R2 by: create target, dump, restore, swap
the credential reference, restart. No rebuild, no release, no code review.
A service meeting these moves P1 → P2 by: create target, dump, restore, swap
the credential reference, restart.
**Decision:** these four properties are mandatory at every rung, including
R3. A business app at R3 that hard-codes its database name has forfeited
consolidation, which §1.3 of the business-app contract exists to preserve.
**Decision 7.1:** mandatory at P1 and above. At P3, SHOULD rather than MUST —
the consolidation `business-app-service-contract` §1.3 preserves is a real
benefit, but a per-client instance that never moves is not misconformant for
naming its own database.
### 6. Placement triggers are recorded in advance
## 8. Placement triggers
A rung change decided under pressure is decided badly. Each service records,
at provisioning time, what would move it up:
A level change decided under pressure is decided badly. Each service records
at provisioning time what would move it:
- Noisy neighbour on a latency-critical path
- A compliance or residency requirement
- A plan tier that sells isolation (see §7)
- A plan tier requiring a higher minimum (§11)
- Backup retention or RTO no longer fitting
- Connection or memory ceiling reached (see §9)
- Connection or memory ceiling reached (§16)
**Decision:** placement ownership is `railiance-platform`'s. This ADR
proposes the ladder and the triggers; it does not appoint per-workload
placement, which remains that repo's call and is the substance of the
2026-08-16 request still open there.
**Decision 8.1:** triggers MUST be *monitored*, not merely recorded. A trigger
in a YAML comment that nobody re-reads is documentation, not control. The
`platform-pg` ceiling in §16 is the first that requires an actual alert.
### 7. Isolation as a product property — the commercial hazard
**Decision 8.2:** placement policy ownership is proposed to
`railiance-platform`. This framework supplies the ladder, the triggers, and
the declaration; it does not appoint per-workload placement. See §18.2 — this
remains their call and their open request.
Plan tiers that "sell isolation" are anticipated. The hazard is precise and
worth stating before revenue depends on it:
## 9. Credentials as a tenancy control
**Selling a move from R1 to R2 as stronger tenant isolation would be false.**
Per §4 it changes nothing about the tenant boundary. A customer buying
"isolated" and receiving R2 gets better noisy-neighbour behaviour and an
independent restore path — real value, and not what they think they bought.
Only R3 changes the answer to "can another tenant's data reach mine".
Short-lived leased credentials from OpenBao, re-read at connection checkout,
with overlap-first rotation, bound the residual risk at every E level below
E4: a leaked credential expires rather than persisting. This is stronger than
the industry norm of a long-lived per-service secret and belongs in the
tenancy model, not only in credential-management canon.
**Decision:** any plan tier naming isolation MUST cite the rung it delivers
and MUST NOT describe a lower rung in R3's language. `adaptive-pricing` owns
plan terms and `tenant-engine` owns which plan a tenant holds; neither
currently maps a tier to a rung, and §12.5 asks them to.
**Decision 9.1:** static long-lived database credentials are not a sanctioned
path for any service above E0.
## 8. Alternatives considered
## 10. Blast radius must be published
**Schema-per-consumer in one database.** Cheaper on connections, allows
cross-consumer queries. *Rejected on catalog leakage:* `pg_catalog` is
readable per-database, so every co-resident can enumerate every other's
table and column names regardless of `GRANT`s. Table names alone leak
business structure and no privilege setting removes that.
**Decision 10.1:** every platform holding consumer data MUST publish, in
concrete terms, what a leaked runtime credential can and cannot reach at the
levels it operates. `rapp-postgres` ADR-0001 §5 is the reference
implementation. Where the model cannot provide a guarantee, the platform says
so and names the escalation, rather than describing a weaker control in
stronger language.
**Row-level security as the consumer boundary.** *Rejected:* puts a
correctness-critical boundary inside query predicates, where one missing
policy or a `SECURITY DEFINER` function silently removes it. Retained as
opt-in defence in depth for the tenant boundary only (§3).
## 11. Commercial expression — room to be successful
**Platform-enforced tenant RLS by default.** *Rejected:* with a pooled
per-consumer role it depends on the consumer setting a session variable
correctly, producing the appearance of a database guarantee over what is
still an application check. A control that misrepresents where it is enforced
is worse than an honest absence.
Isolation will be sold. The framework must let commercial and presentation
work rather than obstruct them, while preventing a claim that cannot be
evidenced.
**Dedicated instance per consumer as the default (R2 everywhere).**
*Rejected as a default* on operational cost — every instance multiplies
backup, upgrade, and monitoring surface — but it is the honest answer for a
consumer that cannot accept §9's residual risk, and it stays available as an
escalation rather than being argued away.
