Multi-Tenant SaaS Architecture

Acknowledgement: Shared by Peter Bamuhigire, techguypeter.com, +256 784 464178.

Use When

• Designing tenancy for a new SaaS, or hardening an existing SaaS against cross-tenant leakage.
• Splitting a SaaS into three panels (super admin, tenant/franchise admin, end user) with distinct scopes.
• Designing per-tenant role overrides, permission priority ordering, and audit trails for privileged access.
• Planning the session, JWT, and tenant_id plumbing that downstream database, API, security, and delivery skills rely on.

Do Not Use When

• The task is a single-tenant app with no plan for tenancy — this skill's constraints add overhead with no benefit.
• The work is about business-module composition rather than tenant boundaries — use modular-saas-architecture.
• The work is pure schema shaping inside an already-defined tenant model — use database-design-engineering and mysql-best-practices.

Required Inputs

• Context map and critical-flow table from system-architecture-design.
• Access-pattern list from database-design-engineering (so isolation model matches real queries).
• Threat model / abuse cases from vibe-security-skill.
• Panel and user-type list from product requirements (super admin, tenant admin, end user, and their sub-types).

Workflow

• Read this SKILL.md first, then load only the referenced deep-dive files that are necessary for the task.
• Apply the ordered guidance, checklists, and decision rules in this skill instead of cherry-picking isolated snippets.
• Produce the tenant-isolation map, panel layout, permission model, and audit plan as the deliverables named in Outputs.

Quality Standards

• Keep outputs execution-oriented, concise, and aligned with the repository's baseline engineering standards.
• Preserve compatibility with existing project conventions unless the skill explicitly requires a stronger standard.
• Prefer deterministic, reviewable steps over vague advice or tool-specific magic.

Anti-Patterns

• Treating examples as copy-paste truth without checking fit, constraints, or failure modes.
• Loading every reference file by default instead of using progressive disclosure.

Outputs

• Tenant-isolation map, panel definitions, permission priority model, and audit plan (see the Outputs table below).
• Clear assumptions, tradeoffs, or unresolved gaps when the task cannot be completed from available context alone.
• References used, companion skills, or follow-up actions when they materially improve execution.

Evidence Produced

Category	Artifact	Format	Example
Data safety	Tenant isolation note	Markdown doc covering row-level vs schema-level isolation, RLS policies, and tenant-scoped indexes	docs/saas/tenant-isolation-note.md
Security	Tenant authorization audit	Markdown doc covering super-admin vs tenant-admin vs user privilege boundaries and authz tests	docs/saas/tenant-authz-audit.md

References

• Use the references/ directory for deep detail after reading the core workflow below.
• Use the documentation/ directory for supporting implementation detail or migration notes.

Production-grade multi-tenant SaaS architecture with three-panel separation, zero-trust enforcement, strict tenant isolation, and comprehensive audit trails. This skill defines the tenant and auth boundaries that every downstream module, API, and data query inherits.

Prerequisites

Load these first, in order:

1. world-class-engineering — repository-wide quality bar.
2. system-architecture-design — produces the context map and critical flows this skill consumes.
3. database-design-engineering — produces the access-pattern list that shapes the isolation model.
4. vibe-security-skill — produces the threat model and auth rules this skill encodes.

When this skill applies

• Designing tenancy for a new SaaS from scratch.
• Converting a single-tenant app to multi-tenant (see documentation/migration.md).
• Auditing an existing SaaS for cross-tenant leakage, missing tenant_id filters, or un-audited admin actions.
• Planning the three-panel file and route layout (/, /adminpanel/, /memberpanel/).
• Designing the permission priority ladder (user deny > user grant > tenant override > role > default deny).
• Defining what actions must be audited and how long audit records are retained.

