Multi-Tenancy Architecture: Shared vs Isolated Patterns for SaaS

Every SaaS app serves multiple customers (tenants) from the same infrastructure. The architecture decision — how much isolation between tenants — affects security, cost, performance, and complexity.

The Three Models#

1. Shared Database (Row-Level Isolation)#

All tenants in the same database, same tables. tenant_id column on every row.

SELECT * FROM orders WHERE tenant_id = 'acme' AND status = 'active';

Pros: Cheapest, simplest, easy to manage Cons: Noisy neighbor risk, one bad query affects all, data leak if you forget WHERE clause Best for: Early-stage SaaS, < 1000 tenants, non-sensitive data

2. Schema-Per-Tenant#

Same database server, separate schema (namespace) per tenant.

-- Tenant A
SELECT * FROM tenant_acme.orders WHERE status = 'active';
-- Tenant B
SELECT * FROM tenant_corp.orders WHERE status = 'active';

Pros: Better isolation, easier backup/restore per tenant, no tenant_id risk Cons: Schema migration across N schemas is complex, connection pooling harder Best for: Mid-stage SaaS, 100-10K tenants, moderate isolation needs

3. Database-Per-Tenant#

Each tenant gets their own database instance.

Tenant A → postgres-acme.rds.amazonaws.com
Tenant B → postgres-corp.rds.amazonaws.com

Pros: Maximum isolation, per-tenant scaling, easy compliance (data residency) Cons: Expensive (N databases), complex provisioning, cross-tenant queries impossible Best for: Enterprise SaaS, regulated industries (HIPAA, finance), large tenants

Comparison#

Modern Approach: Database Branching#

Tools like Neon and Turso make database-per-tenant affordable:

Neon (Serverless Postgres)#

• Branch creation: < 1 second (copy-on-write)
• Scale to zero when tenant is idle (no cost)
• Thousands of databases per project

Turso (Edge SQLite)#

• Per-tenant embedded replicas (in-app, zero latency reads)
• 10K+ databases per account on free tier
• Global replication per tenant

// Turso: create database per tenant
const db = createClient({
  url: `libsql://${tenantId}-myapp.turso.io`,
  authToken: process.env.TURSO_TOKEN,
});

Row-Level Security (RLS)#

PostgreSQL's built-in tenant isolation:

-- Enable RLS
ALTER TABLE orders ENABLE ROW LEVEL SECURITY;

-- Policy: users can only see their tenant's data
CREATE POLICY tenant_isolation ON orders
  USING (tenant_id = current_setting('app.tenant_id'));

-- Set tenant context per request
SET app.tenant_id = 'acme';
SELECT * FROM orders; -- Only returns acme's orders

Supabase uses RLS as its primary security model. Every query is automatically filtered.

Application-Level Patterns#

Tenant Resolution#

How to identify the tenant for each request:

1. Subdomain:   acme.myapp.com → tenant = "acme"
2. Path:        myapp.com/acme/dashboard → tenant = "acme"
3. Header:      X-Tenant-ID: acme
4. JWT claim:   { "tenant_id": "acme", ... }
5. API key:     sk_acme_... → lookup tenant from key

Subdomain is most common for customer-facing SaaS. JWT claims for APIs.

Tenant Context Middleware#

// Express middleware
app.use((req, res, next) => {
  const tenant = req.hostname.split(".")[0]; // acme.myapp.com → acme
  req.tenantId = tenant;
  // Set database context
  db.raw(`SET app.tenant_id = '${tenant}'`);
  next();
});

Tenant-Aware Caching#

Cache key: tenant:{tenantId}:resource:{resourceId}
Example:   tenant:acme:user:123

Never share cache between tenants. Always include tenant_id in cache keys.

Handling the Noisy Neighbor#

One tenant's heavy query shouldn't slow down everyone:

1. Rate limiting per tenant — 100 req/min per tenant, not global
2. Query timeout — kill queries running > 5 seconds
3. Connection pooling — limit connections per tenant
4. Resource quotas — CPU/memory limits per tenant (K8s)
5. Queue isolation — separate queues for heavy tenants

Architecture Examples#

Startup SaaS (Shared DB)#

Client → API → PostgreSQL (shared, RLS)
                → tenant_id on every table
             → Redis (tenant-prefixed keys)
             → S3 (tenant-prefixed paths)

Growth SaaS (Hybrid)#

Client → API Gateway (tenant from subdomain)
           → Shared services (auth, billing)
           → Tenant router
               → Small tenants → Shared PostgreSQL (RLS)
               → Large tenants → Dedicated PostgreSQL
           → Redis Cluster (tenant-namespaced)

Enterprise SaaS (Full Isolation)#

Client → API Gateway
           → Tenant A → Dedicated K8s namespace
                          → Own PostgreSQL, Redis, workers
           → Tenant B → Dedicated K8s namespace
                          → Own PostgreSQL, Redis, workers
           → Shared: Auth, billing, monitoring

Generate your multi-tenant architecture →

Migration Path#

Stage 1 (0-100 tenants): Shared DB + RLS
Stage 2 (100-1K tenants): Schema-per-tenant for large customers
Stage 3 (1K+ tenants): Database-per-tenant for enterprise

Don't over-engineer from day one. Start shared, isolate when you need to.

Summary#

1. Start shared — row-level isolation with RLS is simple and cheap
2. RLS is your friend — PostgreSQL enforces isolation at the database level
3. Subdomain routing for tenant resolution
4. Always prefix cache/storage keys with tenant_id
5. Rate limit per tenant — prevent noisy neighbors
6. Modern tools (Neon, Turso) make DB-per-tenant affordable
7. Migrate gradually — shared → schema → dedicated as tenants grow

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