**Decision 11.1:** plan tiers are expressed internally as **minimum levels**,
not as implementation labels. A tier may require `E3 P2`; it need not print
that string anywhere customer-facing.
**Instance-per-tenant everywhere (R3 as the fleet default).** *Rejected:* at
the estate's tenant grouping taxonomy — which includes `consumer` (private
individuals) and `family` — R3 per tenant is economically impossible. The
taxonomy itself is evidence that pooling is required; you cannot run a
cluster per private individual.
**Decision 11.2:** marketing and product language is free. There is no
requirement to expose level labels, ladder names, or this document to
customers. "Dedicated infrastructure", "isolated tenancy", "private instance"
all remain available.
**One tenancy model for the whole estate.** *Rejected:* the ratified
business-app contract already commits client-facing apps to R3, and platform
services genuinely cannot follow it. The honest answer is two defaults with a
stated rule for which applies, not a single default that one half violates.
**Decision 11.3:** the constraint is on **evidence, not vocabulary**. Any
customer-facing isolation or availability claim must map to a minimum level
the delivering service actually holds, recorded once when the tier is defined.
The review is internal and happens at tier definition — not per campaign, per
page, or per conversation.
## 9. Challenge against outside practice
**Decision 11.4:** the one hard line — a claim that another tenant *cannot*
reach the customer's data requires **E4**. Everything softer than that
absolute has room. This is the claim that reaches compliance questionnaires
and contracts, and it is the one that must be true.
Held against the common industry framing (AWS's silo/pool/bridge model,
cell-based architectures), the estate lands as follows.
`adaptive-pricing` owns plan terms; `tenant-engine` owns which plan a tenant
holds. Neither currently maps a tier to minimum levels; §18.5 asks them to,
for tiers making isolation or availability claims only.
**Where the model is stronger than typical.** Most SaaS multi-tenancy
literature models exactly one boundary — tenant-to-tenant. This estate has
**two stacked boundaries**: platform-service-to-platform-service (the
consumer boundary) and tenant-to-tenant (the princedom boundary). Naming them
separately and refusing to enforce both with one mechanism is a genuine
strength and is uncommon.
## 12. Methodology — analyze, establish, improve, guard
The refusal to overstate the guarantee is also better than typical. Vendors
routinely describe pool-model RLS as tenant isolation. ADR-0001 §5 states the
blast radius in concrete terms instead, which is the practice worth keeping.
The framework is operated as a cycle, not consulted as a reference.
**Where the model is weaker than typical, and this is the finding that
matters.** The pool model's standard mitigation is a *verified* enforcement
layer — a shared data-access library, or RLS, that every service is
demonstrably routed through. This estate has neither. §3 obliges every
consumer to enforce the tenant boundary in its own code, and there is
currently **no fleet mechanism that verifies any consumer actually does**.
**Analyze.** Assess a repo against the four ladders and produce its current
vector. Output: the `tenancy.current` block with the reasoning recorded.
Applies to new services and absorbed ones alike.
`audit-core` has AUDIT-WP-0004-T03; `tenant-engine` has its own checks;
`rapp-postgres` runs 15 adversarial probes against the *consumer* boundary
and none against the tenant boundary inside a consumer. A single missing
`WHERE tenant_id = ?` in any consumer is a cross-tenant data breach that no
platform control would catch, no probe would fail, and no log would show as
an error. This is the highest-severity known gap in the model and §12.3
proposes an owner for it.
**Establish.** Declare the target vector and the gap. The target is set by the
data class held, the tenant groupings served, and any plan tier carried — not
by ambition. Output: `tenancy.target` plus a recorded gap per plane.
**Improve.** Move one plane at a time. Cross-plane moves are where mistakes
happen — raising P while leaving E untouched is exactly the misstep §4.4
warns about. Each step is ordinary repo work with ordinary evidence.
**Guard.** Verify continuously that the declared posture still holds. Guarding
checks a service **against its own declaration**, not against a universal
maximum. This is what makes the verification problem tractable: nobody has to
prove every service is at E4; the check is that nobody is below what they
declared, and nobody claims what they cannot evidence.
Regression found by guarding is a defect; regression declared in advance is a
decision. The estate has been bitten twice by silent pin rollbacks that
produced ordinary-looking 403s and 404s rather than errors. Posture regression
will look the same, so guarding must be designed for invisible failure rather
than for crashes.