Inputs

Artifact	Produced by	Required?	Why
Context map	system-architecture-design	required	defines services, panels, ownership
Critical-flow table	system-architecture-design	required	shapes auth and session boundaries
Access-pattern list	database-design-engineering	required	chooses shared-DB-with-tenant_id vs DB-per-tenant
Threat model	vibe-security-skill	required	informs audit rules and abuse cases
Panel and user-type list	product requirements	required	determines role and scope per panel

Outputs

Artifact	Consumed by	Template
Tenant-isolation map (isolation model + tenant_id rules per table)	database-design-engineering, api-design-first	inline (this skill)
Panel definitions (super admin, tenant admin, end user)	api-design-first, kubernetes-saas-delivery	inline
Permission priority model (user/tenant override/role/default deny)	vibe-security-skill, api-design-first	inline + references/permission-model.md
Session and JWT plan (prefixing, lifetimes, rotation)	api-design-first, vibe-security-skill	inline
Audit event set and retention policy	observability-monitoring, reliability-engineering	inline
Migration plan (single-tenant to multi-tenant)	database-design-engineering, deployment-release-engineering	documentation/migration.md

Non-negotiables

• Every franchise/tenant-scoped table has tenant_id (or franchise_id) NOT NULL with a foreign key to tenants.
• Every query on a tenant-scoped table includes tenant_id in the WHERE clause.
• tenant_id is always extracted from session or JWT — never from client input (body, query, header).
• Default authorisation decision is deny. Every action requires an explicit permission check.
• Every super-admin action that touches tenant data is audited with actor, target tenant, and justification.
• Cross-tenant access that should not exist returns 404 Not Found, never 403 Forbidden (403 confirms the resource exists).

Decision rules

Isolation model

Tenants share data schema AND tenants < ~10k AND regulated data not tenant-partitioned
    -> Shared DB, row-level tenant_id (default choice for this skill)

Strong regulatory boundary (HIPAA/PCI) per tenant
    -> Schema-per-tenant OR DB-per-tenant; app injects connection by tenant

Single very large tenant needs isolation from the rest
    -> Hybrid: shared DB for small tenants, dedicated DB for large tenant

Wrong choice failure modes:
- Shared DB without tenant_id discipline -> data leakage, impossible to audit
- DB-per-tenant at small scale -> migration and operational cost explodes
- Schema-per-tenant without tooling -> migrations drift across schemas

Panel placement of a new feature

Feature controls platform config, billing, tenant lifecycle, or impersonation
    -> /adminpanel/ (super_admin only, audited)

Feature is tenant-admin workspace (manage own tenant, staff, catalogue)
    -> /public/ root (tenant owners/staff, scoped by tenant_id)

Feature is self-service for end user (customer/member/student/patient)
    -> /memberpanel/ (own records only, scoped by tenant_id AND user_id)

Wrong choice failure modes:
- Admin feature in /public/ -> tenant users see platform levers
- Tenant feature in /memberpanel/ -> end users escalate to admin-only data
- End-user feature in /public/ -> collides with tenant admin routes

Permission resolution priority

1. actor is super_admin            -> ALLOW (always audit)
2. user_permissions.denied match   -> DENY
3. user_permissions.granted match  -> ALLOW
4. tenant_role_overrides match     -> ALLOW/DENY per is_enabled
5. role -> permission via template -> ALLOW
6. no match                        -> DENY

Wrong ordering failure mode: if role is checked before user deny, an
explicit revocation never takes effect. Always deny-before-grant at each tier.

Session prefix vs no prefix

One SaaS codebase hosts multiple apps on the same origin  -> prefix required
Only one app on origin AND no shared subdomain sessions   -> prefix optional
SSO / impersonation across apps planned                   -> prefix required

Without a prefix, two SaaS apps on the same host collide in $_SESSION and the
bug only appears in production when both apps are logged in.

Three-panel architecture

+-------------------------------------------------------------+
|                Shared Infrastructure Layer                  |
|  Data (tenant isolated) | Business Logic | Session system   |
+-------------------------------------------------------------+
         |                       |                    |
+--------v--------+    +---------v------+   +---------v-------+
|   /public/      |    |  /adminpanel/  |   |  /memberpanel/  |
| (ROOT)          |    |                |   |                 |
| Tenant Admin    |    |  Super Admin   |   |  End User       |
|  Workspace      |    |  System        |   |  Portal         |
| owner, staff    |    |  super_admin   |   |  member/student |
+-----------------+    +----------------+   +-----------------+

Important: /public/ root is the tenant-admin workspace, not an end-user panel. Confusing the two is the most common mistake.