## 13. Evidence per level
Guarding requires each level to have a defined evidence artifact. Proposed:
| Level | Evidence |
|---|---|
| **I2** | Identifiers validated against the ratified vocabulary; rejection test for a malformed tenant id. |
| **I3** | Live re-query demonstrated on an `aal2`-class path; cached-claim path shown not to be used there. |
| **A2** | Single choke point identified; test that an unbound request is refused. |
| **A3** | Live decision against `flex-auth` with a denial observed at the endpoint, not only at the decision surface. |
| **E1** | Every tenant-owned table carries the tenant key. Schema-level check, mechanical. |
| **E2** | Choke point identified; adversarial test that an identity bound to tenant A cannot read tenant B. |
| **E3** | RLS policies present on every tenant-owned table; probe that a session without the GUC set reads nothing; probe that a wrong GUC reads nothing. |
| **E4** | Per-tenant credential demonstrated unable to connect to another tenant's substrate. |
| **P1–P4** | Provisioning declaration plus the platform's own isolation probes. |
**Decision 13.1:** a level is claimed only with its evidence artifact present.
This turns §6's accuracy rule from an honour system into a check.
**Decision 13.2:** the E2 and E3 evidence artifacts do not exist anywhere in
the estate today. `rapp-postgres` runs 15 adversarial probes, all against the
*consumer* boundary, none against the tenant boundary inside a consumer. This
is the framework's largest live gap and §18.3 seeks an owner.
## 14. Alternatives considered
**One fixed model with a single set of characteristics** (draft-1). *Rejected
by the operator:* it cannot describe a repo that is not there yet, forcing
absorbed repos either to misrepresent their posture or to stay outside the
framework. A framework that can only describe its own end state is not a
framework.
**A maturity model with a single overall level** (bronze/silver/gold).
*Rejected:* collapses the plane separation that makes the framework useful. A
service strong on identity and weak on enforcement has a specific, actionable
gap; one composite score hides it and invites averaging.
**Schema-per-consumer in one database** (sanctioning P0). *Rejected:*
`pg_catalog` is readable per-database, so every co-resident enumerates every
other's table and column names regardless of `GRANT`s. Retained in the ladder
as a describable state, never as a sanctioned target.
**Row-level security as the *consumer* boundary.** *Rejected:* puts a
correctness-critical boundary between two services inside query predicates.
Retained as E3 for the *tenant* boundary, where the realistic alternative is
application code rather than a database.
**Mandating E4 for everyone.** *Rejected:* the tenant grouping taxonomy
includes `consumer` (private individuals) and `family`. A cluster per private
individual is economically impossible; the taxonomy is itself evidence that
pooling is required.
**Leaving each repo to its own model.** *Rejected:* this is the status quo,
and it produced two contradictory ratified defaults plus an unowned placement
question.
## 15. Challenge against outside practice
**Where the framework is stronger than typical.** Most SaaS multi-tenancy
literature (AWS's silo/pool/bridge framing among them) models exactly one
boundary — tenant-to-tenant. This estate has **two stacked boundaries**:
platform-service-to-platform-service, and tenant-to-tenant inside a consumer.
Naming them separately and refusing to enforce both with one mechanism is
uncommon and correct.
Graduated per-plane levels are also stronger than the usual silo/pool/bridge
trichotomy, which describes deployment shape only. Silo/pool/bridge is
approximately this framework's P plane with the other three missing — which is
why it cannot express "pooled infrastructure, structurally enforced
boundary", or the reverse.
**Where it is weaker, and this is the finding that matters.** The pool model's
standard mitigation is a *verified* enforcement layer that every service is
demonstrably routed through. The estate has the concept (E2/E3) and none of
the verification (§13.2). A single missing `WHERE tenant_id = ?` in any
consumer is a cross-tenant breach that no platform control catches, no probe
fails, and no log shows as an error.
**Where the estate has quietly adopted a good pattern without naming it.**
Short-lived leased credentials from OpenBao, re-read at connection checkout,
with overlap-first rotation, is stronger than the industry norm of a
long-lived per-service secret. It bounds §9's residual risk by credential
lifetime. It deserves to be stated as a tenancy control, not just a
credential-management one.
Short-lived leased credentials re-read at checkout, with overlap-first
rotation, beat the industry norm of a long-lived per-service secret. §9
promotes it to a tenancy control.
**Cell-based architecture is the unexplored option.** R2 and R3 partition by
consumer or by tenant. Neither partitions by *cell* — a fixed-size slice of
infrastructure serving a bounded set of tenants, sized so that one cell's
failure is survivable and cell count scales linearly. Given the estate spans
railiance, NetKingdom, HelixForge and Coulomb with tenants layered on top,
cells may fit better than per-tenant clusters at the point where R1 runs out.
Not proposed here; flagged in §12.6 as worth evaluating before the ceiling in
§10 is hit rather than after.
**Cell-based architecture remains unexplored.** Neither P nor E describes a
cell — a fixed-size slice of infrastructure serving a bounded set of tenants,
sized so one cell's failure is survivable and cell count scales linearly.
Given the estate spans railiance, NetKingdom, HelixForge and Coulomb, cells
may fit better than per-tenant clusters at the point P1 runs out (§18.8).
## 10. Scaling demands — the ceiling is closer than it looks
## 16. Scaling demands
Measured against the live `platform-pg` specification, not estimated.
@ -293,90 +471,84 @@ Measured against the live `platform-pg` specification, not estimated.
instances: 1 (no HA; single-node rail)
max_connections: 100
memory limit: 1Gi
cpu limit: 1
storage: 20Gi
per consumer: 14 connections (12 runtime + 2 migration)
```
**Connection ceiling: roughly six consumers.** At 14 connections per consumer
declaration, seven consumers request 98 of 100 connections, before CNPG's
instance manager, the metrics exporter, and superuser-reserved slots. The
practical ceiling is about **six**. Two are provisioned today
(`audit-core`, `tenant-engine`) plus the isolation probe. **We are at
roughly a third of capacity, and the third consumer request will not feel
like a scaling event.**
**Connection ceiling: roughly six consumers.** Seven consumers request 98 of
100 before CNPG's instance manager, metrics exporter, and reserved slots. Two
are provisioned today. **We are at roughly a third of capacity and the third
request will not feel like a scaling event.**
**Memory is likely to bind before connections do.** 100 PostgreSQL backends
against a 1Gi limit is roughly 10MB per backend for all of shared buffers,
work_mem, and per-backend overhead. Connection exhaustion produces a clear
error; memory pressure produces OOM kills and degraded latency across every
co-resident consumer at once.
**Memory likely binds first.** 100 backends against 1Gi is ~10MB per backend
for shared buffers, work_mem and overhead. Connection exhaustion errors
clearly; memory pressure OOM-kills and degrades every co-resident at once.
**Restore time couples all consumers.** Physical backup is instance-wide, so
restoring one consumer means restoring the whole instance to scratch, taking
a logical dump, and importing. A consumer's RTO is therefore a function of
**total instance size**, not its own. One large consumer silently degrades
every co-resident's recovery promise — and per the storage draft §7.3, a
consumer must not promise retention or recovery its platform cannot back.
**Raising E is not free on a shared substrate.** RLS with a per-checkout GUC
does not itself increase connection count, but it forecloses aggressive
transaction-level pooling for consumers that adopt it. E3 on P1 interacts with
the ceiling above and should be planned against it.
**`instances: 1` means no tenant on `platform-pg` has HA.** Deliberate on the
current rail and honestly documented, but it means every shared consumer's
availability story is "restart recovery", not high availability. A plan tier
promising uptime cannot be satisfied at R1 as currently built.
**Restore time couples all consumers.** Physical backup is instance-wide, so a
consumer's RTO is a function of *total* instance size, not its own. One large
consumer silently degrades every co-resident's recovery promise.
**What this implies.** The ladder is not a long-term growth path on the
present substrate — it is a two-to-four-consumer runway. The triggers in §6
must be monitored, not merely recorded, and connection and memory
utilisation belong on the alert list *before* the next consumer is accepted.
**`instances: 1` means no P1 tenant has HA.** A tier promising uptime cannot
be satisfied at P1 as currently built — an availability floor belongs in §11's
minimum-level vocabulary alongside isolation.
## 11. Consequences
## 17. Consequences
- The estate gains one vocabulary for a conversation currently held in four
incompatible ones.
- The estate gains one vocabulary and, more importantly, a way to be honest
about partial adoption.
- Absorbed repos get a described state and a path instead of a failing grade.
- `tenantIsolation` in `PostgresConsumer` is revealed as a mislabelled field
and should be renamed or moved once this ADR settles (§12.1).
- Two ratified defaults (R1 for platform services, R3 for business apps) are
reconciled by scope rather than by one overriding the other, at the cost of
needing a rule for classifying a new service.
- The unverified tenant boundary (§9) becomes a named, owned gap rather than
an assumption distributed across every repo.
- The `platform-pg` ceiling becomes a planning input rather than a discovery.
- Nothing in this ADR changes a running system. It is descriptive of what is
built, plus five decisions that need owner sign-off.