File layout:

public/
  index.php           # Landing
  sign-in.php         # Login
  dashboard.php       # Tenant admin dashboard
  adminpanel/         # Super admin panel
    includes/
  memberpanel/        # End user portal
    includes/
  includes/           # Shared includes
  assets/

Panel definitions

Panel	Path	Users	Scope	Notes
Tenant Admin	/public/	owner, staff	single tenant	all queries include tenant_id = ?; cannot touch platform config
Super Admin	/public/adminpanel/	super_admin	cross-tenant	tenant_id may be NULL; every action audited
End User	/public/memberpanel/	member, student, customer, patient	own records within tenant	queries scoped by tenant_id AND user_id

Tenant isolation model

tenant_id rules per user type

User type	tenant_id column
super_admin	nullable
tenant owner	required, NOT NULL
tenant staff	required, NOT NULL
member / student / customer / patient	required, NOT NULL

Table pattern

CREATE TABLE students (
    id            BIGINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    tenant_id     BIGINT UNSIGNED NOT NULL,
    first_name    VARCHAR(100) NOT NULL,
    last_name     VARCHAR(100) NOT NULL,
    email         VARCHAR(100),
    FOREIGN KEY (tenant_id) REFERENCES tenants(id) ON DELETE CASCADE,
    INDEX idx_tenant (tenant_id),
    INDEX idx_tenant_email (tenant_id, email)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

Composite indexes lead with tenant_id. utf8mb4_unicode_ci everywhere. Match file case exactly (Linux file systems are case-sensitive).

Query enforcement

// Extract tenant from session/JWT. Never from client input.
$tenantId = getSession('tenant_id');

// Regular users: filter by tenant_id
$stmt = $db->prepare('SELECT * FROM students WHERE tenant_id = ? AND id = ?');
$stmt->execute([$tenantId, $studentId]);

// Super admin: cross-tenant access is allowed but always audited
if (getSession('user_type') === 'super_admin') {
    auditLog('CROSS_TENANT_ACCESS', [
        'admin_user_id'      => getSession('user_id'),
        'target_tenant_id'   => $requestedTenantId,
        'action'             => 'VIEW_STUDENTS',
        'justification'      => $request['justification'] ?? null,
    ]);
}

Session and JWT plan

Session prefix system

define('SESSION_PREFIX', 'saas_app_');  // e.g. 'school_', 'restaurant_'

setSession('user_id', $userId);      // writes $_SESSION['saas_app_user_id']
$userId = getSession('user_id');     // reads $_SESSION['saas_app_user_id']

Session cookie settings

Setting	Value
HttpOnly	true
Secure	auto-detect HTTPS (port 443 or $_SERVER['HTTPS'])
SameSite	Strict
Lifetime	30 min idle
Regeneration	on login and on privilege change

$isHttps = (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off')
    || (int) $_SERVER['SERVER_PORT'] === 443;
ini_set('session.cookie_secure', $isHttps ? '1' : '0');

JWT for mobile and API

Setting	Value
Access token lifetime	15 min
Refresh token lifetime	30 days
Rotation	on every refresh
Revocation	persisted table keyed by jti

Permission model

Priority (highest first): user deny > user grant > tenant override > role permission > default deny. See references/permission-model.md for full schema, resolution algorithm, caching strategy, and seed data.

function hasPermission(userId, tenantId, permission) {
  if (user.type === 'super_admin') {
    auditLog('PERMISSION_BYPASS', { userId, permission });
    return true;
  }
  if (userPermissions.denied(userId, tenantId, permission)) return false;
  if (userPermissions.granted(userId, tenantId, permission)) return true;
  for (const role of getUserRoles(userId, tenantId)) {
    const override = tenantRoleOverride(tenantId, role.id, permission);
    if (override !== null) return override.isEnabled;
    if (roleHasPermission(role, permission)) return true;
  }
  return false; // default deny
}

Zero-trust checklist

• tenant_id in every query on a tenant-scoped table (except super_admin with audit).
• Prepared statements only — no string concatenation into SQL.
• Permission check before every mutating operation.
• MFA required for super-admin access.
• Passwords stored as Argon2ID with per-user salt (32 chars) and server-wide pepper (64+ chars).
• Account lockout after 5 failed attempts with exponential backoff.
• Rate limiting: tenant 1000 req/min, user 100 req/min.
• HTTPS + HSTS on all panels.
• Super-admin actions audited with justification.
• Cross-tenant access returns 404, not 403.

API design

# Tenant-scoped (default)
GET    /api/v1/orders          # list within tenant
POST   /api/v1/orders          # create within tenant
GET    /api/v1/orders/{id}     # show; 404 if wrong tenant
DELETE /api/v1/orders/{id}     # delete; 404 if wrong tenant

# Super admin (cross-tenant, audited)
GET    /api/v1/admin/tenants
POST   /api/v1/admin/impersonate

Response envelope:

{ "success": true, "data": {}, "meta": { "page": 1, "total": 100 } }

Error envelope:

{ "success": false, "error": { "code": "PERMISSION_DENIED", "message": "..." } }

Audit and compliance

Always audit: all super-admin actions, impersonation, permission changes, tenant creation/suspension, data exports, failed auth, cross-tenant access attempts.

{
  "id": "uuid",
  "timestamp": "2026-04-07T10:30:00Z",
  "actor_user_id": 123,
  "actor_type": "super_admin",
  "action": "IMPERSONATE_USER",
  "target_tenant_id": 456,
  "justification": "Support request #12345",
  "ip_address": "203.0.113.1",
  "changes": { "before": {}, "after": {} }
}

Retention: security logs 1 year, audit trails 7 years, operational logs 90 days.

Monitoring alerts

• Cross-tenant access attempt (target = zero).
• Super-admin login from new IP.
• Failed auth spike > 100/min.
• Database query without tenant_id on a tenant-scoped table.
• API error rate > 5%.

Tenant lifecycle

PENDING -> ACTIVE -> SUSPENDED -> ARCHIVED

Archival is reversible until a configured retention cut-off; archived tenants are then hard-deleted via cascade.

Anti-patterns

• Client-supplied tenant identifier. $franchiseId = $_POST['franchise_id']; — any user can set this to another tenant. Fix: always getSession('tenant_id') or read from the verified JWT claim.
• Missing tenant scope on a query. SELECT * FROM students WHERE id = ? — returns any student across any tenant. Fix: SELECT * FROM students WHERE tenant_id = ? AND id = ? and treat a missing row as 404.
• Super-admin mutation without audit. deleteStudent($studentId); run from the super-admin panel with no log. Fix: wrap every super-admin mutation with auditLog('ADMIN_DELETE_STUDENT', [...]) capturing actor, target tenant, and justification.
• Returning 403 for cross-tenant access. Confirms the resource exists and enables tenant-ID enumeration. Fix: map cross-tenant misses to 404 so the existence of the resource is hidden.
• tenant_id nullable on a business table. tenant_id BIGINT NULL — allows rows with no owner, which every tenant query then excludes silently. Fix: tenant_id BIGINT UNSIGNED NOT NULL with a foreign key; backfill before flipping NOT NULL.
• Session keys without a prefix in a multi-app host. Two apps on the same origin overwrite each other's $_SESSION['user_id']. Fix: SESSION_PREFIX per app, always go through setSession() / getSession().
• Index order (status, tenant_id) instead of (tenant_id, status). Query planner cannot use the composite index to isolate one tenant's slice. Fix: lead every composite index with tenant_id.
• Big-bang migration of every table at once. Raises rollback risk and produces hours of downtime. Fix: migrate one domain at a time with the six-step pattern in documentation/migration.md and verify no NULL tenant_id before flipping the column to NOT NULL.

Read next

• database-design-engineering — schema, indexes, migration discipline once the isolation model is chosen.
• api-design-first — REST conventions, auth model, and error envelope that this skill's panels depend on.
• vibe-security-skill — threat modelling, abuse cases, and auth/authz matrix that feed this skill.
• kubernetes-saas-delivery — per-tenant deploy, namespace isolation, and progressive rollout on top of this model.
• modular-saas-architecture — when the platform also composes pluggable business modules inside the tenancy defined here.
• mysql-best-practices — MySQL engine-specific execution of the schema patterns here.

References

• references/database-schema.md — tenant, user, audit, and tenant-scoped table schemas with indexes and partitioning.
• references/permission-model.md — RBAC schema, permission resolution algorithm, caching, middleware, hierarchical and conditional permissions.
• documentation/migration.md — adding tenant_id safely, zero-downtime migration, single-to-multi-tenant phases, rollback.