(§18.1).
- The two ratified defaults are reconciled by scope, at the cost of needing a
classification rule.
- The verification problem becomes tractable: guard against declaration, not
against a universal maximum.
- Draft-1's prohibition on RLS is reversed; `rapp-postgres` acquires an
obligation to define and offer the E3 mechanism.
- Nothing here changes a running system.
## 12. Open questions for ratification
## 18. Open questions
1. **`tenantIsolation` field** — `rapp-postgres`: rename to name its plane
(`tenantBoundaryEnforcement`?), or move it out of the storage declaration
entirely.
2. **Placement ownership** — `railiance-platform`: accept the ladder and
triggers, and appoint a recorded placement owner per workload. This is the
2026-08-16 request, still open.
3. **Tenant-boundary conformance** — *owner needed*. Who builds and runs a
fleet check that a consumer cannot leak tenant A's rows to tenant B?
`audit-core` and `tenant-engine` have both declined fleet-scope work on
correct boundary reasoning, so this needs appointing rather than
volunteering. Highest-severity gap in the model.
4. **Business app vs platform service** — *Custodian canon*: a definition, so
a new repo knows whether §1.2's R3 default or this ADR's R1 default
applies to it.
5. **Plan tier → rung mapping** — `adaptive-pricing` and `tenant-engine`:
no tier may name isolation without citing a rung (§7).
6. **Cell-based partitioning** — evaluate before the §10 ceiling is reached.
7. **Engine neutrality** — whether the ladder should be stated
engine-neutrally or admit that R1's guarantee rests on a PostgreSQL
property (per-database catalogs). Inherited unresolved from the storage
draft.
1. **`tenantIsolation` field** — `rapp-postgres`: rename to name its plane and
carry a level (`tenancy.E: 2`), or move it out of the storage declaration.
2. **Placement ownership** — `railiance-platform`: accept the ladder, the
triggers, and the monitoring obligation in §8.1, and appoint a recorded
placement owner per workload.
3. **E2/E3 evidence artifacts** — *owner needed.* Who builds the adversarial
cross-tenant probes and the RLS conformance check? Both `audit-core` and
`tenant-engine` have declined fleet-scope work on correct boundary
reasoning, so this needs appointing. Highest-severity gap (§13.2).
4. **Business app vs platform service** — Custodian canon: a classification
rule, so a new repo knows whether P3 or P1 is its default. Candidate: reuse
`repo-classification-standard_v1.0` rather than invent a second taxonomy.
5. **Tier → minimum level mapping** — `adaptive-pricing` and `tenant-engine`:
required only for tiers making isolation or availability claims (§11).
6. **E3 mechanism** — `rapp-postgres`: what exactly is the GUC contract, and
is it provisioned per consumer on request or offered as a standard option?
7. **Engine neutrality** — the P ladder rests on a PostgreSQL property
(per-database catalogs). State it engine-specifically and say so, or
abstract it and risk a non-Postgres implementation that silently differs?
Inherited unresolved from the v0.1 draft.
8. **Cell-based partitioning** — evaluate at consumer #4 rather than now?
Neither P nor E describes a cell.
## 13. Ratification path
## 19. Ratification path
The predecessor draft stalled because it was routed without one. This ADR:
The predecessor draft stalled because it was routed without one.
1. Is reviewed by `tenant-engine`, `flex-auth`, `rapp-postgres`,
`railiance-platform`, and `adaptive-pricing` against §12.
2. On acceptance, **supersedes** the routing of
1. Reviewed by `tenant-engine`, `flex-auth`, `rapp-postgres`,
`railiance-platform`, and `adaptive-pricing` against §18.
2. Each of those repos publishes its own posture vector (§5) as part of
review. **The framework is validated by whether it can describe them
accurately** — if a repo cannot express itself in these four ladders, the
ladders are wrong and this document changes, not the repo.
3. On acceptance, **supersedes** the routing of
`rapp-postgres/docs/canon-drafts/shared-platform-relational-storage_v0.1-draft.md`,
whose §§3–8 are absorbed above. That draft should then be withdrawn rather
than left pending, so the estate does not carry two overlapping proposals.
3. On acceptance, `rapp-postgres/docs/adr/ADR-0001` moves from `proposed` to
`accepted` and is annotated as the PostgreSQL implementation of this model.
whose §§3–8 are absorbed here. That draft is then withdrawn rather than
left pending.
4. On acceptance, `rapp-postgres/docs/adr/ADR-0001` moves to `accepted` and is
annotated as the PostgreSQL implementation of the E and P ladders.